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Microprocessor-controlled nicad battery charger This intelligent charger does everything a nicad charger should. It automatically checks the condition of the battery, then discharges it or charges it at 500mA or 1A. Design by WARREN BUCKINGHAM This is the first intelligent battery charger that we have presented. Previously, we have featured units which discharge nicads down to 1.1V per cell but then you have to recharge them with your own charger. By contrast, the “Nicad Battery Service Module” is an automatic microprocessor controlled unit which combines the functions of discharging and charging, together with an analysis of battery condition. 16  Silicon Chip Furthermore, you can power it from an AC plugpack or from the cigarette lighter socket in your car. Most users of nicad batteries have experienced poor battery performance at some time and generally this is brought about by incorrect charging. The most common fault is what is called “memory effect” and is brought about because the cells in the battery pack have not been correctly dis- charged before they are recharged. In effect, nicad batteries cannot be used in shallow discharge cycles otherwise their capacity is reduced. They must be discharged to the “end-point” voltage which is typically 1.1V per cell. On the other hand, if the battery is discharged too far, damage can be done to the cells and in fact can reverse the polarity of the cells, thereafter making it virtually impossible to charge the battery with a conventional charger. A few chargers on the market have a discharge button to discharge the battery while others simply discharge every time the battery is connected to the charger. This works but every time the battery is dis­charged it reduces the life of the battery. Another major problem is overcharging. When a near fully charged +V1 RLYA +5V D5 1N4004 10k 10k 10k 1. 2  5W RLY1 A LED1 RED 0.1 10k 4DIP SWITCH Q3 BC547 B 5 18 4.7k 3 17 K 16 +5V 330  4.7k 2.2k 1% 3k 1% TEST VR1 5k 10T 2 8 3 IC2 LM358 2 30k 1% Q5 BC557 1 B X1 3.579MHz E 18pF 0.27  K COND. LED3 RED RLYB  1k 1% 100  1% 10k 1% 0.1 ZD1 18V 400mW BATTERY 2.2k 1% Q4 BC547 B D6 1N4004 C K 10 C C A 10 25VW 18pF Q1 TIP32C E 9 7 13 1k 12 11 E 16VAC 1.5A A CHARGE LED4 RED 6 C 4 IC1 Z8 4 470W 1W Q2 BC547 B 430  430  E B 1 100 25VW 1. 2  5W HIGH LOW C E 15 CURRENT S1  8 430  14 FAULT LED6 OR D1-D4 4x1N4004 +V1 430  A  K READY LED5 GRN A  K B 7805 1000 25VW 2.7k POWER LED2 RED 1000 25VW +5V E C VIEWED FROM BELOW 100 25VW B CE I GO  NICAD BATTERY SERVICE MODULE battery is put on charge, it becomes hot which again reduces its life. In effect, no simple charger is ideal as far as nicad batteries are concerned. Table 1 indicates some of the problems which can occur with different modes of charging nicad batteries. This intelligent charger, or “Nicad Battery Service Module”, actually checks the condition of the battery when it is first connected. First, it places a load on the battery and then checks the slope of the discharge curve. This indicates two aspects of the battery’s condition: (1) it gives an indication of its capacity and state of charge; and (2) it indicates whether the battery is showing symptoms of memory effect. These show up as very Fig.1: the circuit for the Nicad Battery Service Module is based on IC1, a Z8 microprocessor. When the battery is first connected, it is load tested at either a 500mA or 1A rate via Q1, D6 & the associated 1.2Ω 5W resistors. Depending on the battery condition, the processor then either continues to discharge the battery to its end-point voltage or switches straight over to the charge mode. Table 1: Common Problems Function Problem Trickle charge Overcharging. Timed charge Overcharging. Delta V Under or overcharging possible. Most units switch off after the Delta V point reached, or switch off before this, due to battery chemical action. Temperature sensing Overcharging possible; not suitable for most batteries unless they have a heat sensor built in or are charged in a special housing. Manual discharge & charge If not required, time wasted and battery life reduced. Note: overcharging causes the battery to become hot and reduces its life. September 1993  17 All the parts except for transistor Q1 are mounted on a single PC board & this mounts inside a standard plastic case. Q1 is mounted on a U-shaped aluminium heatsink which fits under the board. slight fluctuations on the discharge curve. This load test lasts for up to 30 seconds after which the processor decides either to discharge the battery to the end-point voltage or switch straight over to charging. For a charger with such fancy functions, the Nicad Battery Service Module does not have a fancy appearance. Table 2: Charger Functions Discharge Remove memory. Charge To max. capacity. Flash fault LED Wrong battery, reversed cell, unable to charge. Table 3: Fault Light Indications Steady Below maximum capacity, shorted cell, charged on wrong setting, set too high. Flashing Charge cycle taking too long, battery already charged, reversed cell in battery. 18  Silicon Chip It is housed in a small black plastic instrument case measuring 93 x 56 x 135mm. On the front panel it has a single toggle switch to select the charging rate and on the top of the case are five LEDs which indicate the following: Power, Conditioning, Charging, Ready and Fault. On the rear panel are two sockets, one for AC or DC input and one for connection to the battery to be charged. The unit comes with a 16VAC 1.5A plugpack for charging from the mains supply and a cigarette lighter socket for battery charging in a car. Now let’s have a look at the circuit which is shown in Fig.1. Circuit description The heart of the circuit is the Z86EO (IC1), a member of the Z8 micro­controller family. It is clocked at 3.579MHz, as set by the crystal connected between pins 6 and 7. The Z86EO has an OTP (one time programmable) ROM, a RAM and a couple of inbuilt comparators which are used in this circuit. The ROM holds the algorithms for analysis, discharging and charging of nicad cells, as well as providing all the control functions to drive the LEDs and external circuitry. The two internal comparators of the Z86EO have been config­ured to build a 12-bit A/D converter. With an 8-bit processor such as the Z8, this is done by storing eight bits of the con­ verter output in one register and the remaining four bits in another register. The converter uses a time relationship to convert the battery voltage into a digital code. The battery voltage is applied via a voltage divider to pin 9 of IC1. This voltage is fed to the internal comparators which use it to gener­ate a sawtooth voltage at pin 10. This sawtooth is developed in the following way. Op amp IC2, in conjunction with transistor Q5, forms a con­stant current source which charges the 0.27µF capacitor at pin 10 of IC12. When the voltage at pin 10 rises above the voltage at pin 9, the comparator output at pin 11 goes high. This turns on transistor Q4 which then discharges the capacitor at pin 10, whereupon Specifications Input........................................ 12V to 16V DC or AC, 1.5 amps Output..................................... 500mA or 1A switchable Cells........................................ 1-10 selectable by DIP switch Discharge................................ Voltage end-point. Charging.................................. Switches off when Delta Peak reached. Battery Condition.................... Determined by discharge curve method. Fault Indication........................ Battery below approx. 90% of capacity. Charging Times....................... 500mAh battery, 60 minutes from dead flat; ................................................ 1000mAh battery, 60 minutes from dead flat; ................................................ 1400mAh battery, 84 minutes from dead flat. the cycle repeats itself. In effect, the circuit works as a voltage to frequency converter with an inverse frequency relationship – the higher the battery voltage, the lower the frequency. Typically, when a 7.2V battery pack is being charged, the sawtooth voltage at pin 10 will be about 2.2kHz. The processor then converts the frequency at pin 10, repre­senting the battery voltage, to a digital value. This value is compared to an algorithm selected by the DIP switch at pins 15, 16, 17 & 18. Initially, when the battery is first connected, it is sensed by the processor which sends pin 1 high. This turns on Q2 and Q1. Q1 and LED 1 form a constant current circuit that con­trols both the discharge and charging currents. LED 1 is biased on when Q2 turns on and it provides a reference voltage of about 2V to the base of Q1. Q1 then acts as an emitter follower and pro­duces a voltage of close to 1.2V at its emitter (ie, the base-emitter Where to buy the kit The complete kit for the Nicad Battery Service Module is available only from Cessnock Instru­ men­ tation and Electronics. They own the copyright for the design.The kit contains all com­ponents in­cluding the 16VAC plugpack and the silk screened and drilled plastic case. The cost is $135 plus $10 for packing and postage. Adapters to suit various batteries are available from $25 each. Send orders to CIE, 524 Abernethy St, Kitchener, NSW 2325. voltage of Q1 will be close to 0.8V). This 1.2V is ap­plied to the emitter resistors of Q1 which will be 1.2Ω or 2.4Ω, depending on the setting of switch S1. Thus, Q1 is forced to carry a current of 500mA or 1A, as selected by switch S1. So Q1 operates at this current setting, both when the charger is in charge or discharge mode. OK, so far we’ve connect­ed the battery and it has been sensed by the processor which has turned on the constant current source. This starts sucking cur­rent out of the battery which is monitored all the time by the processor. After the initial discharge test, during which time the conditioning LED (LED 3) will be on, the processor will either decide to continue discharging the battery down to its end-point voltage of about 1V per cell or it will decide to charge the battery. When the latter occurs, pin 3 of IC1 will go high and turn on Q3 which controls DPDT relay RLY1. This changeover relay connects Q1 to the incoming supply so that it now charges the battery at the current selected by S1. Charge cycle Depending on the size of battery and its initial state of discharge, the time to fully charge it can range from less than 15 minutes for the full cycle to several hours. During the charge cycle, the battery is monitored constantly and the processor detects the slight dip in voltage that each cell gives when it reaches full charge. This is the so-called “Delta V” charging method but here there is a refinement. Instead of looking for a dip in the total battery voltage, the processor actually detects the voltage dip for each cell. Since it knows how PARTS LIST 1 plastic case, 135 x 95 x 45mm 1 PC board, 110 x 75mm 1 16V AC 1.5A plugpack with 2.5mm plug 1 cigarette lighter plug & lead with 2.5mm plug 1 DPST toggle switch with cranked leads (S1) 1 3.5mm jack socket 1 2.1mm DC socket 1 4-way DIP switch 1 miniature DPDT switch 1 3.579MHz crystal 1 multi-turn 5kΩ trimpot (VR1) 1 18-pin IC socket Semiconductors 1 Z86EO microcontroller (IC1) 1 LM358 dual op amp (IC2) 1 7805 5V regulator 3 BC547 NPN transistors (Q2,Q3,Q4) 1 BC557 PNP transistor (Q5) 1 TIP32C NPN transistor (Q1) (see text) 4 red LEDs (LED1, LED2, LED3, LED4) 1 green LED (LED5) 1 orange LED (LED6) 1 18V 400mW zener diode (ZD1) 6 1N4004 silicon diodes (D1-D6) Capacitors 2 1000µF 25VW electrolytic 2 100µF 25VW electrolytic 1 10µF 25VW electrolytic 1 0.27µF 63VW MKT polyester 2 0.1µF 63VW MKT polyester 2 18pF ceramic Resistors (0.25W, 1%) 1 30kΩ 2 1kΩ 6 10kΩ 1 470Ω 2 4.7kΩ 4 430Ω 1 3kΩ 1 330Ω 1 2.7kΩ 1 100Ω 1 2.2kΩ 2 1.2Ω 5W wirewound many cells are connected, by virtue of the DIP switch settings, it knows how many voltage dips to look for. Consequently, each battery will end up being charged to a different voltage. For example, we charged three 7.2V 1200mAH nicad racing packs. Two of these were ultimately charged to just over 9V while one was charged to September 1993  19 AC INPUT D1-D4 1000uF O G 100uF I 1000uF 7805 1k 10k 10k 10k 10k 4DIP SWITCH 1. 2  5W RELAY 2.7k 1 430  D6 4.7k D5 0.1 X1 330 A A LED1 K 18pF 18pF 430  10uF K A VR1 1. 2  5W 1k LED2 K LED3 B E 470  5W Q2 Q4 LED4 A .027 K 10k 2.2k 100  2.2k 1 100uF K LED5 A 30k 3k TO Q1 MOUNTED ON HEATSINK C IC1 Z82 4.7k Q3 Q5 IC2 LM358 430  ZD1 430  0.1 S1 LED6 A K Fig.2: install the parts on the board as shown here. The parts shown dotted (link, DIP switch & 0.1µF capacitor) mount on the underside of the board. Note that the two 1.2Ω 5W resistors should be mounted clear of the board, to aid heat dissipation. 10.4V. By the way, while the nominal cell voltage for nicads in 1.2V, it can go substantially higher than this while on charge. This is quite normal. It can happen that one or more cells in a battery pack may have almost identical voltage dips at the end of charge and this can make it difficult for the processor to detect the individual cell voltage dips. This is overcome by having the processor look at the total battery voltage for an overall decline in value at the end of charge, while also taking into account the elapsed time. When the processor decides that charging is complete, it pulls pins 1 and 3 low. This de-energises the relay and turns off the current source involving Q1. At the same time, pin 13 goes high to light the green Ready LED (LED 5). It can also happen that batteries will not charge properly due to internal open or shorted cells or perhaps due to wrong settings of the DIP switches for a particular battery. These cases are indicated by the orange fault LED (LED 6). It indicates the conditions shown in Table 3. Note that if a battery is connected the wrong way around, the charger will not work. Only the Power LED will light. Let’s now recap the sequence of a charging cycle. When power is applied, LED 2 (red) lights and when a battery is connected, the charger goes into the load test phase and the red Conditioning LED lights. When the unit subsequently goes over to charge mode, the red Charge LED lights as well. Finally, when it has finished RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 6 2 1 1 1 2 1 4 1 1 2 20  Silicon Chip Value 30kΩ 10kΩ 4.7kΩ 3kΩ 2.7kΩ 2.2kΩ 1kΩ 470Ω 430Ω 330Ω 100Ω 1.2Ω 4-Band Code (1%) orange black orange brown brown black orange brown yellow violet red brown orange black red brown red violet red brown red red red brown brown black red brown yellow violet brown brown yellow orange brown brown orange orange brown brown brown black brown brown not applicable 5-Band Code (1%) orange black black red brown brown black black red brown yellow violet black brown brown orange black black brown brown red violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown yellow orange black black brown orange orange black black brown brown black black black brown not applicable The power transistor (Q1) is supplied mounted on the heatsink with three wires connected: green for the emitter, blue for the base & white for the collector. These are connected to the underside of the board, as shown in Fig.2. power dissipation of transistor Q1, otherwise it will become very hot. Construction charging, the green Ready LED lights and if a fault occurs, the orange Fault LED lights. If power is disconnected and then reconnected while a bat­tery is being charged, the charger takes 60 seconds to reset itself and then it beings the cycle again with a conditioning test before flicking into charge mode. Power for the circuit comes either from an AC plugpack or from a 12V battery via a cigarette lighter socket in a car. The AC or DC is fed via a bridge rectifier comprising diodes D1-D4 and filtered with two 1000µF capacitors before being fed to a 7805 3-terminal 5V regulator. When supplied with 12V DC, the charger can charge batteries consisting of up to eight cells (ie, 9.6V nominal). When powered by a 16VAC plugpack, the unit can charge batteries with up to 10 cells. Ideally, if the charger is to be used to charge batteries of 7.2V or less at the 1A rate, it should be used with a 12VAC 1.5A plugpack to reduce the Table 2 Switch Number of Cells Battery Voltage 1 2 3 4 1 1.2 1 0 0 0 2 2.4 0 1 0 0 3 3.6 1 1 0 0 4 4.8 0 0 1 0 5 6.0 1 0 1 0 6 7.2 0 1 1 0 7 8.4 1 1 1 0 8 9.6 0 0 0 1 9 10.8 1 0 0 1 10 12.0 0 1 0 1 0 = OFF, 1 = ON. Note: always turn the power off and wait 60 seconds before adjusting the DIP switches. The charger is housed in a standard plastic case. This has two halves which clip together. Inside is a single-sided PC board which measures 110 x 75mm. This has all the components mounted on it apart from transistor Q1 which is mounted on a U-shaped alu­minium heatsink in the base of the case. All the components will be available in a complete kit which will include a 16VAC plug­pack adapter, a cigarette lighter plug lead and a battery output lead fitted with a 3.5mm jack. The component wiring diagram for the charger is shown in Fig.2. Assembly can begin with the 0.25 watt resistors, small capacitors and the transistors. The four 10kΩ resistors associat­ed with the DIP switch are mounted “end-on” while the DIP switch mounts under the board, on the copper side. There is a long link installed on top of the board and four contacts on one side of the DIP switch are actually soldered to this link. Next, fit the diodes, the electrolytic capacitors, the LM358 (IC2), multiturn trimpot VR1 and the 3-terminal regulator. In each case, make sure that the component is correctly oriented on the board. The two 1.2Ω 5W resistors should be mounted so that they stand September 1993  21 the base. The TIP32C transistor and heatsink assembly is sandwiched between the PC board and the base with the aid of two 5/16-inch nuts which act as spacers.The method of assembly is as follows: (1) place a nut over the central pillar in the base of the case, then fit the transistor heatsink over it. (2) Place another nut over the central pillar and then an insu­lating spacer. (3) Place an insulating spacer over the other pillar and then secure the board with the two self tapping screws. Do not over-tighten the screws and fit the front and rear panels of the case before they are fully driven home. Now comes setting up and calibration. Before fitting IC1 into its socket, connect the AC plugpack to the charger and measure the voltage at pin 5 (of the socket). It should be +5V DC. Check also that +5V is present at pin 8 of IC2 and at the collector of Q3. If not, check that the 5V regula­tor is OK. This done, turn the power off and wait at least 60 seconds before inserting IC1 into its socket. Make sure you get it the right way around. The pin 1 end should face the regulator end of the board. Next, set all the DIP switches to off before turning the power on again. Apply +7V from a power supply to the battery output and adjust trimpot VR1 until both pins 2 and 4 of IC1 are high; ie, +5V. The charger is now ready for use. Battery voltage selection A nut is fitted over the central pillar on the bottom of the case before the heatsink assembly is fitted. A second nut & an insulating spacer are then fitted to the pillar & an insulating spacer also fitted to the other pillar before the PC board is secured in position. about 6mm clear of the board, to aid heat dissipation. LED 1 can be mounted with short leads but the five indica­tor LEDs need to be mounted with long leads, so that their bodies are 20mm above the PC board. This is done so that they will protrude slightly through the lid of the case when it is clipped together. An 18-pin IC carrier is used for the Z8 (IC1) but this chip should not be installed until later. A 0.1µF capacitor is con­nected underneath the processor socket (on the copper side of the board) between pins 5 and 14. Also connected 22  Silicon Chip to the underside of the board are the leads to the 3.5mm battery socket. The input power socket and the DPST toggle switch S1 are mounted on the top of the PC board. The power transistor Q1 is supplied mounted on the heatsink with three wires connected: green for the emitter, blue for the base and white for the collector. These are connected to the underside of the board, as shown in Fig.2. The PC board is assembled into the case and secured by two self tapping screws with go into integral pillars in Always turn off the power and wait 60 seconds before ad­justing the DIP switches which are accessed via a hole on the underside of the case. The settings are shown in Table 4. Charge rate selection Select 500mA or 1A, which ever is the value closest to the rating of your battery. It is not recommended to charge at a rate higher than 1.2 times the battery capacity. For example, if you have a 500mAh AA cell, choose the 500mA rate. If you have a 7.2V 1200mAh racing pack, choose the 1A rate. If you wish to charge at a lower rate, then replace the 1.2Ω 5W resistor across switch S1 with a 10Ω 0.25W resistor. This will result in a charging current of 100mA instead of 500mA. This makes it suitable for charging 9V SC 100mAh batteries.