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Get rid of the dreaded memory effect Build this nicad battery discharger occurs when you recharge a partially charged battery repeatedly. This fools the battery into thinking that it was fully flat before you started charging it, even though it wasn't. When you use it next, the battery only discharges to the point from which it was charged and then stops delivering current as though it had gone flat. CROZMAN Thus, the capacity of the battery is vastly reduced and this leads to a constant S0mA for a period of 14 hours corresponding reduction in operation for a standard charge, although they time. can be fast charged if certain precauThis is the reason why so many tions are taken. people have trouble with nicads. They In either case, they should be fully use their camcorder to shoot half an discharged before you start to charge hour of the kids playing under the them if the best performance is to be sprinkler, then later want to shoot an obtained. If they are not correctly dis- hour of the kids playing local compecharged, the dreaded "memory effect" tition football. So they put the camcomes into play. corder on charge so that it will be "fully charged", ready for the footMemory effect ball. Little do they know that they The memory effect in nicad cells have just shortened the discharge time of the battery. Similar problems occur when people recharge parCOMPLETE tially flat nicad packs from LED1 04 mobile telephones. 1N4004 Our Nicad Discharger is both the prevention and the cure for the "memory effect" problem. It is powered by the battery under discharge and Are you having battery problems with your video camera or mobile telephone? This low cost device will solve your problems. It will correctly discharge a nicad battery pack so that it can then be recharged to full capacity. By MARQUE People with mobile telephones or camcorders often rush out to buy a new battery pack in the belief that the one they have is a "dud". This is often a complete waste of money. In many cases, the "dud" battery is still OK and just needs to be revived. Nicad batteries are rather touchy things when it comes to recharging. They like to be recharged by a constant current that is 1110th their full discharge capacity. For example, S00mA cells should be charged at a a~m~v DISCHARGE NICAD DISCHARGER 22 SILICON CHIP A0ADJ VIEWED FROM BELOW Fig.1: the circuit is powered by the battery under discharge. The battery voltage is fed to a voltage divider, sampled by S1 & compared using ICla with a 2.5V reference from ZDl. ICla then drives constant current source Ql to discharge the battery, while IClb drives the DISCHARGE indicator LED. PARTS LIST The Nicad Discharger can be used with standard battery packs ranging from 6V to 12V. By correctly discharging a battery pack, you get rid of the memory effect & this allows the pack to be recharged to its full capacity. prevents the memory effect from occurring by discharging the pack to its correct endpoint voltage before you connect it to your charger. It can also restore older battery packs to their former glory by a method called deep cycling. Deep cycling of nicads is carried out by charging and then fully discharging the battery repeatedly. The reason for doing this is to eliminate any memory effect present in the battery. A few cycles is enough to rejuvenate most battery packs but really stubborn ones may take longer. 1 PC board, code SC14207921, 80 x 45mm 1 Dynamark front panel label, 45 x93mm 1 metal diecast case, 98 x 50 x 25mm 1 single-pole 5-position rotary switch 1 400mm length of medium duty figure-eight cable 1 19mm plastic knob 1 red alligator clip 1 black alligator clip 2 LED mounting bezels 1 TO-220 insulating kit (mica washer plus insulating bush) 1 cordgrip grommet 1 machine screw, nut and lock washer 5 PC stakes 1 10kQ linear trimpot (VR1) Semiconductors 1 LM358 dual op amp (IC1) 1 LM336-2.5 2.5V voltage reference (IC2) 1 BD679 NPN Darlington transistor (01) 3 red LEDs (LEDs 1,2&3) 1 1N4004 diode (D1) 3 1N914 diodes (D2,D3,D4) Polarity reversal Our Nicad Discharger discharges batteries at a constant 200mA to this endpoint and then indicates (via a LED) that the pack is fully discharged. Five voltage settings are provided to accommodate different battery packs: 6V, 7.2V, 8.4V, 9.6V and 12V. To use the unit, you simply select the rated voltage of the battery pack and connect the flying leads. The DISCHARGE LED now comes on to indicate that the battery pack is discharging. When the endpoint is reached, the COMPLETE LED lights to indicate that the battery can be recharged and the DISCHARGE LED then goes out after a delay of several minutes. When nicad batteries are connected in series and discharged as a pack, the weakest cell will always be the first to fully discharge. If the discharge then continues, the weakest cell will be recharged by the others and this will cause it to reverse polarity, thus leading to permanent damage. To avoid this risk, an endpoint voltage is chosen for the pack. This represents the point at which all the cells have virtually totally discharged but occurs before any damage can occur to the weakest cell. In practice, this endpoint voltage is 1.1 V per cell (note: nicad cells maintain a virtually constant output voltage until they are fully discharged). Thus, for a 7.2V battery pack, the endpoint is 6.6V. constant 2.49V output using VRl. the two associated diodes, Dl and D2, The circuit is based on ICla, a corn- . provide temperature compensation for parator wired with a small amount of the voltage reference. positive feedback (via an 820kQ resisIf the battery voltage is above the endpoint voltage, pin 1 ofICla will be tor) so that it acts as a Schmitt trigger. The hysteresis produced by this posihigh and thus Darlington transistor Ql will be on. Note the presence of tive feedback stops the circuit from oscillating at the transition point. Fig.1 LED 3 and D3 in series between Ql's shows the details. base and the negative rail. These comAs shown, the battery voltage is ponents form a zener diode so that Ql applied to a resistive divider string acts as a constant current sink to-discharge the battery. via reverse polarity protection diode D4 and tapped off using switch S1. The way in which this works is as This tapped voltage is then fed to pin follows. Because the voltage on Ql 's 3 ofICla and compared with the volt- base is held at approximately 2.3V when pin 1 of IC la is high, it follows age generated by ZDl. that Ql 's emitter must be at about 1V ZDl is an LM336-2.5 voltage refer(since Ql is a Darlington transistor). ence which is adjusted to provide a Resistors (0.25W, 1%) 1 820kQ 1 750Q 1 5.1kQ 1 620Q 1 4. ?kQ 1 560Q 1 2.4kQ 1 470Q 1 2.2kQ 1 4.?Q 1W 2 820Q Circuit details JULY 1992 23 Fig.2(a): install the parts on the PC board as shown here but note that Qt, LED 1 & LED 2 must first be mounted on the case lid (see text). Fig.2(b) at right is the full size etching pattern for the PC board. Thus, a constant 213mA (approx.) discharge current flows through the 4. 7.Q 1W resistor while ever pin 1 of IC1a is high. (Note: LED 3 and D3 were chosen in preference to a conventional zener diode because they give a much sharper knee characteristic). MICA INSULA TIIG WASHER -jl:Q ' T0220 DEVICE Comparator IC1b is wired in parallel with IC1a. Its output (pin 7) is high when IC1a's output is high and this drives the DISCHARGE LED (LED 2) . When the battery subsequently discharges to its endpoint, pin 1 of IC1a switches low. This turns Qi off to end the discharge cycle and lights LED 1 to show that the discharge cycle has been completed. Both LEDs will now be on until, after a delay of several minutes , pin 7 of IC1b also switches low and turns LED 2 off. Note that a separate feedback resistor is not necessary for IC1b. That's because IC1b derives its positive feedback from the resistor across IC1a. Construction LID Fig.3: mounting details for the BD679 Darlington transistor. It must be isolated from the lid of the case using a mica washer & insulating bush. Smear both sides of the mica washer with heatsink compound before bolting the assembly together, then bend the leads of the transistor down to mate with the .stakes on the PC board. Construction is straightforward as all the parts are mounted on a small PC board coded SC14207921. Fig.2(a) shows the wiring details. Before mounting any of the parts, check that the switch fits into its holes on the PC board. Enlarge the holes to 1mm diameter if necessary. The resistors and diodes can now be installed on the PC board as shown in Fig.2(a). Make sure that the diodes are correctly oriented. The 4. 7.Q 1W resistor is mounted slightly proud of the board to aid heat dissipation. PC stakes are required to terminate the Darlington transistor leads and the battery leads. These should be soldered in next, followed by voltage reference ZD1, IC1 and the trimpot (VR1). Note that the flat side of the voltage reference faces the switch, while pin 1 of the IC faces away from the switch. Push these components as far down onto the board as they will comfortably go before soldering their leads. LED 3 can be installed next. This goes in next to the switch and should be mounted as close to the board as possible so that it doesn't later foul the lid of the case. Note that, unlike the other two LEDs, LED 3 is enclosed in the case. Don't put the other two LEDs in yet; we'll come to those later. The rotary switch needs to be the sealed PC-mount variety, as the open types are too deep to fit in the case. As purchased, the switch will have 12 positions but can easily be changed to a 5-position type by moving the locking ring at the front (behind the mounting nut). Check that the switch is RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 24 No. 1 1 1 1 2 1 1 1 SILICON CHIP Value 4-Band Code (1%) 5-Band Code (1%) 820k.Q 5.1 k.Q 4.7k.Q 2.2k.Q 820.Q 750.Q 620.Q 560.Q 470.Q 4.7.Q grey red yellow brown green brown red brown yellow purple red brown red red red brown grey red brown brown purple green brown brown blue red brown brown green blue brown brown yellow purple brown brown yellow purple gold brown grey red black orange brown green brown black brown brown yellow purple black brown brown red red black brown brown grey red black black brown purple green black black brown blue red black black brown green blue black black brown yellow purple black black brown yellow purple black silver brown TABLE 1 Battery Voltage Endpoint Voltage 6V 5.5V 7.2V 6.6V 8.4V 7.7V 9.6V 8.8V 12V 11 V seated properly against the board before soldering all the pins. The PC board can now be put aside while the necessary holes are drilled in the metal diecast case. First, attach the adhesive label to the lid, the drill holes to accept the rotary switch, the bezels for LEDs 1 & 2, and the mounting screw for the Darlington transistor. The hole for the rotary switch is best made by first drilling a small hole and then enlarging it using a reamer. The hole for the transistor mounting screw should be in line with the collector pin on the PC board and about 20mm from the lefthand edge of the lid (see photo). You will also have to drill a hole in one end of the case to accept a cordgri p grommet for the battery leads. Fig.3 shows the mounting details for the Darlington transistor. It must be electrically isolated from the lid of the case using a TO-220 mounting kit (mica washer plus insulating bush). Make sure that the mounting area is free of metal swarf and smear heatsink compound on both sides of the mica washer before bolting the assembly together. The leads of the transistor are then bent at right angles so that they mate with the PC stakes on the board. The indicator LEDs can now be pushed into their bezels on the lid. Orient each LED so that its anode (longest) lead is closest to the outside of the panel. This done, mount the board on the lid by sliding the leads of the LEDs into their mounting holes and doing up the lock nut of the rotary switch. Finally, solder the LED leads, cut off the excess lead lengths, and connect the leads of the transistor to their matching PC stakes. All that remains now is to connect This view shows the PC board after the lid has been removed (in practice, Ql & LEDs 1 & 2 are mounted on the lid first, as described in the text). LED 3 is pushed all the way down into the board, so that it sits below the switch body. The PC board is secured to the lid of the case by the switch, indicator LEDs & the mounting screw for Ql. Make sure that Ql's tab is correctly isolated from the lid (see Fig.3) before completing the assembly. the battery leads. These are made from a short length of medium-duty figure8 cable terminated with alligator clips. Secure the free end of the cable to the case using the cordgrip grommet and connect the positive and negative leads to the board as shown in Fig.2. Test & calibration To test the unit, first check the wiring and component orientation carefully, then connect a variable power supply in the place of the battery under discharge. Set both the supply and the Nicad Discharger to 12V, then slowly wind the supply back to see if the LEDs change state. If they do, then everything is OK. If they don't, go back and recheck the circuit board for errors. Finally, the unit can be calibrated by setting the supply to 11 V and adjusting VRl until the DISCHARGE LED just goes out and the COMPLETE LED just comes on (note: leave the Nicad Discharger set to 12V during this procedure). The remaining four ranges can then be checked. They should be very close to the cut-off points shown in Table 1. SC ]ULY1992 25