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Get maximum life & capacity from your nicad cells Build this nicadcell discharger Are you having problems with nicad cells in your battery packs? This low-cost discharger will avoid most of those hassles. It will correctly discharge any nicad cell so that it can be recharged to full capacity. Design by JOHN CLARKE Back in July 1992, we published a simple but effective nicad battery discharger for battery packs ranging from 6V to 12V. This has proved immensely popular and so has a more complicated automatic discharger design featured in the November 1992 issue of SILICON CHIP. The latter design was able to discharge battery packs ranging from 3.6V to 12V. As good a:s these designs are, they do suffer from one major problem. Ideally, to obtain maximum capacity from a nicad battery pack, each cell should be separately discharged to the "end point" voltage of 1.1V and then each cell should be individually recharged. The reason for this is that individual cells in a battery pack will have differing capacities and therefore when the battery pack is nominally discharged, each cell will have a different voltage. When they are subsequently recharged, maximum capacity will not be obtained. This problem has no solution when the cells in a battery pack are permanently connected and the connections are inaccessible; ie, when they are sealed inside the package. So for nicad battery packs, one of the discharger 20 SILICON CHIP circuits referred to above is the only practical solution. However, if you make up your own battery packs or use single nicad cells in any configuration, you can now obtain the best results with this single cell discharger. It will discharge any size single nicad cell down to 1.1 V whereupon it will turn itself off automatically. After that, the amount of current drawn from the discharged cell is minute (less than 20 microamps, in fact), so you don't have to worry if you leave the cell in the discharger and forget it. Features • Discharges either AAA, AA, C or D cells • Stops discharging at 1.1V cell voltage • Flashing LED to indicate discharging • Adjustable discharge current • 20µA cell current once discharged • Reverse polarity protection • Compact design l '-<" \1~~! .·• ·.,........... _ The discharger requires no external power supply as it gets its power from the cell it is discharging. This is the same concept as for the discharger circuits presented in the July and November 1992 issues of SILICON CHIP. Design approach A self-powered single cell discharger presents something of problem because the battery voltage is nominally 1.2V and this falls to 1.1V at the end of discharge. This is not enough to power conventional op amps or voltage reference sources, both of which are necessary if the circuit is going to do the job properly. Our solution to this problem is to use a DC-DC converter to step-up the cell voltage to around 9V. Fig.1 shows the complete circuit. IC1 is a TL496 DC converter. Power for IC1 comes from the nicad cell via Mosfet Ql. When START switch Sl is pressed, the cell voltage is applied to IC1 and it delivers close . to +8.8V across the 470µF electrolytic capacitor at its pin 8 output. This then becomes the supply rail for the remainder of the circuit which comprises a 5.1V zener diode, an LM358 dual op amp (IC2) and a few other minor components. IC2a is used as a comparator to compare the nicad cell voltage against a reference voltage. The nicad cell voltage is applied to the non-inverting input at pin 3 via a 2.7kQ resistor. This is compared to the inverting input at pin 2 which is set to 1. 1V. This voltage is derived from 5.1 V zener diode ZD1 via trimpot VRl. - START S1 2.7k STEP-UP VOLTAGE CONVERTER 01 MTP3055 +1.2V L1 50uH 470 + 16VWi 1'l 3 2 10k IC1 TL496 G NICAD+I CELL - 4 +8 .SV ..__ ._.- - ,~ 470 16VW + ! 7 REVERSE POLARITY PROTECTION 10k *R1 1.Sk 18k REFERENCE EXTRA DISCHARGE....__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___, 10 + l&VWi 7 *SEE TABLE L1: 33T, 0.5mm ECW ON NEOSID 17•732·22 TOROID NICAD CELL DISCHARGER K 7 B EOc VIEWED FROM BELOW Fig.1: the circuit uses DC converter ICl to step up the battery voltage to 8.8V when the START switch (S1) is pressed. This rail is then used to power comparator stage IC2a & LED flasher stage IC2b. When the battery discharges to 1.lV, pin 1 ofIC2a goes low, Qt switches off & removes power to ICl, & the LED stops flashing. Nate that the 5 .1 V zener diode has a zero temperature coefficient so that the 1.1 V reference will not vary with changes in temperature. With a charged cell connected to the circuit, pin 3 ofICZa will be higher than the reference voltage at pin 2. The op amp output at pin 1 will therefore be high. This voltage is applied to the gate of Q1 which feeds the nicad cell voltage through to IC1 when switch S1 is released. We have used a Mosfet here since it has only about 40mV across it when it is turned on. To compensate for this 40mV, a 470kQ resistor is connected between pins 1 and 3 of IC2a. This applies a small amount of hysteresis to the comparator. When the voltage across the nicad cell drops to just slightly below 1.1 V, the output of IC2a goes low, turning offMosfet Qt. Power is removed from the circuit and Q2 switches off. We haven't described the function of Q2 yet but we'll come to it in a moment. After Q1 and Q2 switch off, the only power drawn from the nicad cell is the leakage current of Q1 and Q2 which is negligible. We estimate the leakage to be about 40µA. LED1 DISCHARGING DISCHARGING FLASHER lows. The non-inverting input at pin 5 is tied to three 10kQ resistors - one to the +8.8V supply, one to 0V and the third to the output at pin 7. This voltage divider network means that the 10µF capacitor at pin 6 will constantly be charged and discharged between about +5.9V and +2 .9V. At the same time, the output at pin 7 will be a square wave with a frequency of about 3Hz and this drives LED 1 which then GDS functions as a discharge indicator. Notice the 2.2kQ resistor at the output of IC2b. This is connected to the +8.8V rail and current is drawn through it when ever the output at pin 7 is low. At other times, when pin 7 is high, current flows through LED 1. Thus, there is current flow via the output of IC2b whether the LED is on or off. This helps prevent voltage fluctuations from the nicad cell which could otherwise upset the operation ofIC2a. Nicad load current The total current drawn from the Discharge indicator Op amp IC2b is wired as a Schmitt trigger oscillator which is used to flash LED 1 on and off. It operates as fol- Most of the parts are mounted on a small PC board which clips into a plastic case. The battery holder is mounted on the lid of the case & is connected back to the board via flying leads. Note that the resistors are all mounted "end-on". MAY 1993 21 Fig.3: this is the full-size etching pattern for the PC board (code 14305931). nicad cell while it is discharging comprises the current drawn by the LED flasher, the zener diode reference, the comparator circuit and the step-up converter. This totals about 150mA. This is adequate for discharging AAA and AA size cells but more current is needed to discharge larger cells within a reasonable time. Transistor Q2 provides this extra discharge current. So if you want to discharge C or Dsize nicad cells, you will need to include resistors Rl and RZ, plus transistor QZ. Table 1 shows the values for Rl and RZ to suit the various cell capacities. If you wish to discharge cells of different capacities, you will need a rotary switch to switch Ql and RZ into or out of circuit. Alternatively, you could take the simple approach and build a separate discharger to suit each type of cell in use. Some readers may question how the addition of QZ and Rl will increase the current drain from the nicad cell by 70 milliamps. After all, a Mosfet Ql must be bent forwards as shown in simple calculation for the this photograph so that it clears the lid of the current through Rl will give case. the current as close to 5.5mA (I = [8.8V - 0.6V]/1.5kQ = 5.5mA). The point to remember here is that the 8.8V supply is being derived via the inverter based on ICl. The voltage step up ratio is about 7.3 and after the efficiency of the circuit is taken into account, the nicad cell has to deliver about 12mA for every lmA drawn from the 8.8V rail. Hence, when QZ and Rl are present, the circuit draws an extra 70mA. In order to draw an extra 180mA, as required for D cells, RZ is installed and a total of about 400mA is then drawn from the cell. To protect ICl against incorrect connection of the nicad cell, diode D1 and two lQ resistors are included in the circuit. These restrict the reverse voltage across ICl to less than 1V and thus protect it against damage. Construction We built our prototype Nicad Cell Discharger to suit AA size cells. It is housed in a plastic case measuring 83 x 54 x 30mm and has an AA size cell holder on top. A front panel label RESISTOR COLOUR CODES 0 0 0 0 0 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 1 1 3 470kQ 18kQ 15kQ 10kQ 6.8kQ 2.2kQ 1.5kQ 680Q 10Q 6.8Q 1Q yellow purple yellow brown brown grey orange brown brown green orange brown brown black orange brown blue grey red brown red red red brown brown green red brown blue grey brown brown brown black black brown blue grey gold brown brown black gold brown yellow purple black orange brown brown grey black red brown brown green black red brown brown black black red brown blue grey black brown brown red red black brown brown brown green black brown brown blue grey black black brown brown black black gold brown blue grey black silver brown brown black black silver brown 0 0 0 0 0 0 0 22 1 2 SILICON CHIP TABLE 1 Cell Capacity 1B0mAh (AAA) Discharge Current 150mA Q no R1 R2 - - 500mAh (AA) 150mA no - - 1.2Ah (C) 220mA yes 1.5k - 2Ah (C,D) 220mA yes 1.5k - 4Ah (D) 400mA yes 1.5k 6.8 ohms measuring 48 x 26mm is fitted to the end of the case, along with the pushbutton START switch (S1) and the discharge LED. Apart from LED 1 and switch S1, all the circuit components are mounted on a PC board coded 14305931 and measuring 61 x 46mm - see Fig.2. This board is a press fit into the plastic case. After checking the PC board for any defects such as shorted or open circuit tracks, you can begin the assembly by inserting six PC stakes at the external wiring points, the two ICs and the diodes. Check the orientation of the ICs and diodes before soldering them in place. This done, insert trimpot VR1 and the capacitors. Note that the 470µF capacitors are quite squat - their length is only 12.5mm. We used capacitors from Altronics (Cat. R-5162) . If other types are used, they must not be longer than 13mm so that the PC board will fit into the case. All the resistors are mounted end on to conserve space. You should use your digital multimeter to make sure that you are inserting the correct value resistor in each position. Use Table 1 to check if you need to insert QZ and resistors Rl and RZ. Mosfet Ql is mounted with its leads bent so that it can lie horizontally over IC1 , while QZ must be inserted so that it does not sit higher than the electrolytic capacitors. Note that the photograph of our prototype board shows transistor QZ and resistors Rl and RZ in place, although they would only be needed if large cells were to be discharged. Inductor 11 is made by winding 33 turns of0.5mm enamelled copper wire on a Neosid toroid. Pass half the length through the centre of the core and wind on about 16 turns, then use the other end to wind on the rest of the turns. This done , strip the lead ends and solder them into the holes on the + DISCHARGING + START NICADCELL DISCHARGER Fig.3: this full-size artwork can be used as a template when drilling holes for the switch & LED indicator. PC board. The wound toroid is held in place using a transistor insulating bush and a 3mm machine screw through the centre. Secure it with a 3mm nut on the underside of the PC board. The front panel label can now be affixed to the end of the case and the holes drilled for the LED bezel and switch. After attaching the wires to the switch, sleeve the switch end with heatshrink tubing or insulating tape to prevent the bottom of the PC board from shorting to the switch contacts. Next, connect the switch wires to the PC board, then install the PC board in the case and push the LED into its bezel. The LED leads can now be soldered to the its stakes on the PC board. Make sure that the LED is connected with the correct polarity. Its anode lead is the longer of the two. The cell holder is attached to the lid of the case using contact adhes~ve or super glue. We do not recommend using screws and nuts since these could easily short between resistors on the PC board. Drill small holes so that you can pass the cell holder wires through the lid of the case and solder the wires to the PC board pins, as shown in Fig.2. Testing To test the unit, rotate VR1 fully anti-clockwise, insert a charged nicad cell into the holder and press S1. The discharging LED should immediately begin flashing. Check that the voltage between pins 4 and 8 of ICZ is about PARTS LIST 1 plastic utility box, 83 x 54 x 30mm 1 PC board, code 14305931 , 61 x46mm 1 front panel label, 48 x 26 1 single cell holder plus clip lead (see text) 1 momentary pushbutton switch (S1) 1 15mm LED bezel 1 Neosid 17-732-22 iron powdered toroid (L 1) 1 1-metre length of 0.5mm enamelled copper wire 1 transistor insulating bush plus screw & nut 1 10kn horizontal trim pot (VR 1) Semiconductors 1 TL496 DC converter (IC1) 1 LM358 dual op amp (IC2) 1 MTP3055A or MTP3055E N-channel Mosfet (01) 1 BC338 NPN transistor (02) 1 1N4002 1A diode (01) 1 BZX79C5V1 5.1V 500mW zener diode (ZD1) 1 5mm red LED (LED 1) Capacitors 2 470µF 16VW PC electrolytic (13mm length, see text) 1 10µF 16VW PC electrolytic 2 0.1 µF MKT polyester Resistors (0.25W, 1%) 1 470kQ 1 18kQ 1 15kn 3 10kQ 1 6.8kQ 1 2.2kQ 1 1.5kQ 1 6800 1 1 6.80 0.5W 2 10 10n Miscellaneous Heatshrink tubing for body of S1, tinned copper wire, hookup.wire, solder, machine screws & nuts. 8.8V and that the voltage across ZD1 is about 5.1 V. If all is well, rotate trimpot VRl clockwise until LED 1 stops flashing to verify that the comparator is operating. VR1 should then be adjusted so that the voltage between pins 2 & 4 of ICZ is 1.1 V (this sets the cell discharge voltage). The Nicad Cell Discharger is now ready to discharge your cells and help keep them in top condition. SC MAY 1993 23