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If you own equipment which uses nicad batteries, then this discharger is for you. Used correctly, it will maintain the full capacity of your battery pack & extend its useful life. It can even rejuvenate an old battery pack that’s suffering from the memory effect. Automatic discharger for nicad battery packs By JOHN CLARKE While nicad batteries are designed to provide reliable power over many charge/discharge cycles, most people find that their new battery pack starts to give trouble after only a few such cycles. This problem is particularly prevalent in mobile tele­phone battery packs. The pack provides a reasonable “talk-time” when new but this quickly diminishes after a few weeks of use. When this happens, many people assume that the battery pack is crook and buy a new one. But that’s normally a complete waste of money. As a general rule, the battery will still be quite OK and just needs to be revived. 18  Silicon Chip The problem can usually be attributed to the so-called “memory effect”. This is a characteristic of the nicad cell whereby it ceases providing current when it has discharged to the level from which it was last charged. Thus, if a nicad battery pack is repeatedly recharged with half its capacity still remaining, it will eventually stop delivering power at half capacity. This means that, for the example given above, the battery’s capacity is effectively halved. Of course, if the battery is continually recharged from its 75% level, the problem is exacer­ bated. It will now only provide 25% of its capacity. With that in mind, it’s not hard to understand why mobile telephone batteries “run out of steam” so quickly from new. The only solution to this problem is to ensure that the nicad pack is fully discharged before recharging commences. This will ensure that the pack can deliver its full capacity every time. That’s where this Nicad Discharger comes in. It discharges the nicad pack until it reaches its full discharge voltage of 1.1V per cell, at which point it automatically switches off. The pack can then be removed and recharged to its full capacity on a charger. By adopting this technique, the dreaded mem­ory effect is avoided. 2x1N4004 D2 D3 Q1 BC327 C E 33k 3.6V 1.5k 910  560  430  330  430  1.8k A D1 1N4004 B 3 S1 NOMINAL BATTERY VOLTAGE 2 8 IC1a LM358 1 9.6V 10k Q2 BC338 B  K 680 C K LED2 REVERSE POLARITY  A 680 CELLS UNDER DISCHARGE D4 1N4004 S2 START E 4 7.2V 8.4V LED1 DISCHARGING 33k 4.7k 4.8V 6V 27 5W 2.7k REF1 LM3362.5 VR1 ADJ 100k 5 6 7 IC1b 470  Q3 BD679 B E 12V ADJUST VR1 FOR 0.49V 1.8k C 2. 7  B PLASTIC SIDE E E C B C VIEWED FROM BELOW ADJ K A 3-10 CELL NICAD DISCHARGER Fig.1: the circuit is powered by the battery under discharge. When the START switch (S2) is pressed, Q1 turns on & the battery voltage is fed to a resistive divider. The voltage selected by S1 is then compared with a reference voltage using IC1a, which turns on Q2 to maintain power when S2 is released. IC1b & Q3 form a constant current source which discharges the battery to an end point of 1.1V per cell. When this point is reached, Q2 turns off & the discharge cycle ceases. Note, however, that several full discharge/charge cycles may be necessary to fully rejuvenate a battery pack that is already suffering from the memory effect. This technique is called “deep cycling”. Provided that the pack is OK in other respects, this treatment is usually completely effective and leads to a dramatic increase in battery life and performance. And, of course, you will save money – nicad batteries are expen­sive. Discharge rate Our Nicad Discharger discharges batteries at a nominal 200mA rate until the end point voltage of 1.1V per cell is reached. During this time, a LED on the front panel glows to indicate that the pack is discharging. When the end point of 1.1V per cell is reached, the discharger switches itself off and the LED goes out to indicate the end of the discharge cycle. Thus, for a 7.2V battery pack, the end point voltage is 6.6V. That’s because there are six cells in a 7.2V pack (ie, each cell is at 1.2V when fully charged). Similarly, the end point voltage for a 12V pack is 11V. Note that nicad cells maintain a virtually constant 1.2V output until they are almost fully dis­charged. The unit is very easy to operate – all you have to do is connect the positive and negative leads to the battery pack, set the range switch to the rated battery voltage, and press the START button. The rest all happens automatically and you simply wait until the DISCHARGE LED goes out before removing the pack for recharging. A second LED on the front panel lights to warn you if the pack is accidentally connected with reverse polar- ity. No damage to the Nicad Discharger (or to the pack) will occur if you do this – just reverse the connections to correct the problem. In fact, this design is based substantially on the Nicad Discharger published in July 1992. This was a popular unit but, following publication, we received many requests for two extra voltage ranges below 6V. This new circuit adds these ranges and can now handle nicad packs ranging from 3.6V to 12V over seven ranges. In addition, the new design includes the aforementioned automatic switchoff feature and the reverse polarity indicator – items that were missing from the previous design. Circuit details Fig.1 shows the circuit details of the Nicad Discharger. It’s based mainly on Main Features • Seven ranges; suitable for 3.6V, 4.8V, 6.0V, 7.2V, 8.4V, 9.6V & 12V nicad battery packs • • Discharges battery down to 1.1V per cell • • • Discharge indicator LED Automatic switch-off with negligible current drawn after end point voltage is reached Reverse current protection & LED indicator Self-powered from discharging cells August 1994  19 430  1.8k 430  330  REF1 D3 33k LED1 10k VR1 A K Q2 560  33k B C E 1.5k Q3 Q1 2. 7  D2 680  S2 D1 2.7k 470  910  680  1.8k 4.7k 1 IC1 LM358 S1 K D4 27W 5W TO CELLS A LED2 Fig.2: install the parts on the PC board as shown in this wiring diagram. Make sure that all polarised parts are correctly oriented & mount the 27Ω 5W resistor slightly proud of the board to allow the air to circulate beneath it for cooling. Fig.3: check your PC board against this full size etching pattern before installing any of the parts. dual op amp IC1, transistors Q1-Q3, and voltage reference REF1. The op amp is an LM358 which can operate from a supply rail as low as 3V. This allows the circuit to operate correctly while discharging a 3.6V battery pack to an end point of 3.3V. Initially, when a battery pack is connected, no current flows in the circuit since all transistors are off. The circuit is turned on simply by pressing momentary pushbutton switch S2. When this happens, base current for Q1 flows via its 4.7kΩ base resistor, the base emitter junction itself and the 27Ω emitter resistor. Q1 thus turns on and applies power to pin 8 of IC1, to voltage reference REF1 via a 1.8kΩ resistor, and to a resistive divider string (33kΩ - 1.8kΩ). REF1 is an LM336-2.5 voltage reference and this device provides a constant 2.5V output over a wide current range from 400µA to 10mA. This voltage is fed to trimpot VR1 which is adjusted to provide a 0.49V Fig.4 (above): here are the mounting details for Darlington transistor Q3. It must be isolated from the front panel using an insulating washer & its leads bent at right angles to mate with the pins on the PC board – see photo at left. 20  Silicon Chip reference for the inverting input (pin 2) of comparator stage IC1a. IC1a compares the voltage at the wiper of switch S1 with the 0.49V reference on pin 2. If the voltage on pin 3 is greater than 0.49V (ie, the battery is not fully discharged), pin 1 of IC1a switches high and turns on transistor Q2 via a 10kΩ base resistor. This in turn ensures that Q1 remains on and that the circuit remains powered up when S2 is released. At the same time, LED 1 (the DISCHARGE indicator) turns on, since there is a path to ground via the 680Ω resistor and Q2. IC1b and Darlington transistor Q3 form a constant current source which discharges the battery at a nominal 180mA. The non-inverting input of IC1b (pin 5) is set at 0.49V (the reference voltage from VR1), while the inverting input (pin 6) monitors Q3’s emitter voltage. IC1b’s output appears at pin 7 and drives Q3 via a 470Ω resistor. As a result, a voltage of 0.49V is maintained across Q3’s 2.7Ω emitter resistor and this sets the current through Q3 to about 180mA. This current flows via diode D4 to discharge the cells. In addition, some discharge current also flows through LED 1 and IC1, so that the total discharge current adds up to a nominal 200mA. The resistive divider network sets the cutoff voltages for the various battery packs. This network is tapped off using switch S1 and the sampled battery voltage then fed to pin 3 of IC1a which operates as described previously. In practice, the resistor values were selected so that, for each range, the voltage on S1’s wiper is at 0.49V when the pack has discharged to 1.1V per cell. These resistor values take into account the fact that the voltage across the 27Ω 5W resistor increases by about 30mV for every volt applied to the circuit. When the voltage at S1’s wiper subsequently drops just below 0.49V (ie, when the battery pack drops just below its end point voltage), pin 1 of comparator IC1a switches low and removes the drive to Q2. Q2 thus turns off and so Q1 also turns off and interrupts the power to the circuit. This also turns off the DISCHARGE LED and transistor Q3 (since there is no longer any drive from IC1b), and so the bat­tery ceases discharging. PARTS LIST 1 PC board, code 14306941, 101 x 49mm 1 plastic case with aluminium lid, 115 x 65 x 40mm 1 front panel label, 64 x 126mm 2 alligator clips (1 red, 1 black) 1 150mm-length of red hook-up wire 1 150mm-length of black hookup wire 1 small cordgrip grommet 1 knob to suit 1 single-pole 7-position rotary switch 1 momentary pushbutton switch 1 TO-126 mica or silicone insulating washer 1 3mm screw & nut to mount Q3 1 100kΩ vertical trimpot (VR1) 5 PC stakes This view shows how the fully-assembled PC board appears after the front panel has been removed. Note that Darlington transistor Q3 should be mounted on the front panel before soldering its leads to the stakes on the board. Reverse polarity protection for the circuit is provided using diodes D1-D4 and the 27Ω resistor. If the battery is con­nected with reverse polarity, D1 clamps the voltage across IC1a to just 0.6V, D2 and D3 conduct to prevent destructive reverse breakdown of Q1, and D4 prevents reverse current flow through Q3. The 27Ω resistor provides current limiting under reverse polarity conditions. This device dissipates about 3.4W when a 12V battery is incorrectly connected, hence its 5W rating. Finally, LED 2 is forward biased under reverse polarity conditions and so lights to provide a visual warning. Board assembly The Nicad Discharger circuit is built on a PC board coded 14306941. Fig.2 shows the wiring details. Begin the construction by installing PC stakes at the external (plus & minus) lead positions and at the BCE positions for Q3. This done, install the resistors, taking care to ensure that you have the correct value in each position. Table 1 shows the resistor colour code but it’s also a good idea to confirm each value using a digital multimeter, as some of the colours can be difficult to decipher. The 27Ω 5W resistor should be mounted about 2mm above the PC board so that the air can circulate beneath it for cooling. Now install the IC, the diodes and transistors Q1 & Q2. Make sure that these components are all correctly oriented – pin 1 of the IC is adjacent to a small notch in one end of its plastic body. Note that Q1 is a PNP transistor Semiconductors 1 LM358 dual op amp (IC1) 1 LM336-2.5 reference (REF1) 1 BC327 PNP transistor (Q1) 1 BC338 NPN transistor (Q2) 1 BD679 NPN Darlington transistor (Q3) 4 1N4004 1A diodes, (D1-D4) 2 3mm red LEDs (LED 1,LED 2) Resistors (0.25W, 1%) 2 33kΩ 2 680Ω 1 10kΩ 1 560Ω 1 4.7kΩ 1 470Ω 1 2.7kΩ 2 430Ω 2 1.8kΩ 1 330Ω 1 1.5kΩ 1 27Ω 5W 1 910Ω 1 2.7Ω TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  2 ❏  1 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 Value 33kΩ 10kΩ 4.7kΩ 2.7kΩ 1.8kΩ 1.5kΩ 910Ω 680Ω 560Ω 470Ω 430Ω 330Ω 2.7Ω 4-Band Code (1%) orange orange orange brown brown black orange brown yellow violet red brown red violet red brown brown grey red brown brown green red brown white brown brown brown blue grey brown brown green blue brown brown yellow violet brown brown yellow orange brown brown orange orange brown brown red violet gold brown 5-Band Code (1%) orange orange black red brown brown black black red brown yellow violet black brown brown red violet black brown brown brown grey black brown brown brown green black brown brown white brown black black brown blue grey black black brown green blue black black brown yellow violet black black brown yellow orange black black brown orange orange black black brown red violet black silver brown August 1994  21 This view shows how the front panel is attached to the PC board & secured via the switch bushes. Note the mounting details for transistor Q3. Discharge Current 12V 11V 210mA 8.8V 200mA 8.4V 7.7V 7.2V 6.6V 6.0V 5.5V 4.8V 4.4V 3.6V 3.3V 190mA 180mA Turn-off accuracy: within 10mV per cell. Leakage current after discharge: <2µA at 11V; <0.25µA below 6V. Reverse battery polarity current: 370mA <at> -12V; 140mA <at> -6V; 70mA <at> -3.6V. BATTERY VOLTAGE 3.6V . + . 8.4V . 9.6V . 12V + own label from the published artwork – see Fig.5. Because it handles most of the current, Q5 requires a modest amount of heatsinking and this is achieved by mounting it on the lid of the case – more on this later. PRESS TO START + End Point Voltage 9.6V + + REVERSE DISCHARGE POLARITY NICAD DISCHARGER The unit is housed in a plastic utility case which has an aluminium lid. This is fitted with an adhesive label measuring 64 x 126mm or you can make your Range (Batt. Voltage) 7.2V . Final assembly Specifications 6.0V . 4.8V . while Q2 is an NPN type, so don’t get these two transistors mixed up. Trimpot VR1 and REF1 can be installed next (watch the orientation of REF1) but leave Q3 off for now since it must be mounted on the metal lid of the case. Switches S1 and S2 are soldered directly to the PC board – see Fig.2 and the photos. The two LEDs can now be installed in the respective loca­tions but don’t solder their leads yet. That step comes later, after they have been pushed through their mounting holes on the front panel of the case. Be sure to orient the LEDs correctly – the anode lead of each LED is the longer of the two (see pinout diagram on Fig.1). Fig.5: this full-size artwork can be used as a drilling template for the front panel. 22  Silicon Chip Begin the case assembly by attaching the label to the lid, then drill holes to accept the two 3mm LEDs, switches S1 and S2, and a 3mm mounting screw for Q3. Note that it’s best to drill small pilot holes for the two switches and then slowly enlarge them to the correct size using a tapered reamer. Deburr all holes after drilling and pay particular atten­ tion to the area around the transistor mounting hole –it must be perfectly smooth and free of metal swarf to avoid punch through of the insulating washer used later to isolate the transistor from the lid. A hole should also be drilled in one end of the case to accept a cordgrip grommet for the battery leads. As supplied, the rotary switch will have 12 positions, so you will have to adjust the selector ring to change it to a 7-position type. To do this, simply remove the nut and lockwasher from the threaded bush, then lift the selector ring and rotate it so that the locating pin goes in slot seven. Check that the switch does indeed now have seven positions, then trim the length of the shaft to suit the knob. Fig.4 shows the mounting details for tran­ s istor Q3. It must be electrically isolated from the front panel using a mica washer. Make sure that the mounting area is perfectly smooth and smear both sides of the mica washer with heatsink compound (not necessary if a silicone washer is used) before bolting the assembly together. The leads of the transistor are then bent at right angles so that Test & adjustment To test the unit, you will need to drill a small access hole through the front panel immediately above VR1. Alternative­ly, you will have to temporarily remove the front panel. Next, connect a 6V battery pack (or variable supply) to the circuit and wind VR1 fully anticlockwise to ensure that the discharge transistor (Q3) remains off. This done, set S1 to the 4.8V range and press S2 to start the discharger. Check that there is now 6V between pins 8 & 4 of IC1 and 2.49V across REF1 when S2 is released. The DISCHARGE LED (LED 1) should also be alight. If everything checks out so far, connect your multimeter across the 2.7Ω resistor (next to Q3) and adjust VR1 for a read­ing of 0.49V. This adjustment sets the reference voltage applied to IC1a and ensures correct operation of the constant current source (IC1b & Q3). Switch off immediately after making this adjustment and re-attach the front panel (if necessary). If you have a variable power supply, check that the discharger switches off at the correct voltage for each range selected (see specifications). The reverse polarity indica­tor circuit can be tested by reverse connecting the power and checking that LED 2 lights. Finally, always be sure to set S1 (the range selector switch) to the nominal voltage of your battery pack before press­ing the START switch to begin the discharge cycle. For example, if your nicad pack has a nominal output of 7.2V when fully charged, then set SC S1 to the 7.2V range. SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM PRICE ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏ 3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________ Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Street ___________________________________________________________ Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). ✂ they mate with the BCE PC stakes on the board. The front panel can now be attached to the PC board by fitting the matching holes over the switch bushes and doing up the locking nuts. Note that S1 is fitted with a large star washer, while S2 has a flat washer fitted to its bush (these washers all go behind the front panel). This done, the two LEDs can be pushed into their front panel holes and their leads sol­dered. The assembly can now be completed by attaching the battery leads to the PC board (red for positive, black for negative). These leads pass through the cordgrip grommet in one end of the case and are terminated with alligator clips or with some other suitable connector for your battery pack. August 1994  23