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Burp Charge Your Batteries for better cell health By JOHN CLARKE Most readers know that Nicad and NiMH batteries can be fast charged but an even better way of doing it is to “burp” charge them. This is a rapid alternate charge and discharge process that reduces pressure and temperature build-up in the cells and as a result, increases the charging efficiency. T HIS VERY versatile Nicad & NiMH Burp Charger can charge a single cell or up to 15 series-connected cells. All the standard charge cycles such as fast charge, top-up and trickle are available, together with the added benefits of burp charging. Built-in safeguards include temperature sensing of the cells to prevent overcharging, as well as sensing inside the charger itself for over-temperature protection. The concept of burp charging has been known since the late 1960s. At that time though, the many benefits 66  Silicon Chip thought to be associated with it were largely unsubstantiated. A specialised IC (the ICS1702) was developed that incorporated burp charging (see http://www.klaus-leidinger.de/mp/ RC-Elektronik/Reflexlader/ics1702. pdf) and this became the basis for commercial burp chargers and for chargers used by NASA for nicad cells in space applications. However, these chargers were used without any real understanding as to why burp charging was beneficial. It wasn’t until 1998 that an exhaus- tive investigation compared standard charging with burp charging in a research paper entitled “Investigation of the Response of NiMH Cells to Burp Charging” by Eric C. Darcy (see http://corsair.flugmodellbau.de/ files/elektron/NASA-II.PDF or in condensed form at ntrs.nasa.gov/search. jsp?R=20000086665). This research proved that burp charging improved cell performance compared to other charging techniques. Basically, it was found that the burp process caused the oxygen bubbles siliconchip.com.au + 1.0 CHARGE BATTERY CURRENT 0.5 CHARGE PAUSES TIME 0 0.5 1.0 1.5 2.0 BURP 2.5 940ms 1ms 1 SECOND 30ms 29ms Fig.1: the charge, pause & discharge (burp) cycles for the SILICON CHIP Nicad/NiMH Burp Charger. It comprises a 940ms charge period followed by a 1ms pause, then a 30ms discharge period, followed by a 29ms pause, giving a total cycle of one second. – BURP CHARGE CYCLE produced during charging to be reabsorbed back in the electrochemical process. With oxygen levels lowered, there is less pressure build up inside the cell. In addition, the lack of oxygen bubbles increases the available surface area on the cell electrodes and results in more efficient charging. The research also found that while many commercial burp chargers, including those that use the ICS1702 IC, used a 5ms discharge (burp) period, a period of 30ms was more beneficial. That’s because a longer discharge period allows more complete recombination of generated oxygen. For that reason, a 30ms burp period is used in the new design described here. By the way, the term “burp” charging is something of a misnomer as the oxygen is not “burped” or released. Instead, it is recombined or consumed at the positive electrode surface. very simple circuit design which used the positive half of the AC waveform from a low-voltage transformer for charging and the negative half of the AC waveform for discharging. The circuit was designed so that the discharge current was much less than the charge current, otherwise charging wouldn’t have occurred. However, this charge/discharge cycle was far from ideal. Burp chargers are not commonly available but standard NiMH/Nicad chargers can be obtained just about anywhere. However, the latter usually only charge two or four AA cells at a time and they charge at quite a slow rate, typically taking 4-15 hours for a full recharge. But what if you want to charge at a much higher rate, or you want to charge more than four cells at a time, or if you want to use burp charging? Or what if you want to cater for ‘C’ and ‘D’ cells or battery packs? The answer is to build the SILICON CHIP Nicad/NiMH Burp Charger. This new unit can charge from 1-15 NiMH or Nicad cells simultaneously; ie, battery packs up to 18V. In addition, the charging rate can be set from just a few milliamps up to 2.5A and it includes reliable end-of-charge detection (using temperature sensing), with extra safeguards to prevent over-charging. Safety is important when charging NiMH and Nicad cells because they can have their life seriously shortened if the charger is left on for too long after the battery pack has reached full charge. Worse still, the cells can be destroyed or explode if over-charged. To see why over-charging can destroy a battery pack, take a look at Fig.2. This shows the typical voltage, temperature and internal pressure rise of a cell or battery pack during charging. Once charging goes past the 100% point, the temperature and internal pressures rapidly rise, while the voltage initially rises and then falls. Continual overcharging will damage the cells due to the elevated temperature. This accelerates chemical reactions that contribute to the cell’s ageing process. In extreme cases during overcharging, excessive internal pressure can open the safety vents to release the pressure. These vents 75 1.50 100 65 1.46 80 siliconchip.com.au CELL VOLTAGE 45 60 1.42 1.38 40 PRESSURE 55 CELL VOLTAGE Fig.1 shows the sequence of charge, pause and discharge (burp) for the SILICON CHIP Nicad/NiMH Burp Charger. It comprises a 940ms charge period followed by a 1ms pause, then a 30ms discharge period, followed by a 29ms pause, giving a total cycle of one second (1s). On this figure, a charging period is shown as having a value of ‘1’ while a discharge (burp) period is assigned a value of -2.5. This simply means that the discharge current is 2.5 times the charge current. This cycle differs markedly from that used in the Burp Charger published in the August 1995 issue of “Electronics Australia”. In that circuit, the charge and burp discharge periods were the same at 10ms each. This was due to the TEMPERATURE (°C) Charge/discharge cycle PRESSURE TEMPERATURE 35 25 1.30 20 1.34 0 0 50 100 STATE OF CHARGE (%) Fig. 2: typical charging curves for NiMH/Nicad batteries. Cell temperature (green) and voltage (red) rate of change are often used to detect the “end point” (100% charge), although voltage rate detection is not reliable in NiMH cells. March 2014  67 Main Features •  Designed for charging NiMH and Nicad cells •  •  Optional burp charging •  Adjustable charge current •  Charging time-out •  dT/dt (temperature change rate) for Optional top-up and trickle charging end of charge detection •  Over and under cell-temperature detection •  Power, charging and temperature indication LEDs •  Adjustable charging time-out limit •  Adjustable dT/dt setting •  Adjustable top-up and trickle charge currents •  Over-temperature cut out for charger will then re-close after the pressure has been released but by that time the cells will already have been damaged. Full charge detection Full charge can be determined in one of two ways. The conventional way has been to monitor the voltage across the battery pack and detect the point at which the voltage suddenly begins to rapidly rise and then fall. This form of charge end-point detection is called dV/dt (ie, change in voltage with respect to time). In practice, the critical end-point can be difficult to detect at low currents, particularly with NiMH cells. In fact, dV/dt end-point detection with NiMH cells is neither safe nor practical. The only safe way is to monitor the temperature of the cells but very few chargers do this. Basically, this latter method of endpoint detection monitors the temperature rise of one or two cells within the battery pack. During charging, the cells do not heat up much because most of the incoming power is converted into stored energy. However, once the cells become fully charged, the charging power is converted to heat and so the cells quickly rise in temperature. This temperature change at the charging end point is called dT/dt, ie, change in temperature over time. The critical rate is of the order of 2°C per minute and this is the point where 68  Silicon Chip normal charging should cease. Some chargers, this one included, include a top-up charge after the end-point to ensure full charging. The top-up charge rate is less than the main charge current and is set at four times the trickle current setting. Finally, after the top-up cycle, the cells can be trickle-charged at low current to maintain full charge. In this situation, the cells are deliberately left connected to the charger so they are fully charged when needed. Our new burp charger monitors the cell temperature using a small thermistor. This is installed in the battery pack or cell holder, in close contact with one of the cells. The beauty of this system is that it will reliably detect the end of charge (end-point) of any type of cell, regardless of whether it was initally completely flat or only partially discharged. Note that when charging very cold batteries, there may be a rapid rise in temperature during charging. This could cause a false dT/dt end of charge indication. To circumvent this, the dT/dt measurement for end of charge detection is only enabled when the cell temperature is at least 25°C. Should the thermistor end-point detection fail, a timer is included that will switch off charging after a preset period. Further safeguards to protect the cells are also included. For example, charging will not start or will cease if the NTC thermistor is disconnected or if the temperature is under 0°C or over 50°C. In addition, if the charger itself becomes too hot, charging will pause and the temperature is measured after two minutes to check if it has cooled sufficiently to restart. Select the features you want In its simplest form, the charger includes only the temperature detection feature, after which charging ceases. However, you can add top-up and trickle charging if you want. In addition, the charging rate can be set for both the main charge current and the trickle charge, along with the time-out period and dT/dt values. In practice, the main charging rate can be set from about 40mA up to 2.5A, while trickle-charging can be set from 10-500mA. The time-out can be set from between 0-25 hours, while dT/ dt can be selected from between 0.5°C per minute to 5°C per minute. Further details concerning these ad- justments are included in the settingup section of this article. Three front-panel LEDs are used to indicate the status of the charger. First, the Power LED is lit whenever power is applied to the charger, while the Thermistor LED lights if the thermistor is disconnected or if there is an over or under-temperature detection. For over-temperature (>50°C), the Thermistor LED will flash once a second (1Hz) while for under temperature (<0°C), the LED will flash once every two seconds (0.5Hz). Over-heating of the charger itself causes the Thermistor LED to flash once every four seconds. Finally, the Charging LED is continuously lit during the main charging cycle and switches off when charging is complete. If top-up or trickle charging are selected, the charging LED will flash at 1Hz during top-up and at 0.5Hz during trickle charge (ie, at 1s and 2s intervals respectively). Note that if the Thermistor LED is lit or flashing, the charging LED will be off, indicating that charging has either paused or ceased. Circuit details Now take a look at Fig.3 for the circuit details. It’s based on IC1, a PIC16F88-I/P microcontroller, plus Mosfets Q1 & Q2. Q1 is used for charging, while Q2 is used for the burp discharging. In addition, two NTC thermistors, TH1 & TH2, are used. TH1 monitors the temperature of the cell or battery pack being charged. It’s connected via a 3.5mm jack plug and socket (CON3) and together with 20kΩ trimpot VR5, forms a voltage divider across the 5V supply. VR5 is adjusted so that the voltage across the thermistor is 2.5V at 25°C (note: NTC stands for “negative temperature coefficient” and means that the resistance of the thermistor is progressively reduced as the temperature rises). The voltage across TH1 is monitored at the AN4 input (pin 3) of IC1 via a 47Ω resistor and 100nF filter capacitor. These are included to remove RF (radio frequency) signals and noise that could be present due to the thermistor being connected remotely from the circuit. The voltage at the AN4 input is then converted into a digital value and monitored for dT/dt changes. It is also compared by IC1 against stored over and under-temperature values. siliconchip.com.au 7 – 30V DC INPUT D3 1N5819 S1 REG1 LM317T K A TP5V OUT IN POWER ADJ CON1 LM317T 120Ω 10 µF 35V OR 50V V1 OUT ADJ 220 µF TP GND VR6 500Ω IN OUT 0.1Ω 5W 1k +7 – 30V SWITCHED K ZD2 5.1V 100k 1W A C B 2 E 4 D4 1N4148 (5V LESS THAN +7V – 30V SWITCHED) C Q3 BC337 E IC2b +5V λ VR4 10k TRICKLE VR3 10k TP4 CHARGE 1 2.5V = 2.5A 5V = 500mA 18 17 5V = 5h 470Ω 8 TP1 470Ω A CHARGE LED3 THERMISTOR A LED2 λ K TP5 RA5/MCLR Vdd 3 16 RA7 RA4/AN4 13 6 RB0 (PWM) RB7/AN6 AN2/RA2 7 15 λ RA3 /AN3 ∆T/T VR2 10k RB6 RB5 RB2 RB4 RB1 RB3 CON3 3.5mm SOCKET TH2 A 12 1 11 2 10 3 9 D1 1N5822 K 4 V1 LEDS DIP SWITCH Vss K 5 K 100nF HEATSINK TEMP S2 AN0/RA0 RA6 TP2 5V = 5°C/min AN1/RA1 TO TH1 (CELL/BATTERY TEMPERATURE) 47Ω 2 IC1 PIC16F88 PIC1 6F8 8 –I/P TP3 TIMEOUT K VR5 20k 10k 14 4 K 10 µF 100nF 10k 9.1k +5V K +5V θ LED1 K A A POWER D2 MBR735 D1, D3 10k 10k 470Ω D A 6 IC2: LM358 100k VR1 10k K 5 K 7 A 10k B MBR735 10k D5 1N4148 Q2 SPP15P10 A 1W A TP6 S G CONSTANT CURRENT SHUNT ZD3 10k 12V 10 µF BUFFER A 1k 7 IC3b 5 K + – 100nF 6 11k 1 IC3a K Q5 BC337 DIVIDER 8 3 3.9k 1 µF 1.5k 100k 0.5W CON2 IC3: LMC6482AIN TPV+ TO BATTERY A 10 µF 35V 8.2k 3 10Ω SWITCH S2 ON = 1 TIMEOUT x5 2 TOP UP 3 TRICKLE 4 BURP B SC  20 1 4 4 ZD1 16V 100nF C 1k 1 µF 1W IC2: LM358 K G S A 1k E D4, D5 A 2 Q1 IRF540 OR IPP230N06L3 1k 1 IC2a 10k Q4 BC337 D 8 K 0.22Ω ZD1, 2, 3 A BURP CHARGER FOR NICAD/NiMH BATTERIES 5W K Q1, Q2 BC 33 7 B E G C D D S Fig.3: the circuit is based on IC1. This accepts inputs from TH1 & TH2, trimpots VR1-VR5 and DIP switch S2, sets the charge rates and the time-out, and controls the charging current through Q1 via its PWM output (RB0). IC1’s PWM output also drives Q5 & IC3a which then drive a current shunt based on IC3b & Q2 to provide the discharge circuit. siliconchip.com.au March 2014  69 Parts List 1 PCB, code 14103141, 105 x 87mm 1 119 x 94 x 34mm diecast case (Jaycar HB-5067 or equivalent) 1 2.5mm DC socket (Jaycar P-0621A, Jaycar PS-0520 or equivalent) (CON1) 1 3.5mm stereo PCB mount jack socket (Altronics P0092, Jaycar PS-0133 or equivalent) (CON3) 1 3.5mm mono line jack plug 1 SPDT toggle switch, PCB mount (Altronics S1421 or equivalent) (S1) 1 2-way screw terminal, 5.08mm spacing (Altronics P2040, Jaycar HM-3130) (CON2) 1 4-way DIP switch (Altronics S3050, Jaycar SM-1020 or equivalent) (S2) 2 DIL 8-pin sockets (optional) 1 DIL18 IC socket 2 10kΩ <at> 25°C NTC thermistors (Jaycar RN-3440 or equivalent) (TH1,TH2) 1 crimp eyelet, 5.3mm ID (to mount TH2) 3 TO-220 silicone insulating washers 3 TO-220 insulating bushes 1 cable gland for 3-6.5mm cable 4 rubber feet 4 M3 x 6.3mm tapped spacers 8 M3 x 5mm screws 5 M3 x 10mm screws 5 M3 nuts 1 M3 star washer 1 200mm length of single-core screened cable 7 PC stakes Hook-up wire, heatshrink, etc Semiconductors 1 PIC16F88-I/P microcontroller programmed with 1410314A.hex (IC1) 1 LM358 dual op amp (IC2) 1 LMC6482AIN CMOS dual op amp (IC3) 1 LM317T adjustable regulator (REG1) 1 IRF540 or IPP230N06L3 N-channel Mosfet (Q1) 1 SPP15P10PLH P-channel logic level Mosfet (Q2) 3 BC337 NPN transistors (Q3-Q5) 1 1N5822 3A Schottky diode (D1) 1 MBR735 7A Schottky diode (D2) TH2 is connected to the AN6 input of IC1 and monitors the charger’s heatsink temperature. This allows IC1 to shut the charger down if the heatsink temperature exceeds a preset value. Trimpots VR1, VR2 & VR3 are used to set the time-out, dT/dt and trickle charge values. These trimpots connect to AN0, AN3 & AN1 of IC1 respectively and are be set to apply between 0V and 5V to these inputs. Trimpot VR4 sets the charging cur- rent. This trimpot connects to the +5V supply via a 9.1kΩ resistor and this restricts the adjustment range to a nominal 2.5V maximum at IC1’s AN2 input (pin 1), corresponding to a 2.5A maximum charge rate. The voltage inputs are all converted to digital values within IC1 so that the settings can be processed in software. Test points TP1-TP5 are provided for setting the trimpots when using a multimeter. There is also a TP GND + 0.1Ω RESISTOR, Q2, D2 CELL OR BATTERY DISCHARGE D1, Q1, 0.22Ω RESISTOR – CHARGE CHARGE & DISCHARGE CURRENT FLOW 70  Silicon Chip Fig.4: the basic charge and discharge current paths for the unit. During charging, current flows from the power supply, through the cell or battery and then via diode D1, Mosfet Q1, and a 0.22Ω resistor to ground. Conversely, during discharge, current flows from the cell or battery through Mosfet Q2, diode D2 and a 0.1Ω resistor. 1 1N5819 1A Schottky diode (D3) 2 1N4148 diodes (D4,D5) 1 16V zener diode 1W (ZD1) 1 5.1V zener diode 1W (ZD2) 1 12V zener diode 1W (ZD3) 2 3mm green LEDs (LED1, LED2) 1 3mm red LED (LED3) Capacitors 1 220µF 35V or 50V PC electrolytic 4 10µF 35V or 50V PC electrolytic 2 1µF 16V PC electrolytic 4 100nF 63V or 100V MKT polyester Trimpots 4 10kΩ horizontal trimpots (VR1-VR4) 1 20kΩ horizontal trimpots (VR5) 1 500Ω horizontal trimpot (VR6) Resistors (0.25W, 1%) 3 100kΩ 5 1kΩ 1 11kΩ 3 470Ω 8 10kΩ 1 120Ω 1 9.1kΩ 1 47Ω 1 8.2kΩ 1 10Ω 1 3.9kΩ 0.5W 1 0.22Ω 5W 1 1.5kΩ 1 0.1Ω 5W terminal which is useful when checking these voltages. The voltages measured at each test point directly relate to the setting’s value. For example, setting VR1 to give 4V at TP1 will set the time-out to four hours. This time-out value can be multiplied by a factor of five by setting the No.1 switch in DIP switch S2 to the ON position. This ties pin 12 (RB6) of IC1 to ground. Conversely, with this switch open, pin 12 is pulled to +5V via an internal pull-up resistor within IC1 and the time-out is set to x1. Switches 2, 3 & 4 in DIP switch S2 work in a similar manner. The No.2 switch enables the top-up, the No.3 switch enables the trickle charge mode and the No.4 switch enables the burp charge. Outputs RB1 and RB2 of IC1 drive the Thermistor and Charge indicator LEDs (LED2 & LED3) respectively via 470Ω resistors. These indicate the charger’s status, as described previously. Charge & discharge Two separate circuits are used for siliconchip.com.au the charge and discharge functions. To understand how this works, refer to Fig.4 which shows the basic charge and discharge current paths. During charging, current flows from the power supply through the cell or battery and then via diode D1, Mosfet Q1 and a 0.22Ω resistor to ground. Conversely, during discharge, current flows from the cell or battery through diode D2, Mosfet Q2 and a 0.1Ω resistor. Note, however, that this is a simplified diagram and the currents through Q1 and Q2 are controlled so that charge and discharge rates are correct for the cell or battery that’s connected to the charger. Refer back now to Fig.3 for the full details. A constant current source comprising op amp IC2a and Mosfet Q1 charges the battery via CON2. IC1’s RB0 output provides a 5V 3.9kHz PWM (pulse width modulated) signal which is fed to a divider and filter network comprising 8.2kΩ and 1kΩ resistors and a 1µF capacitor. This filter network smooths the pulse output to give a DC voltage. This smoothed DC voltage sets the current provided by Q1 to the battery and the 5V PWM signal has its duty cycle adjusted over a wide range, from trickle to full charge. The 5V level is effectively reduced to 543mV via an 8.2kΩ and 1kΩ voltage divider. As a result, the maximum voltage that can be applied to pin 3 of IC2a is 543mV when the PWM duty cycle is 100% (ie, full charge). For a 50% duty cycle, the average voltage from RB0 is 2.5V, or 271.5mV after passing through the divider. This filtered voltage is applied to pin 3 of IC2a and this sets the charge current. When pin 3 is at 543mV, IC2a’s pin 1 output adjusts the gate drive to Mosfet Q1 so that the voltage across the 0.22Ω source resistor (as monitored at pin 2 of IC2a) is also 543mV. The charge current is therefore 2.47A (ie, 543mV ÷ 0.22Ω). Diode D1 is included to prevent current flow via Q1’s intrinsic reverse diode if power is connected with reverse polarity. D1 is a 3A Schottky type, specified because it has less than half the forward voltage of a normal power diode. Typically, it has about 380mV across it (at 2.5A) compared with a standard diode which would have 0.84V across it at 2.5A. That also means less power loss in the diode; 0.95W for the Schottky diode siliconchip.com.au Specifications •  Maximum input voltage: 30V. •  Maximum charge current: 2.5A. •  Charge current adjustment: from 0-2.5A, corresponding to 0-2.5V at TP4 using VR4 (in approximately 40mA steps). •  Time-out adjustment: from 0-5 hours, corresponding to 0-5V at TP1 using VR1. 0-25 hour with x5 selected (when DIP switch 1 closed). •  dT/dt adjustment: from 0.5-5°C rise/minute, corresponding to 0.5-5V at TP2 as set by VR2. •  dT/dt measurement interval: once every minute when cells reach 25°C or more. •  Top-up and trickle charge: top-up available when DIP switch 2 is closed; trickle enabled when DIP switch 3 is closed. •  Trickle charge: adjustable using VR3 from 0-500mA, corresponding to 0-5V at TP3. Adjustable in approximately 5mA steps. •  Top-up charge: 4 x trickle setting for one hour. •  Burp discharge: enabled when DIP switch 4 is closed. Discharge current is 2.5 times the charge current. Time-out is increased by 13% to compensate for reduced charge period and added discharge period. •  Cell over-temperature cut-out: 50°C. •  Cell under-temperature cut-out: 0°C. •  Charger over-temperature cut-out: 40°C. •  Charging cycle with burp selected: charge period 940ms, pause 1ms, burp discharge 30ms and pause 29ms (all over a 1s period). compared to 2.1W in a standard diode. IC1’s RA6 output drives transistor Q4. This transistor is used to pull the voltage at pin 3 of IC2a to a very low level, so that the charge current is effectively reduced to near zero. This shut down is required during pause (when the PWM is also dropped to zero) and also during discharge when the PWM is still present to provide the discharge current setting. Burp discharge Another constant current circuit is employed for the burp discharge function. This comprises op amp IC3b and Mosfet Q2, with a 0.1Ω source resistor used for current monitoring. This circuit is connected to the positive supply (instead of the 0V supply as for the charge circuit) and so Q2 is a P-channel Mosfet. In addition, the PWM signal for IC1 is inverted and referenced to the positive supply. The same PWM signal from RB0 (pin 6 of IC1) is also used to control IC3b & Q2. However, because we now have a P-channel Mosfet, the signal is inverted and level-shifted by transistor Q5. When the PWM signal is at 5V, Q5 switches on and its collector goes low, pulling one side of the 3.9kΩ resistor low. This 3.9kΩ resistor limits the current flow through 5.1V zener diode ZD2, This zener diode clamps the inverted voltage to within 5V of the switched supply rail. As a result, the 5V PWM signal is now inverted and referenced below the positive supply which can be as high as 30V, depending on the number of cells being charged. IC3a is powered from a 5V supply; ie, between the 30V positive rail at its pin 8 and a rail 5V below this at pin 4. A 100kΩ resistor couples Q5’s output to pin 3 of IC3a and this resistor limits the current into clamp diode D4. D4 prevents the voltage applied to pin 3 going much below the pin 4 rail, thereby preventing damage to this op amp. IC3a essentially buffers the PWM signal before feeding it to op amp IC3b via an 11kΩ/1.5kΩ divider. A 1μF capacitor filters the divider’s output. This divider is designed to automatically provide a discharge current that’s 2.5 times greater than the charge current. March 2014  71 CON2 4148 10k 10k 10k 12V ZD1 100nF IC2 LM358 D1 Q1 1k 1k IRF540 0.22 Ω 5W 1 µF C 2014 (UNDER PCB) TH2: OFF BOARD – SEE TEXT 5822 16V 10Ω 10k 8.2k 9.1k 20k 500Ω 10k + D5 14103141 Q4 + 10 µF D2 MBR35 BC337 VR4 10k VR2 10k BATTERY 10k PIC16F88 IC1 TP+5V TP GND TP5 – 1 2 3 4 100nF TP2 TP4 VR5 SPP15P10 + BC337 10 µF 100nF 100nF 11k 1.5k 5.1V 10k 1k VR6 47Ω TP3 VR3 10k VR1 10k (UNDER PCB) 1k 1k DIP SWITCH S2 10 µF ZD3 CON3 10 µF TP1 TP6 100k A LED2 Q2 10k 120Ω S1 470Ω LMC6482 Q3 Q5 BC337 REG1 LM317T 470Ω 4148 D4 3.9k CON1 TPV+ A 0.1 Ω 5W IC3 + 220 µF 470Ω LED3 100k 5819 A D3 100k + LED1 + 1 µF ZD2 (UNDER PCB) 14130141 NiMH, NiCd Burp Charger Fig.5: install the parts on the PCB as shown in this layout diagram and photograph. The text describes the mounting details for Q1, Q2 & D2 (see also Fig.6), thermistor TH2 and the three LEDs. The 5V inverted PWM signal that’s now referenced to the positive supply becomes a 600mV signal (again referenced to the positive supply) after the divider. When the PWM level is at maximum (ie, the charge current is 2.47A), 600mV appears across Mosfet Q2’s 0.1Ω source resistor. This results in a 6A discharge current, ie, close to 2.5 times the charge current. Power supply Power for the circuit comes from a 7-30V DC supply (plugpack or laptop supply) via Schottky diode D3. D3 provides reverse polarity protection for the following 220μF capacitor and 3-terminal regulator REG1, an LM317T set to provide 5V to IC1 and the trimpots. This was chosen in preference to a fixed 5V regulator because it can be adjusted to supply a more precise 5V, using trimpot VR6. An exact 5V rail makes the settings of VR1-VR5 more accurate. The 5V supply for op amps IC3a & IC3b is provided by IC2b. This is connected to invert the 5V from REG1 and level-shift it so that it is 5V below the positive supply rail. 12V zener diode ZD3 prevents IC2b’s output from going more than 12V below the positive supply rail at power up. This protects IC3 from damage as its maximum supply rating is 16V. D5 prevents IC2b’s output from conducting current through the 12V zener diode in the forward direction if the power supply is reversed in polarity. This also protects IC3 from damage. Supply voltage requirements In order to fully charge a battery, we need up to 1.8V per cell from the plugpack even though the nominal Table 1: Resistor Colour Codes   o o o o o o o o o o o o o o o No.   3   1   8   1   1   1   1   5   3   1   1   1   1   1 72  Silicon Chip Value 100kΩ 11kΩ 10kΩ 9.1kΩ 8.2kΩ 3.9kΩ 1.5kΩ 1kΩ 470Ω 120Ω 47Ω 10Ω 0.22Ω 0.1Ω 4-Band Code (1%) brown black yellow brown brown brown orange brown brown black orange brown white brown red brown grey red red brown orange white red brown brown green red brown brown black red brown yellow violet brown brown brown red brown brown yellow violet black brown brown black black brown red red silver brown brown black silver brown 5-Band Code (1%) brown black black orange brown brown brown black red brown brown black black red brown white brown black brown brown grey red black brown brown orange white black brown brown brown green black brown brown brown black black brown brown yellow violet black black brown brown red black black brown yellow violet black gold brown brown black black gold brown black red red silver brown black brown black silver brown siliconchip.com.au Fig.6: diode D2 and Mosfets Q1 & Q2 are mounted on the base of the case and are insulated from it using insulating bushes and silicone washers. Make sure that the metal tab ends of the devices cannot short against the side of the case. MOSFETS, DIODE D2 PCB INSULATING BUSH CASE SILICONE WASHER M3 x 10mm SCREW DIODE & MOSFET MOUNTING DETAIL battery voltage. The maximum charging current is also limited by the mAh capacity of the cell or battery (see Table 2) and the rating of the DC plugpack or power supply. So in order to charge at 2.5A, the power supply or plugpack must be able to deliver this current. Construction terminal voltage shown on the battery pack is 1.2V per cell. So, to charge a 6V battery which has five cells, we need a DC input voltage of 5 x 1.8V = 9V. Similarly, an 18V battery has 15 cells and so this requires a 15 x 1.8V = 27V supply to fully charge it. Charging only one, two or three cells nominally requires up to 5.4V. In practice though, more than 7V is required at the input to ensure that the LM317T regulator (REG1) operates correctly, ie, remains in regulation. For operation in a car, the input voltage will be around 12V with the engine stopped and up to 14.4V with the engine running. A 12V supply can charge up to six cells (ie, a 7.2V battery), while a 14.4V supply (with the engine running) can charge up eight cells (ie, a 9.6V battery). Note also that using a supply voltage that is significantly higher than required to charge the cells will cause the charger to heat up more than necessary. For example, at 2.5A and with a supply that’s 10V higher than the battery voltage, around 25W will be dissipated in the charger. In that case, the charger will certainly become hot and will shut down when its heatsink (ie, the case) reaches 40°C. Basically, this means that the charge current may have to be reduced if the supply voltage is high compared to the siliconchip.com.au The assembly is straightforward since all the parts are mounted on a PCB coded 14103141 and measuring 105 x 87mm. This is housed in a metal diecast case measuring 119 x 94 x 34mm. Fig.5 shows the assembly details. Begin construction by checking the PCB for any defects such as shorted tracks, breaks in the copper and incorrect hole sizes. Also, check that the corners at the lefthand end of the PCB have been shaped to clear the internal corner posts. It’s rare to find any problems but it’s always a good idea to check before installing any of the parts. Next, place the PCB inside the case and mark out the corner mounting holes in the base, noting that the PCB must sit as far to the left as it will go. This is necessary so that switch S1 and the 3.5mm socket later protrude through the case side. Drill these mounting holes out to 3mm and deburr them using an oversize drill. Now for the PCB parts. Install the small resistors first, taking care to fit the correct value in each location. Table 1 shows the resistor colour codes SILICON CHIP + Power but it’s always a good idea to use a digital multimeter check each one before installing it (some colours can be difficult to read). The 0.1Ω and 0.22Ω 5W resistors can go in next. These should be mounted about 1mm above the PCB to allow air to circulate beneath them for cooling. That’s easily done by pushing them down onto a 1mm-thick cardboard spacer before soldering their leads (don’t forget to remove the spacer afterwards). Next, install the diodes (but not D2), then fit IC sockets for IC1, IC2 & IC3. Be sure to orientate each socket correctly, ie, with its notched end to the left. Once these are in, install the correct op amp in each position but leave the PIC16F88 micro out for the time being. Follow with DIP switch S2, making sure that its No.1 switch goes to the left. The zener diodes can then be installed. ZD1 is a 16V 1W type and may be marked as a 1N4745; ZD2 is 5.1V 1W and may be marked as a 1N4733; and ZD3 is 12V 1W and may be marked as a 1N4742. Again, the orientation of these parts is important. The capacitors can now be fitted, making sure that the electrolytics go in with the correct polarity. That done, install PC stakes for TP GND, TP +5V and test points TP1-TP5. The three LEDs are next on the list, starting with LED1 (green). First, orientate it as shown on Fig.5, then bend its leads down at right angles 6mm from Nicad & NiMH Burp Charger Off + DC In 7-30V + + On Charge M3 NUT + + In Thermistor Fig.7: this fullsize artwork can be used as a drilling template for the front side panel of the case. NOTE: POSITION LABEL SO THAT POWER SWITCH IS 16.5mm DOWN FROM TOP EDGE OF BOX BASE March 2014  73 Table 2: Typical Settings For A Range Of Cell Capacities Standard Charge (5h) Fast Charge 20mA (VR4 <at> 20mV) 60mA (VR4 <at> 60mV) 200mA (VR4 <at> 200mV) 10mA (VR3 <at> 100mV) 400mAh 40mA (VR4 <at> 40mV) 120mA (VR4 <at> 120mV) 400mA (VR4 <at> 400mV) 20mA (VR3 <at> 200mV) 700mAh 70mA (VR4 <at> 70mV) 210mA (VR4 <at> 210mV) 700mA (VR4 <at> 700mV) 35mA (VR3 <at> 350mV) 900mAh 90mA (VR4 <at> 90mV) 270mA (VR4 <at> 270mV) 900mA (VR4 <at> 900mV) 45mA (VR3 <at> 450mV) 1000mAh 100mA (VR4 <at> 100mV) 300mA (VR4 <at> 300mV) 1.0A (VR4 <at> 1.0V) 50mA (VR3 <at> 500mV) 1500mAh 150mA (VR4 <at> 150mV) 450mA (VR4 <at> 450mV) 1.5A (VR4 <at> 1.5V) 75mA (VR3 <at> 750mV) 2000mAh 200mA (VR4 <at> 200mV) 600mA (VR4 <at> 600mV) 2.0A (VR4 <at> 2.0V) 100mA (VR3 <at> 1.0V) 2400mAh 240mA (VR4 <at> 240mV) 720mA (VR4 <at> 720mV) 2.4A (VR4 <at> 2.4V) 120mA (VR3 <at> 1.2V) 2500mAh 250mA (VR4 <at> 250mV) 750mA (VR4 <at> 750mV) 2.5A (VR4 <at> 2.5V) 125mA (VR3 <at> 1.25V) 2700mAh 270mA (VR4 <at> 270mV) 810mA (VR4 <at> 810mV) 135mA (VR3 <at> 1.35V) 3000mAh 300mA (VR4 <at> 300mV) 900mA (VR4 <at> 900mV) 3300mAh 330mA (VR4 <at> 330mV) 990mA (VR4 <at> 990mV) 4000mAh 400mA (VR4 <at> 400mV) 1.2A (VR4 <at> 1.2V) 4500mAh 450mA (VR4 <at> 450mV) 1.35A (VR4 <at> 1.35V) 2.5A (1.6h) (VR4 <at> 2.5V, VR1 <at> 1.6V) 2.5A (1.8h) (VR4 <at> 2.5V, VR1 <at> 1.8V) 2.5A (2h) (VR4 <at> 2.5V, VR1 <at> 2.0V) 2.5A (2.4h) (VR4 <at> 2.5V, VR1 <at> 2.4V) 2.5A (2.7h) (VR4 <at> 2.5V, VR1 <at> 2.7V) 5000mAh 500mA (VR4 <at> 500mV) 1.5A (VR4 <at> 1.5V) 2.5A (3h) (VR4 <at> 2.5V, VR1 <at> 3.0V) 250mA (VR3 <at> 2.5V) 9000mAh 900mA (VR4 <at> 900mV) 2.5A (5.4h) (VR4 <at> 2.5V, VR1 <at> 1.08V, DIP Switch No.1 ON) 2.5A (5.4h) (VR4 <at> 2.5V, VR1 <at> 1.08V, DIP Switch No.1 ON) 450mA (VR3 <at> 4.5V) Slow Charge (15h) Battery Or Cell Capacity (VR1 <at> 3V, DIP Switch No.1 ON) (Do not select top up) 200mAh its body. That done, solder the LED in place with its horizontal lead sections exactly 5mm above the PCB (hint: use a 5mm-thick spacer to set the height). The remaining two LEDs can then be fitted in exactly the same manner. Trimpots VR1-VR6 are next on the list. Note that the 10kΩ trimpots may be marked 103, the 20kΩ trimpots marked 203 and the 500Ω trimpot marked 501 (ie, instead of the actual ohm values). Regulator REG1 is next and is mounted with its leads bent down at right angles so that its metal tab sits flat against the PCB. Secure this tab to the PCB using an M3 x 10mm screw, nut and shakeproof washer before soldering the leads. That done, install the DC socket (CON1), the 2-way screw terminal block (CON2), the 3.5mm jack socket (CON3) and switch S1. Be sure to push these parts all the way down so that 74  Silicon Chip (1.5h at or below 2.5A) (VR1 <at> 5V, DIP Switch No.1 off) (VR1 <at> 1.5V, DIP Switch (Top up not recommended) No.1 off) they sit flush against the PCB before soldering their leads. That completes the PCB assembly, except for Q1, Q2 and D2. As shown on Fig.5, these three devices are all mounted under the PCB, with their leads bent up at 90° so that they pass through their respective mounting holes. This allows their tabs to be later bolted to the bottom of the metal case for heatsinking. In each case, it’s simply a matter of first bending the two outside leads up by 90° exactly 7mm from the device body. The middle leads of Q1 & Q2 can then be bent up 5mm from the body, after which you can loosely fit all three devices to the PCB but don’t solder their leads yet. Take care not to get the two Mosfets mixed up – Q1 is an IRF540 while Q2 is an SPD15P10. Case preparation It’s necessary to drill some extra Trickle Current (DIP Switch No.3 on) (Top up with DIP Switch No.2 ON will be 4 x trickle setting) 150mA (VR3 <at> 1.50V) 165mA (VR3 <at> 1.65V) 200mA (VR3 <at> 2.0mV) 225mA (VR3 <at> 2.25V) holes in the case, before installing the PCB. The mounting holes for the PCB assembly were drilled in a previous step (ie, before the parts were installed) and the next step now is to use the front-panel artwork (Fig.7) as a drilling template for the front-panel holes. You can either copy the artwork shown in Fig.7 or you can download the artwork in PDF format from the SILICON CHIP website (free for subscribers) and print it out. In either case, it should be cut out and attached to the case using adhesive tape, after which the various holes can be drilled. Be sure to position the label so that the centre of the On/Off switch is exactly 16.5mm down from the top edge of the base. Use a small pilot drill to start the holes, then remove the template and carefully enlarge each one to size using a large drill and/or a tapered reamer. There are six holes in all – three for the siliconchip.com.au LEDs and one each for the DC socket, 3.5mm jack socket and switch S1. Once all the holes have been drilled, print out a final front-panel label, laminate it and attach it to the case using double-sided tape or silicone adhesive. The various holes can then be cut out with a sharp hobby knife. Final assembly Begin the final assembly by securing four M3 x 6.3mm tapped Nylon spacers to the base of the case using M3 x 5mm screws. The PCB assembly (together with the loosely-fitted Q1, Q2 & D2 parts) can then be slipped into the case and secured to the spacers using another four M3 x 5mm screws. The next step is to drill the mounting holes for Q1, Q2 & D2. These devices must be positioned so that the ends of their tabs clear the side of the case by 1-2mm. If a tab does touch the side of the case, you will have to remove the offending device and rebend its leads so that it is clear. Once everything is correct, remove the PCB assembly and drill the device mounting holes to 3mm, then deburr them using a larger drill. It’s vital that the area around each of these holes inside the case is perfectly smooth and free of metal swarf, so that the insulating washers used when mounting the devices will not be punctured. A hole also needs to be drilled and reamed in the adjacent side of the box (ie, at the Q1/Q2 end) to accept a cable gland (position this directly opposite CON2), while a 3mm hole must also be drilled to mount thermistor TH2. Be sure to position the hole for the cable gland down far enough so that the gland doesn’t later interfere with the lid of the case. Mounting TH2 Thermistor TH2 is attached to a 5.3mm crimp eyelet which is then fastened to the inside of the case using an M3 x 10mm machine screw, nut and lockwasher (ie, to detect heatsink temperature). First, remove the plastic insulating piece from the eyelet, then prise open the crimp section using pliers. That done, shape the crimp lugs so that they lightly clamp the thermistor in place but without the leads making contact to the crimp eyelet. Finally, glue the thermistor in place using epoxy resin and heatshrink it, then refit the PCB assembly in the case siliconchip.com.au Determining The Charger Settings Before adjusting the time-out, trickle charge and time-out settings, you need to know the Ah rating (or mAh rating) of the cells or the battery. This will normally be printed on the side. You also need to know the nominal battery voltage (or the number of cells connected in series to calculate this) and the voltage/current ratings of the plugpack. Note that when using slow charging rates (eg, charging over 15 hours), the top-up current would exceed the charge rate. In this case, do not enable top-up. Similarly, at faster charging rates (eg, charging over five hours), the top-up current may be similar to the charge rate and again top-up is not recommended. Charge rate This will depend on the mAh rating of the cells or battery and on the desired charge rate (slow, standard or fast) – see Table 2. The plugpack used must also be capable of supplying the required current. Time-out The time-out should be set to 1.5 times the Ah rating of the battery divided by the charge current. For example, a 2500mAh (2.5Ah) battery charged at 1A should be fully charged after 2.5 hours. In this case, the time-out should be set to 2.5 x 1.5 ÷ 1 = 3.75h. That’s done by adjusting VR1 to give 3.75V at TP1 (see text). Note that any changes made to the time-out value during charging will not take effect until the power is switched off and on again. This also includes any changes to the DIP switch settings. Any changes to other settings will take effect immediately and will affect the current charging cycle. Trickle charge The trickle charge requirement is calculated by dividing the Ah (amp hour) rating of the cells by 20. So, for example, if the cells are rated at 2400mAh, then the trickle charge current should be set to 120mA. Adjusting the dT/dt value The endpoint temperature rise detection adjustment (dT/dt) should initially be set to 2.5°C per minute (ie, by adjusting VR2 for 2.5V on TP2). In some cases, however, the charger may stop before the battery is fully charged or conversely, it may tend to overcharge the battery. Under-charging is indicated if the charging period appears to be too short and the batteries do not deliver power for the expected period. In this case, turn VR2 further clockwise to increase the dT/dt value. Conversely, if the battery pack becomes quite hot after full charge has been reached, turn VR2 anticlockwise to decrease the dT/dt value. and attach the thermistor assembly to the case wall using an M3 x 10mm screw, nut and lockwasher. The thermistor’s leads are then connected to its pads on the top of the PCB – see Fig.5 and photo. Thermistor TH2 Bolting down Q1, Q2 & D2 Mosfets Q1 & Q2 and diode D2 can now be fastened to the bottom of the case. As shown in Fig.6, these devices must each be insulated from the case using a silicone washer and insulating bush. An M3 x 10mm screw and nut is used to secure each device in place, after which its leads are soldered to their pads on the top of the PCB. This view shows how therm­ istor TH2 is attached to a 5.3mm crimp eyelet and fastened to one end of the case. March 2014  75 COVER IN HEATSHRINK THERMISTOR TH1 SINGLE CORE SCREENED CABLE 3.5mm JACK PLUG PLUG COVER to CON1 (positive to the centre of the plug) and switch on. Check that the power LED (LED1) lights, then connect a multi­ meter between TP5V and TP GND and adjust VR6 for a reading of 5.0V. Now check that there is 5V between pins 14 & 5 of IC1’s socket. If so, check that TP6 is at -5V with respect to TPV+. If this is correct, switch off the power, wait a short time and then insert microcontroller IC1 (notched end to the left). Adjustments THERMISTOR TH1 CABLE DETAILS Fig.8: the battery-pack temperature sensor (TH1) is connected to the charger via a length of single-core screened cable and a 3.5mm jack plug. Be sure to heatshrink the thermistor connections so that they cannot short together. Once all these devices are in, use a multimeter to check that the metal tabs of these devices are indeed isolated from the metal case. If you get a low resistance reading between a device tab and the case, dismantle the assembly and check that its insulating washer hasn’t been punctured (eg, by metal swarf). Check also that the device’s tab is clear of the side of the case. Battery-pack thermistor As shown in Fig.8, the batterypack thermistor (TH1) is connected to a 3.5mm jack plug via single-core screened cable. Be sure to sleeve the thermistor connections with heatshrink tubing to prevent any shorts between them or to the battery holder terminals. The thermistor itself needs to be mounted in the battery holder so that it makes contact with the side of at least one of the cells under charge. For our prototype, we drilled a hole in a 2 x AA cell holder so that the thermistor is sandwiched between the cells when they are in place (see photo). Alternatively, depending on the type of battery holder (or if no holder is used), the thermistor can be held in place against the cells using a length of hook and loop material. The shielded lead running to the thermistor is secured to the end of the battery holder using a small cable tie and a couple of self-tapping screws. Setting up It’s now time to make some initial voltage checks. First, with IC1 still out of its socket, connect a DC plugpack The battery pack thermistor (TH1) can be fitted to a 2 x AA cell holder by drilling a hole between the two compartments as shown here. Its leads are attached to a single-core shielded cable and this is secured using a cable tie which wraps around two self-tapping screws that go into the holder at one end. 76  Silicon Chip Now for the final adjustments. This involves adjusting the various trimpots for charge rate, cell/battery temperature cut-out, time-out (ie, the maximum time for which the charger operates before it cuts out) and endpoint temperature detection. The procedures are as follows: •  Charge rate: the charge rate is set using trimpot VR4 and will depend on the mAh rating of the cells or battery. It will also depend on the current rating of the plugpack power supply being used and on the desired charge rate (slow, standard or fast). Table 2 shows the charge settings for cells/batteries ranging in capacity from 200mAh to 9000mAh. It’s just a matter of choosing a charge rate to suit the cells or battery and adjusting VR4 to give the required voltage on TP4. •  Cell/battery temperature cut-out: this involves adjusting trimpot VR5 so that the voltage on TP5 is 2.5V when thermistor TH1 is at 25°C. So, if the ambient temperature is 25°C, simply adjust VR5 for 2.5V on TP5. If the ambient temperature is 20°C, set VR5 for 2.8V on TP5. And if the ambient temperature is 30°C, set VR5 so that TP5 is at 2.2V. Note that some battery packs will have a thermistor already installed. This should not be used unless it has the same resistance characteristics as the one specified for TH1. It should measure about 10kΩ at 25°C and the resistance should fall with increasing temperature. •  Time-out: the time-out is adjusted using VR1. This can be set from 0-25 hours by monitoring the voltage between TP1 & TP GND. The voltage on TP1 directly translates to the time-out in hours, so if it’s set to 2.5V, the timeout will be 2.5 hours. And if it’s set to its 5V maximum, then the time-out will be 5 hours. siliconchip.com.au Fig.9: the waveforms in the above-left screen grab show the operation of the Burp Charger at a sweep speed of 10ms/ div for a 100ms period. The yellow trace is the PWM signal from the microcontroller at pin 6; the pink trace is the 30ms discharge pulse from pin 16 to the base of Q3; and the green trace is the pulse signal from pin 15 to the base of Q4 which turns off Mosfet Q1 while the battery is being discharged and for 30ms after that. The blue trace shows the fluctuation in the battery voltage of a 4-cell Nicad pack. Note that it drops for 30ms (the burp period), then recovers and begins rising again as the charging cycle resumes. The screen grab to the right shows the operation at a much slower sweep speed of 500ms/div (5-second duration). As stated, the No.1 switch in DIP switch S2 acts as a x5 multiplier for the time-out. So if this switch is set to ON and TP1 is set for +5V, the timeout will be 25 hours. Similarly, if TP1 is set to 1.2V, the time-out will be six hours (5 x 1.2). The accompanying panel (Determining The Charger Settings) tells you how to calculate the time-out value required for the cells used. Table 3 also shows the typical settings for cells of various capacities. •  Endpoint temperature rise detection: VR2 is used to adjust the endpoint temperature rise detection (dT/dt). This can be adjusted from between 0.5°C per minute rise to 5°C per minute rise by monitoring the voltage between TP2 and TP GND. Once again, there is a direct correlation between the voltage and the setting. For example, a setting of 2.5V at TP2 will set the dT/dt value to a 2.5°C per minute rise and this should be the initial setting. This can later be changed if you find that the battery pack is either being under-charged or over-charged (see panel). Top-up/trickle charge options Setting the No.2 and No.3 switches in DIP switch S2 to ON enables the top-up and trickle charge modes respectively. These can be activated together or individually. If you want top-up only, set switch No.2 to ON; if you want both top-up and trickle charge, set both No.2 and No.3 to ON; and if you want trickle charge only (without top-up), set switch No.3 to ON (and leave No.2 off). Note that if either top-up and/or trickle charge is enabled, you then need to set the trickle charge rate (the top-up charge rate is fixed at four times the trickle charge rate). That’s done using trimpot VR3, which allows adjustment from 500mA down to less than 20mA. Once again, the panel tells you how to calculate the required trickle charge rate to suit your cells. It’s then just a matter of monitoring the voltage at TP3 and adjusting VR3 accordingly (eg, 1V = 100mA, 3V = 300mA and 5V = 500mA). Finally, as previously stated, you need to choose a power supply (eg, a plugpack) with an output voltage under load that’s at least equal to 1.8 x the number of cells in the battery – eg, 7.2V for a 4-cell (4.8V) battery. Note, however, that the supply must be at least 7V for batteries with less than four cells, to ensure REG1 operates correctly. Refer back to the section titled “Supply voltage requirements” SC for the full details. tel: 08 8240 2244 Standard and modified diecast aluminium, metal and plastic enclosures www.hammondmfg.com siliconchip.com.au March 2014  77