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Build a dry-cell battery rejuvenator Are you sick of throwing away those AAsize dry batter­ies? Well, don’t – rejuvenate them instead with this Dry-Cell Rejuvenator. Depending on the state of the cells, you could get up to 10 times their rated life & save big money! By DARREN YATES That’s right – this circuit allows you to rejuvenate dry-batteries. Of course, you’ve read the warnings printed on dry battery cases quite a few times. While the exact wording may differ from brand to brand, they all say much the same thing: “do not charge this cell”. It’s true that placing a dry cell into 14  Silicon Chip an ordinary nicad charger will create serious problems. These constant current chargers cause heat which produces steam and pressure, and this can easily burst the battery casing. This Dry-Cell Rejuvenator overcomes that problem by using a charging technique that results in very little heat production, thereby greatly reducing internal stress. It can recharge dry cells up to 10 times and save your hard-earned dollars in the process. In addition, this circuit helps our environment. We cur­rently throw away millions of dry cells each year –cells that eventually rust out and release their chemical cocktail of elec­trolytes. So reducing the number of cells we throw away provides definite environmental benefits. Both alkaline and conventional carbon-zinc batteries can be recharged by the Dry-Cell Rejuvenator. And because it employs two independent (and identical) charging circuits, it can charge either a single cell or two cells at the same time. The circuit runs off a standard 12VDC 300mA plug- +9V 4.7k D2 1N914 1M 100k 10k 4 IC1a 2 LM324 15k 100k 1 1k 11 100k 6 10k 7 IC1b A LED1 CHARGE 10k  K E 10k 5 10k Q3 BC547 B 2x1N914 D3 D4 C 1.5V CELL 100  Q4 BC547 C B E 10k E 10k .01 Q2 BC327 C 4.7k Q1  100 BC547 C 1W B 68k 3 E B 100k 2.2 25VW +9V 4.7k D5 1N914 1M 100k 15k 13 IC1c 100k 14 1k 10k 100k 9 10k 8 IC1d A LED2 CHARGE 10k  K E 10k 10 10k 10k .01 Q6 BC327 C 4.7k Q5 100  BC547 C 1W B 68k 12 E B Q7 BC547 B 2x1N914 D6 D7 C 100  Q8 BC547 C B 1.5V CELL E 10k E 100k 2.2 25VW D1 1N4004 B E C E B C VIEWED FROM BELOW I GO IN 12VDC 300mA PLUG-PACK 100 16VW 7809 GND OUT +9V 100 16VW DRY-CELL REJUVENATOR Fig.1: the circuit employs two identical sections to individually charge two 1.5V cells. IC1a is a Schmitt trigger – when the battery voltage is low, its output is high & Schmitt oscillator IC1b drives Q1 & Q3. These transistors in turn switch complementary pair Q2 & Q4 to provide the charge/discharge cycles. pack and will recharge an alkaline cell in about 18 hours. Conventional zinc-based cells are recharged in around 12 hours. Note however, that a recharged cell will not have “as-new” capacity. Provided the cell is in good condition, it will typically recharge to about 60% of the new capacity, at least for the first 7-8 cycles for an alkaline cell and 3-5 cycles for a zinc-carbon cell. After that, its perfor­mance will begin to deteriorate quite markedly. As a point of interest, a recharged alkaline cell will have greater capacity than an equivalent-size nicad cell, with the added benefit that it charges up to 1.6V. This figure is equival­ent to new cell voltage and is much higher than a nicad’s 1.2V rating. A feature of the unit is that it is very easy to use – you simply switch it on, slip the battery into its holder and, after about a second, the circuit will decide if that battery can be charged. If so, an indicator LED on the front panel will light up and the battery will be charged until its voltage rises above 1.65V. At this point, the circuit automatically switches into trickle mode and the indicator LED goes out to signal the end of the charging cycle. Faulty cells What happens if you attempt to charge a cell that has gone open circuit November 1994  15 PARTS LIST 1 PC board, code RAT002, 102 x 57mm 1 zippy box, 130 x 68 x 41mm 1 front panel artwork 1 12VDC 300mA plugpack 2 “AA” size cell holders 1 2.5mm DC panel mount socket 1 Mini-U heatsink Semiconductors 1 LM324 quad op amp (IC1) 6 BC547 NPN transistors (Q1,Q3,Q4,Q5,Q7,Q8) 2 BC327 PNP transistors (Q2,Q6) 1 7809 9VDC regulator (REG1) 1 1N4004 rectifier diode (D1) 6 1N914 signal diodes (D2-D7) 2 red 5mm LEDs (LED1,LED2) Capacitors 2 100µF 16VW electrolytics 2 2.2µF 25VW electrolytics 2 .01µF 63VW MKT polyesters Resistors (0.25W, 1%) 2 1MΩ 14 10kΩ 8 100kΩ 4 4.7kΩ 2 68kΩ 2 1kΩ 2 15kΩ 4 100Ω 1W Miscellaneous Light-duty hookup wire, machine screws & nuts, washers, solder. 16  Silicon Chip 5 1 VOLTAGE or high impedance? In the first case, the circuit will remain in trickle mode and the indicator LED will stay out. The same goes for a cell that’s already fully charged. So if the circuit refuses to start when you install a cell, check its output voltage. If the voltage is close to 0V, that cell has passed the point on no return and should be discarded. On the other hand, the circuit will attempt to charge cells that have discharged to a low voltage (ie, below 1V) and, as a result, have a high internal impedance. Cells in this condition will charge to 1.6V very quickly however, typically in less than five minutes, after which the circuit switches to trickle mode. The cell then quickly loses its charge so that, after a few minutes more, the circuit reverts to the full charging mode again. Any cell which causes the circuit to exhibit this behaviour should also be discarded, since it is obviously Fig.2: the charge/discharge waveform used by Hollows in 1955. The charge/ discharge ratio was about 5:1. incapable of holding any worthwhile charge. General guidelines In order to get the most out of the Dry-Cell Rejuvenator, there are several important guidelines that must be followed. Let’s take a look at these. First, never let the cell voltage fall below 1.0V. This is basically a cell’s “point of no return”. If its output voltage falls below this figure, it will generally not hold a sufficient charge to make recharging worthwhile. Second, recharge the cells as soon as possible when they go “flat”. The longer they are left lying around, the harder it is for the Rejuvenator to recharge them. Similarly, use them again as soon as possible after recharging, otherwise they will begin to deteriorate. This fast recycling technique will allow you to get the most out of your batteries. Third, don’t leave a battery on charge for more than two days (48 hours). If a battery hasn’t charged up in this time, it can be considered a lost cause and should be discarded. If you persist for longer than this, heat will slowly build up and some lesser-quality batteries may begin to leak. Finally, if a cell does begin to leak as a result of charg­ing or was already leaky, it should be discarded at once. The fluid discharge from a leaky cell is highly corrosive and can damage valuable equipment. Note that the Dry-Cell Rejuvenator works best on alkaline and heavy-duty (or super heavy-duty) zinc-carbon cells, so you are definitely better off spending a little extra for these types. Warning: under no circumstances should you try to recharge lithium batteries. Charging principle The charging principle relies on the chemistry inside the cell. If a carbon-zinc cell is charged with plain DC, the zinc is returned to the negative electrode in spongy blobs. Although this results in a cell with reasonable output voltage, it will also have a high internal impedance. Hence, it will be unable to deliver the expected power to the load. Much of the initial study into dry cell recharging was done nearly 40 years ago by R. Hollows and the results published in a 1955 edition of “Wireless World”. Hollows found that if the cell was charged using “dirty DC”, the zinc was distributed more evenly and compacted on the casing. The result was a cell which resembled its original charged state. A similar process occurs in alkaline cells. In this case, the term “dirty DC”, refers to a half-wave rectified DC voltage with a small negative offset. Fig.2 shows the details. When applied to a battery, this resulted in a 5:1 charge/ discharge ratio; ie, the battery was charged during the positive half cycle of the waveform and discharged during the much shallower negative half cycle. In effect, the principle could be called “five steps for­wards and one step back”. Hollow’s work was based on a circuit which used a 3VAC transformer, an item not commonly found these days. In addition, Hollow’s circuit would not have been the most efficient way of recharging a dry cell, due to the low frequency of the charging waveform (50Hz). This circuit overcomes those problems by using a square-wave oscillator to generate the charging waveform and by operating at a much higher frequency (4.5kHz). Circuit details Fig.1 shows the circuit details for the Dry Cell Rejuvena­tor. As already mentioned, it consists mainly of two identical charging circuits, one for each cell. These two circuit sections are powered from the plugpack via reverse polarity protection diode D1 and a 3-terminal regulator which delivers a 9V rail. IC1a is one-quarter of an LM324 quad op amp and is connect­ed as a Schmitt trigger. The 68kΩ, 15kΩ and 1MΩ resistors set the reference voltage on pin 3 to approximately 1.6V, while the in­verting input (pin 2) monitors the battery voltage via a 100kΩ resistor and a 2.2µF filter capacitor. If the cell voltage is less than the LED1 CELL 1 LED2 CELL 2 Fig.3: install the parts on the PC board & complete the wiring as shown here. Be sure to use the correct transistor at each location & note that a small finned heatsink is bolted to the metal tab of the 7809 3-terminal regulator. Q5 reference voltage, pin 1 of IC1a switches high and lights LED 1 to show that charging has begun. At the same time, pin 5 of op amp IC5b is biased to about half supply via a voltage divider consisting of two 100kΩ resistors. This op amp is connected as a Schmitt trigger oscilla­tor. When pin 1 of IC1a switches high, IC1b oscillates at a fre­quency of about 4.5kHz and with a 50% duty cycle. The square-wave output from IC1b appears at pin 7 and drives transistor inverter stages Q1 and Q3. These transistors, in turn, switch the main output devices (Q2 and Q4) on and off. In effect, Q2 and Q4 function as a complementary output pair. When pin 7 of IC1b goes high, Q1 and Q3 turn on, Q4 turns off and Q2 turns 100  100  10k .01 .01 D3 Q3 4.7k 4.7k 10k Q4 10k 100k 68k D2 100k 100k 10k 10k Q2 15k 1M 10k 100uF 1k 2.2uF 100k D7 10k 100k 100k 100k 100  Q7 10k 1 1M 68k 7809 100k 10k 10k D6 4.7k 1k 15k D5 Q8 12VDC PLUG-PACK 100uF 2.2uF IC1 LM324 4.7k Q6 100  D1 D4 Q1 10k 10k 10k 10k on and supplies charging current to the cell. Subsequently, when pin 7 of IC1b goes low, Q1 and Q3 turn off and so Q2 also turns off to end the charging pulse. At the same time Q4 turns on, since diodes D3 and D4 are now forward biased via a 10kΩ pullup resistor (more on these in a moment). The cell now discharges through Q4 and its associated 100Ω collector resistor. Because oscillator IC1b has a 50% duty cycle, Q2 and Q4 also operate with a 50% duty cycle. This means that the cell is charged for half the time and is discharged for the other half of the time. However, when Q2 turns on, its 100Ω collector resistor has about 7.5V across it, whereas when Q4 turns on its 100Ω collector resistor only has about 1.5V (ie, the cell voltage) across it. As a result, about 75mA flows through Q2 to charge the cell, while only about 15mA flows through Q4 to discharge it. This means that the charge-discharge ratio works out to be about 5:1, although this will vary somewhat according to the cell voltage. Trickle mode As the battery charges, its voltage is monitored by pin 2 of IC1a. When it exceeds 1.6V (the reference set on pin 3), pin 1 of IC1a switches low to about 0.7V and this set the bias applied to pin 5 of IC1b to about 0.35V. As a result, IC1b changes its output to a low-duty (1:10) square-wave with a frequency to about 2.2kHz. This change in frequency (from 4.5kHz to 2.2kHz) is due to the different bias, while the lower duty cycle is partly due to the Schmitt trigger action and partly due to asymmetry in the output of IC1b. In operation, IC1b’s output (pin 7) swings closer to RESISTOR COLOUR CODES ❏ No. ❏   2 ❏   8 ❏   2 ❏   2 ❏ 14 ❏   4 ❏   2 ❏   4 Value 1MΩ 100kΩ 68kΩ 15kΩ 10kΩ 4.7kΩ 1kΩ 100Ω (5%) 4-Band Code (1%) brown black green brown brown black yellow brown blue grey orange brown brown green orange brown brown black orange brown yellow violet red brown brown black red brown brown black brown gold 5-Band Code (1%) brown black black yellow brown brown black black orange brown blue grey black red brown brown green black red brown brown black black red brown yellow violet black brown brown brown black black brown brown not applicable November 1994  17 Take care to ensure that the DC socket is wired to suit the plugpack, so that the correct supply polarity is applied to the board. The external wiring connections to the board can be made via PC stakes. ground than to the positive supply rail and this situation is exaggerated when the input threshold is pulled low. What happens now is that IC1b delivers a train of narrow positive-going pulses and these briefly pulse Q1 and Q2 on to trickle charge the battery. Q4 remains off in this mode, however. That’s because D2 now clamps Q3’s collector to a maximum of 1.4V (remember that pin 1 of IC1a in now at 0.7V) and this, coupled with the voltage across D3 and D4, means that there will be insufficient bias to turn Q4 on. This means that the battery is not discharged for part of the time when the circuit in trickle mode. If the battery voltage now subsequently falls below the reference voltage, pin 1 of IC1a switches high again and the circuit reverts to its full 18  Silicon Chip charge/discharge mode of operation. In this mode, D2 is reverse biased and so Q3 is now able to turn Q4 on and off to provide the discharge cycle, as described pre­viously. Note that because IC1a is connected as a Schmitt trigger with about 100mV of hysteresis, the circuit is effectively pre­vented from oscillating when the cell voltage reaches the refer­ence voltage on pin 3. Instead, the cell voltage must fall from 1.6V to 1.5V before the circuit will revert to full charging mode and must then reach 1.6V again before reverting to trickle mode. If the cell is removed, the circuit behaves as if a fully charged cell is in position; ie, it switches to trickle mode and the LED goes out. That’s because the 2.2µF capacitor on pin 2 of IC1a is charged almost to +9V (via the 100kΩ feedback resistor) by the current pulses generated each time Q2 turns on. The second charging circuit, based on op amps IC1c and IC1d and transistors Q5-Q8, functions in exactly the same manner. Construction Most of the parts are in­stalled on a PC board measuring 102 x 57mm and coded RAT002. Begin construction by fitting PC stakes to the external wiring points, then install the various parts as shown on Fig.3. The resistor colour codes are shown in the accompanying table but we also recommend that you check each value using a DMM, as some colours can be difficult to decipher. Take care to ensure that all semiconductors are correctly oriented and don’t get the transistors mixed up – Q2 and Q6 are BC327 PNP types, while the rest are BC547 NPN types. The 7809 regulator must be installed with its metal tab adjacent to the edge of the board – see photo. It is fitted with a TO-220 Mini-U heatsink to aid heat dissipation. Once the board has been completed, it can be installed in a plastic zippy case measuring 130 x 68 x 41mm. Use the board as a template to mark out its mounting holes, then drill the holes to 3mm along with a hole in one end of the case to accept the power socket. This done, attach the front panel label to the lid and drill out the mounting holes for the battery holder and the two LEDs. The latter should be made just large enough so that the LEDs are a push fit. Finally, mount the various items in position and complete the wiring as shown in Fig.3. The PC board is se­cured using machine screws and nuts, with additional nuts used as spacers. Take care to ensure that the LEDs are wired with the cor­rect polarity. The anode lead of each LED is the longer of the two. Testing Before switching on, temporarily disconnect one of the leads to the DC power socket and connect a multimeter set to milliamps across the break. This done, apply power and check Where to buy a kit of parts The Dry-Cell Rejuvenator is only available from RAT Elec­tronics. Complete kits, including all specified components, instructions, case, front panel and 12V DC plugpack, are available for $44.95 ($39.95 without plugpack). Please add $5.05 for post­age and packaging for delivery within two weeks. To place your order, phone or fax RAT Electronics on (047) 77 4745 or send your cheque/money order to: RAT Electronics, PO Box 641, Penrith, NSW 2750. Note: copyright (c) 1994 RAT Electronics. Copyright of the cir­cuit and PC board art associated with this project is owned by RAT Electronics. that the current drawn by the circuit is about 10mA with no cells in place. Note that both LEDs should flash briefly when power is applied. If you now install a single “flat” cell, the circuit should switch to charge mode – the appropriate LED should light to indicate that charging it taking place and the current drain should rise to about 50mA. This should increase to about 90mA if a second “flat” cell is installed. Check also that the second LED is now lit. If you don’t get the correct current readings, switch off immediately and check the board carefully for incorrect parts placement or orientation. Check also that the 7809 3-terminal regulator is delivering +9V and that this voltage appears at pin 4 of IC1 and on the emitters of Q2 and Q6. Finally, remember that a dry cell should not be discharged below 1V if it is to be successfully recharged and don’t leave any cell on full charge for more than 48 hours – if it hasn’t charged up in this time, it can be considered defunct. That’s it – you are now ready to start recharging those expensive dry batteries and do your bit for the enviSC ronment as well. AC/DC digital clamp meter with 4000 count display and bargraph! ● High speed auto-or manual ranging ● High speed sampling for 40 segment bargraph display ● Average, Temperature test, Max hold, Peak hold functions ● Sleep mode to reduce battery con- sumption ● Continuity beeper, Data hold, Diode test and analog signal output ● Battery or AC adaptor operation Brief Specifications Functions : AC/DC current, AC/DC voltage, Ohms, Continuity, Diode test, Frequency, Temp, Data/ Peak/Max hold, Average., Analog signal output Display : LCD 3.5 digits, 4000 (Hz: 9999) count Bar Graph Display : 40 segments Ranges : Auto or manual ranging Aac, Adc : 400, 1000A Vac, Vdc : 40, 400, 650V Frequency : 10.0-999.9Hz Temperature : -50.0 to +150°C Jaw Opening : 55 mm ø or 65 x 18mm busbar Withstand Voltage: 2.5kVac, 1 minute Lloyd’s Register Quality Assurance to ISO-9001 2343 – one of the NEW Generation of Multimeters from Centrecourt D3, 25-27 Paul Street North, North Ryde Call Robyn for more information on (02) 805 0699 or fax : (02) 888 1844 November 1994  19