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If you own a mobile telephone, this simple gadget will dramatically increase the life of your nicad batteries. It does this by correctly discharging the battery pack to its endpoint voltage, so that it can then be recharged to full capacity. By BERNIE GILCHRIST* An autoillatic nicad battery discharger I N RECENT YEARS, lots of people have thrown perfectly good nicad batteries on the scrap heap in the mistaken belief that they had reached the end of their useful service life. Often, however, such batteries are perfectly OK apart from having their apparent capacity drastically reduced by what is known as the "memory" effect. This memory effect is acquired when nicad batteries go through repetitive discharge cycles in which only part of their rated capacity is Research & Development Department, Dick Smith Electronics, North Ryde, Sydney. * 40 SIL/CON CH I P used before recharging. As a result, the battery acquires a "memory" so that it only discharges to the point from which is was recharged and then behaves as though it had gone flat. When you think about it, it is quite easy for this situation to occur. Acommon problem is when people recharge partially flat camcorder batteries or batteries from mobile telephones. After a number of cycles, the batteries acquire a memory effect and this drastically reduces the operational time of the equipment. So how do we prevent this memory effect? The solution is to discharge the battery all the way to its endpoint voltage (1.1 V per cell) before recharging it to full capacity. A few such deep cycles are usually required to rejuvenate a battery that is already suffering from a memory effect, after which it should deliver its full rated capacity. Automatic discharger The easiest way of correctly discharging your nicad battery packs is to use·a special discharger such as the unit presented here. It can discharge nicad battery packs ranging from 3.6V to 12V and, unlike other units on the market, switches itself off when the endpoint voltage has been reached so that no further discharge takes place. This automatic switch-off feature means that you don't have to constantly check the discharger and dis- - START S2 12V 1.2k 16k VR1 Sk 1.2k 47k 1.2k 1.2k 1M + D2 1N4002 7.3k T NICAD BATTERY I 1.2k 1.2k ..a... BATTERY VOLTAGE S1 B B 1.2k 2.2k A 3.6k 3.3k LE01 RED K PLASTIC SIDE B EOC NICAD BATTERY DISCHARGER Fig.1: the circuit is powered by the battery under discharge. When S2 is pressed, pin 6 ofIC1b goes high & turns on Q4, Q5, Q6 & Q1. As the battery discharges, IC1b compares the voltage on its pin 3 input with a reference voltage derived via S1 from IC1a. When the voltage on pin 3 falls below the selected reference voltage, pin 6 ofIC1b switches low again & Q4, Q5, Q6 & Q1 switch off. connect the leads at the appropriate time. This is important, because if discharge continues after the endpoint voltage is reached, the weakest cell in the pack can eventually be forced to reverse its polarity and this causes permanent damage and drastically reduced capacity. To avoid this risk, the Nicad Battery Discharger discharges the pack to 1.1 V per cell and then switches off before any damage can occur. Thus, for a 7.2V battery pack, the endpoint voltage is 6.6V. (Note: nicad cells maintain a virtually constant output voltage until they are fully discharged). During operation, the unit is powered by the battery under discharge. The battery pack is simply connected via two clip-on leads and the battery voltage selected by means of an 8position rotary switch. The DISCHARGING LED now comes on to indicate that the unit is operating correctly and you can set a toggle switch (CURRENT) so that the battery dis- charges at either 50mA or 200mA. When the endpoint voltage is reached, the DISCHARGING LED goes out. The battery can now be disconnected and recharged to full capacity in the usual manner. How it works At the heart of the circuit is ICl , an LMlOCL op amp and voltage reference - see Fig.1. This IC contains a stable Z00mV reference which is permanently connected to the non-inverting input of the reference op amp (ICla). ICla amplifies this reference voltage by an amount depending on the setting of VRl, so that 0. 73V is applied to the following resistive divider chain. This divider chain sets the cut-off voltages for the various battery packs. It is tapped off using switch S1 and the sampled reference voltage is then fed to pin 2 of IClb where it is compared with a sample of the battery voltage on pin 3. Let's look at this in greater detail VIEWED FROM BELOW ~ ECB When the battery pack is first connected, virtually no current flows in the circuit (except for negligible leakage current). The circuit operation is now initiated by pressing the START button (S2). When this happens, power from the battery is applied to pin 7 of IC1 via a lOmA constant current source consisting of transistors QZ and Q3. The current drawn by the LMl0 is only about 500µA at most and so most of the lOmA from the constant current source flows through LED 1. LED 1 serves two purposes. First, it acts to provide a regulated supply of about 1. 9V to ICl. Second, it lights to indicate that the battery is being discharged. Assume initially that the battery voltage is greater than the endpoint voltage. In this case, the sampled battery voltage applied to pin 3 of IClb will be greater than the reference voltage on pin 2. Thus, pin 6 of rc1b swings high; ie, to within about 50mV of the 1.9V supply. This then turns on transistors Q4 and Q5 to discharge the battery. Because Q4's emitter will be at 1.3V, Q6 also turns on and the discharge current will be either 50mA or 200mA, depending on the setting of switch NOVEMBER 1992 41 PARTS LIST 1 plastic case, 41 x 68 x 130mm 1 single pole 8-position rotary switch (S1) 1 momentary contact pushbutton switch (S2) 1 SPOT miniature toggle switch (S3) 1 T0-126 mica washer 1 10 x 3mm machine screw & nut (for transistor mounting) 4 nuts to suit toggle and pushbutton switches 1 star washer to suit switch S1 1 red crocodile clip 1 black crocodile clip 1 400mm-length of red mediumduty hook-up wire 1 400mm-length of black medium-duty hook-up wire 1 60mm length of 3-way rainbow cable 1 knob to suit rotary switch 1 5kQ miniature vertical trimpot (VR1) Semiconductors 1 LM1 0CL op amp & voltage refererice (IC1) 1 BC328 PNP transistor (01) 2 BC557 PNP transistors (Q2,Q3) 2 BC549 NPN transistors (Q4,Q6) 1 8D140 PNP transistor (05) 4 1N4002 silicon diodes (D1-D4) 1 5mm red LED (LED1) Capacitors 1 10µF 16VW PC electrolytic 1 1µF 50VW PC electrolytic Resistors (0.25W, 1%) 1 1MQ 1 2.2kQ 1 47kQ 7 1.2kQ 1 16kQ 1 100Q 1 7.3kQ 2 56Q 1 3.6kQ 1 43Q 1 3.3kQ 1 8.2Q Where to buy the parts A kit of parts for this project is available from any Dick Smith Electronics store or by mail order from PO Box 321, North Ryde, NSW 2113. Phone (02) 888 2105 or, if outside Sydney, (008)226610. The kit comes complete and includes a pre-punched & silkscreened front panel. The price is $29.95 plus $5 p&p. Quote Cat. K3126 when ordering. Note: copyright of the PC artwork associated with this project is retained by Dick Smith Electronics. 42 SILICON CHIP S3. Note that some of this discharge current flows via the constant current source (QZ & Q3} and via Q6 and its associated 56Q resistor. When Q6 turns on, its collector current is sufficient to saturate Ql and so this transistor remains on when the START button (SZ} is released. The circuit thus remains on and the battery continues to discharge at either the 50mA or 200mA rate until it reaches its endpoint voltage. When the endpoint is reached, the voltage on pin 3 of IClb falls below the reference voltage on pin 2 and pin 6 switches low. This removes the bias from transistors Q4, Q5, Q6 & Ql and so the circuit switches off, the LED goes out and the battery ceases discharging. The lMQ feedback resistor between pins 6 & 3 of IClb provides the op amp with a small amount of hysteresis so that it switches cleanly at the transition point. If the battery is already below its endpoint voltage when the START button is pressed, the output of IClb will remain low and so Q4, Q5, Q6 & Ql will remain off. However, the LED will light while ever the button is held down. As soon as the button is released, the LED will go out again and the supply voltage on pin 7 ofICl will quickly fall as the l0µF capacitor discharges. Diodes D1-D4 protect the circuit against reverse battery connection. If the battery is connected the wrong way around, a small reverse current flows via the lO0Q resistor and the base-collector junction of Ql but this cannot damage the transistor and the IC is protected by DZ. The l00Q resistor between the base and emitter of Ql ensures that it fully turns off at the end of the discharge period. Construction Fig.2 shows the wiring details for the Nicad Battery Discharger. Most of the parts, including the switches, are mounted on a PC board (code ZA1373) and this is housed in a small plastic utility case measuring 41 x 68 x 130mm. Start the construction by installing all the resistors and diodes, then install the transistors and the two capacitors. The resistor values should be checked using a digital multimeter, since the colour bands can sometimes be difficult to read. Make sure that all polarised corn- Fig.2: make sure that all polarised parts are correctly oriented when installing them on the PC board & don't mix up the transistor types. Q5 is mounted on the front panel & is connected to the board via flying leads - see Fig.a. ponents (transistors, diodes & capacitors} are correctly oriented and that the correct transistor type is used at each location. If you use the wrong transistor or install a part the wrong way around, that part could self-destruct at switch-on and possibly take other parts out with it. The IC, switches and trimpot can now be installed as shown in Fig.2. An IC socket was used for the prototype but there's no reason why the IC cannot be soldered directly to the board. Be sure to install the IC with the correct polarity. As supplied, the rotary switch will have 12 positions but it can easily be changed to an 8-position type by moving the selector ring at the end of the threaded bush. Check that the switch is seated properly against the board before soldering its pins. Switch S2 should be mounted so that it sits about 4mm above the board surface, while S3 should be pushed all the way down onto the board. At this stage, the only part not fitted to the board will be the indicator LED. We'll come to that later. In the meantime, cut two 400mm lengths of medium-duty hook-up wire for the battery connections (one red and one black} and solder them to the PC board. Similarly, solder three 60mm-long leads to the PC board for the connec- tions to the power transistor (Q5). Because it handles most of the current, Q5 requires a modest amount of heatsinking and this is achieved by mounting it on the metal lid of the case. Fig.3 shows the mounting details. Note that Q5 must be electrically isolated from the lid using a mica washer. Before mounting the power transistor, check that the contact area is perfectly smooth and free of metal swarf. If you detect any roughness, use a small file and a piece of wet and dry paper to smooth the contact area so that there is no risk of the metal punching through the mica washer when the assembly is bolted together. When you are satisfied that the contact area is OK, smear both sides of the mica washer with heatsink compound, then bolt the assembly together as shown in Fig.3. The three connecting leads from the PC board can then be soldered to the transistor pins. Finally, use your multimeter to confirm that Q5 's collector is indeed electrically isolated from the lid. Final assembly The PC board is secured to the lid of the case by the pushing the switch bushes through matching holes and doing up the locking nuts. Before doing this, however, fit the LED to the lid of the case by pushing it into its mounting bezel. This done, rotate the LED so that its anode and cathode leads will mate with the appropriate holes on the PC board (the anode lead is the longer of the two) and fit the locking ring. Two sets of mounting nuts are supplied for switches SZ and S3 and one nut should be fitted to each switch to provide support for the back of the lid. The nut for SZ should be done all the way up, while the nut for S3 TABLE 1 Battery Voltage Endpoint 3.6V 3.3V 4.8V 4.4V 6V 5.5V 7.2V 6.6V 8.4V 7.7V 9.6V 8.8V 10.8V 9.9V 12V 11V The assembled PC board is secured to the lid of the case as shown in this photograph. Note that although LED 1 is shown here mounted on the board, it is normally fitted to its bezel first & its leads slid through matching holes in board when the latter is mounted in position. should be positioned just over half way up the bush. In addition, star washers should be fitted to all the switches (see photo). The PC board can now be mounted on the lid by guiding the switch bushes through their mounting holes and making sure that the leads from the LED pass through their mounting holes in the PC board. This done, install the switch locking nuts, then solder and trim the LED leads. The battery leads exit through a hole in the top of the case. Tie a knot in these leads just before the exit point, so that the leads cannot be pulled out, and terminate their free ends with alligator clips (or some other suitable connector). Use a red alligator clip for the positive lead and a black alligator clip for the negative lead. MICA WASHER ~ NUT \ PLASTIC~r SIDE SCREW ~ -CASE LID / T0128 DEVICE Fig.3: transistor Q5 must be electrically isolated from the lid of the case using a mica washer as shown in this mounting diagram. Check that the mounting surface is smooth & smear both sides of the mica washer with heatsink compound before bolting the assembly together. Test & adjustment The unit is tested by substituting a variable power supply for the nicad battery pack. First, set the discharger to the 12V range, then connect it to the power supply and set the supply to give an output of 15V. Trimpot VR1 should initially be set to its mid-range position. Now press the START button. The DISCHARGING LED should immediately come on and should stay Qn when the button is released. If it does , the circuit is working correctly and you can check that it automatically switches itself off at some point by slowly winding the supply back until the LED suddenly goes out. Finally, the circuit can be calibrated by setting the supply to 11 V (exactly) and adjusting VR1 until the LED just goes out. Make sure that the discharger is set to the 12V range during this procedure. The remaining seven ranges can then be checked. The endpoint voltages should be very close to those listed in Tabie 1. If there are any significant variations from the listed values, check the resistors in the divider chain. SC NOVEMBER 1992 43