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This “intelligent” Nicad battery charger was designed for high-current, rapid-charge applications, such as cordless power tools and model racing cars. It’s just the shot for recharging bat­tery packs ranging from 7.2V to 14.4V. By PETER HAYLES Intelligent Nicad Battery Charger for Power Tools A S A KEEN HANDYMAN, I have a number of power tools, including a few cordless types that run off nicad battery packs. These bat­tery packs range from 7.2V to 14.4V and almost inevitably contain Sanyo or Panasonic nicad cells, regardless of the brand of the tool itself. Properly treated, these battery packs should be good for hundreds of charges and can potentially last many years. Unfor­ tunately, proper nicad chargers are usually expensive and the cheap chargers supplied with the original equipment often incor­rectly charges the cells and dramatically shortens their life. 66  Silicon Chip Recently, I found that my 2-year-old 9.6V cordless drill battery wouldn’t perform to its rated capacity after charging. Unfortunately, battery packs are fairly expensive to replace, sometimes costing almost as much as the entire drill kit – and that’s if you can purchase the battery pack separately at all. Often, you will simply be told to just “buy a new drill”. In fact, it is far cheaper to purchase your own cells and manufacture a “new” battery pack using the old case. This invol­ves soldering leads between the battery tags to connect them in series. Note, however, that you should never solder directly to the cell cases – that can damage them and is quite dangerous. In selecting replacement cells, I researched the manufactur­er’s specifications on charging and guess what? – the battery charger that came with the drill didn’t comply with these speci­fications. Instead, the supplied charger is a very simple device that applies a constant current to the battery pack and doesn’t cut out once the pack is fully charged. As a result, once the cells are fully charged, the battery starts to heat and the internal pressure builds up. This can lead to permanent cell damage and in serious cases, the battery can rupture or vent electrolyte. Having paid good money for a new battery pack, I decided to design a new charger that would not damage the battery. In par­ticular, I wanted a charger that not only met the specifications but would also sense the condition of cells and charge according­ly. In short, I wanted to be able to “throw” the pack on the charger and know that it would be “good” the next time I reached for it. And that meant it had to be fully automatic, with no switches to set. Meeting these requirements also meant that the charger required some inbuilt “intelligence”, so logic control circuitry was required. At the same time, I wanted to keep the design as simple as possible and keep the component count down – after all, reducing the size of a PC board and the number of holes in it leads to major cost savings. In the end, I decided on a very simple 1-chip design based on a PIC microcontroller (PIC is a registered trademark of Micro­Chip and refers to a range of microcontrollers). That way, it’s the software that’s programmed into the PIC that does all the hard work. If you don’t have a PIC programmer, don’t panic! – programmed PICs to suit this design are available inexpensively from the author. Other than this, only a few commonly available components are required to complete this project. The “all-up” cost should be about $60 which is a lot cheaper than your next battery pack! Nicad characteristics Even if you don’t want to build this charger, you can still learn how to get the most from your nicad batteries. To start with, a “cell” is defined as a single vessel containing elec­trodes and electrolyte for generating current. A battery consists of two or more cells. Nicad cells are rated at 1.2V for design purposes, although they normally develop about 1.25V and require a charging voltage of 1.5V (during full charge). Nicad cells can supply very large amounts of current and display a remarkably flat discharge characteristic, maintaining a consistent 1.2V throughout discharge. The voltage then drops quite suddenly and a cell is almost completely flat at 0.8V. This is called the “knee” characteristic because of the shape of the graph of voltage against time. Nicad battery capacity is rated in mAh (milliampere-hours) and is commonly referred to as “C” – ie, it can supply 1C mA for 1 hour, 2C mA for 30 minutes, etc. Three different charging techniques are commonly employed: trickle charging whereby the battery is “topped up” at 3.3% of C to 5% of C; slow charging at 10-20% of C; and fast charging at 50-100% of C. Slow charges are not meant to be continually applied and since nicad Below: the unit is easy to build since virtually all the parts are on the PC board. Keep the wiring neat and tidy by using cable ties and note that the large metal diecast case is necessary for heatsinking. April 2001  67 Fig.1: this flow chart shows the basic operation of the software that’s programmed into the PIC microcontroller. batteries are about 66% efficient, this type of charging normally takes about 8-15 hours. On the other hand, fast charges at 100% of C should be terminated after about 1.5 hours, assuming that the battery is flat to begin with. Once a battery is fully charged, it produces gas and this creates a high internal pressure and a sudden rise in tempera­ ture. At this point, the battery should be switched to trickle charging, otherwise it will begin to vent and release its elec­trolyte. And that permanently damages the cells. As a matter of interest, my old battery was rated at C = 1300mAh and my old charger was rated 400mA (30% of C). This means that the charger should have been switched off after about four hours, provided that the battery was almost flat to begin with. However, there is no way of knowing if C was actually 1300mAh or if it had decreased a bit. Once a battery starts to deteriorate, it becomes a vicious cycle and the battery then deteriorates rapidly due to more and more overcharging. According to the manufacturer, the cells supplied with my drill should have been good for 500-1000 cycles if properly treated! The memory effect Possibly the biggest misconception that surrounds Nicad cells is a result of the so-called “memory effect”. Almost every one quotes it as the reason that cells have to be completely flattened (ie, to 0V) before charging – otherwise they develop some sort of memory and can only hold a partial charge from there on. The “memory effect” was discovered during the early days of satellites. They used solar cells to charge nicad batteries and these batteries were subjected to precise charge/discharge cycles many hundreds of times, as the satellite alternated between darkness and sunlight during its orbit. However, memory effect isn’t a problem if the charge/discharge cycles are varied – it certainly isn’t a significant problem in normal home usage. Although it may be OK (but not really a good idea) to dis­charge individual cells to 0V, this is certainly not recommended for an entire battery of cells. The reason is simple – when a battery is discharged below 0.8V per cell, one of the cells is inevitably weaker than the others and goes to 0V first. If the battery is further flattened, this 68  Silicon Chip Fig.2: the PIC microcontroller (IC1) is at the heart of the circuit. It continually samples the battery voltage and outputs a PWM waveform which controls constant current source REG2 via transistor Q1. battery becomes reverse charged (ie, it reverses polarity) and this weakens it even further. This creates an effect called “voltage depression” and it’s quite common in battery packs that are treated this way. Eventually, the battery’s performance drops off quite sud­ d enly which ironically is the very thing that the user is trying to prevent. Preventing this problem is quite straightforward – don’t discharge the battery to 0V. Most users know where the battery’s “knee” occurs; it is when the tool first starts to show signs that the battery perfor­mance (and hence battery voltage) is suddenly dropping. It is a good idea to immediately recharge the battery from this point. Usually, there will be less than 5% of C remaining anyway. One other thing – Nicad batteries don’t like getting too hot or too cold. They will not take a full charge and they actually discharge (even under no load) much faster when over 40°C or below 0°C. For this reason, you should avoid leaving cordless tools inside a hot car. In addition, a nicad battery pack builds up internal heat when working, so don’t over-work the tool. Nicad batteries should also be left to cool down for a while after discharge before recharging them. Note also that Nicad batteries do self-discharge and the rate is also temperature related. As a rule of thumb, they will hold a full charge (with no load) for about a month or two but when they get old or hot, they might only last a day. So what can you learn from this? The rules are: (1). Don’t flatten a nicad battery below 0.8V per cell. (2). Don’t overcharge your battery beyond 100% of C. (3). Nicads don’t like to get too hot or too cold (0-40°C is usually best). Nicad charging The nicad batteries used in cordless tools and model racing cars generally have a value of “C” ranging from 10003000mAh. The first step is to determine what “C” is for your cells. You can do that either by directly inspecting the cells (assuming that the battery pack can be easily disassembled) or by contacting the manufacturer for the part number. The value for “C” is often included in the part number and its specifications can be checked out on the manufacturer’s website. For my new battery, the value for “C” was 1700mAh. Note that the “C” of the individual cells is the same as the “C” of the complete battery. When designing a charger, you should first consider how the cells are to be used. For power tool and model car applications, the charge use is termed “cycle use” because the battery is repeatedly charged and discharged. In addition, the charge time required is usually as fast as possible – ie, between 1 and 2 hours. My batteries were capable of taking a fast charge of 100% of C, which equates to 1.7A. Despite this, I conservatively selected 1.25A as my charge current because I wanted to be able to charge 1300mAh (1.3Ah) batteries as well. This value should be OK for most readers and it doesn’t really matter if it is a bit less than 100% of C, because the charger will eventually detect a peak anyway. However, some readers will want to adjust the maximum charging current and this procedure is described later on. For “cycle use”, there are two recommended methods of de­tecting charge termination – either using a temperature sensor in the battery pack or using a “negative delta V” cutoff system. The temperature technique relies on detecting the sudden rise in battery temperature when the battery is fully charged and using this to shut down the charger. There is nothing wrong with doing this but battery packs do not always come with temperature sen­sors built in. Furthermore those that do, often sense the temper­ ature of one cell only. The “negative delta V” system reApril 2001  69 Fig.3: follow this wiring diagram to build the Intelligent Nicad Charger. Make sure that all semiconductors are correctly orientated and note that the 1Ω 5W resistor should be mounted slightly proud of the PC board, to aid cooling. lies on the fact that the battery voltage peaks and then drops about 15-20mV per cell when fully charged. This charger will detect a minimum peak of about 84mV and so can be used to charge battery packs ranging from 7.2V to 14.4V (ie, 6-12 cells). Note that the upper limit is determined by the maximum output voltage of the charger. No matter how discharged the battery is, this technique will give enough charge to restore the battery to its full state. The battery is then continually “topped up” with a trickle charge to prevent slow leakage due to its internal resistance. Another thing to consider is the requirement to let the battery cool down before recharging. If a battery is hot, its output voltage will rise slightly as it cools. This battery charger is programmed to wait until the battery voltage is stable for about 30 seconds before starting to charge. If the battery has just come off discharge and is hot, it may take a minute or so for the charge to begin to start. In addition, new batteries may show false peaks during the first four minutes of charging. For this reason, the charger starts with a slow “soft start” charge for four minutes, to allow the battery to cool and get past this point. In order to make the unit fully 70  Silicon Chip automatic, it also automati­cally detects when a battery is connected for charging. There’s just one proviso here – the battery voltage must be above 2V (open circuit) for the charger to recognise it. If a battery pack is discharged to 0V, it won’t be recognised and the charger won’t start. In practice, this isn’t a problem since a cordless tool or model car stops working altogether when the pack gets down to about half voltage (ie, 3.6V for a 7.2V pack, or 7.2V for a 14.4V pack). Of course, no-one uses a tool until it stops working altogether – instead, the battery is placed on charge as soon as there is a marked deterioration in performance. The charging algorithm used by the PIC microcontroller is shown in Fig.1. Note that the first LED is on continually during the “bulk charge” process, while the second LED indicates the type of charge being applied. The operation of the charger is fairly straightforward. Normally, when the charger is switched on, both LEDs flash once. The charger then waits in standby mode until a battery is connected. Once a battery is connected, the charger progresses though several modes: ie, cool, soft, fast and trick­le. At the end of the charging process, the battery can be left on trickle charge indefinitely, or removed from the charger at this point. When the battery is removed, the charger reverts to standby. Basic operation Fig.4: this diagram shows the mounting details for the LM317K regulator (REG2). Make sure that it is electrically isolated from the case. Fig.2 shows the full circuit details of the Nicad Battery Charger. It uses a PIC microcontroller (IC1) to generate a pulse width modu­lated (PWM) waveform and this signal switches a constant current supply based on REG2 which is used to charge the battery. In operation, the PIC microcontroller senses the battery voltage and converts this to a digital value using an internal A/D (analog-to-digital) converter. It then adjusts its PWM output signal to control the charging rate accordingly. It also drives the two LEDs, to indicate the charging status. The smallest and cheapest microcontroller that could be used to perform the A/D conversion and still have the necessary func­ tions and control lines is the PIC16C711. This device is an 8-bit, high-performance 4MHz CPU and it includes four A/D converter stages, a brown-out timer and a watchdog timer. The timers are used to reset the chip if problems occur due to power transients or interruptions. The PIC16C711 comes in an 18pin dual-in-line package and has a “massive” 1K words of program memory and 68 bytes of data. It’s hardly enough to load Windows 2000 but it’s quite enough for a relatively simple control program. Circuit details OK, let’s look at how the circuit works in greater detail. As shown, the circuit runs off an AC plugpack and its output is fed to a bridge recti­fier (BR1) and a 4700µF filter capacitor. This capacitor reduces the DC ripple to about 1V under full load (1.5A). REG1, a 7805 3-terminal regulator, produces the +5V rail for the PIC microcontroller. A 0.1µF capacitor is used to decouple this rail. Crystal X1 and its associated 27pF capacitors provide a stable and accurate 4MHz timebase for IC1. This is necessary to ensure accurate time delay functions for charging. The two LEDs (LED1 & LED2) are driven directly from pins 8 & 9 of IC1 via 100Ω current limiting resistors. Pin 18 (RA1/AN1) is used to “sense” the battery voltage. This input samples the battery voltage via a voltage divider consisting of 3.3kΩ and 1kΩ resistors. These resistors are neces­ sary to “divide” the battery charging voltage of about 0-21.5V down to 0-5V, which is the range of the PIC’s A/D converter. Note that the PIC uses an 8-bit A/D converter, so we have 256 (28) possible values. This gives us a resolution of 21.5/256 = 84mV which means that a 6-cell (7.2V) pack is the smallest pack that the charger will peak detect. The PWM waveform from IC1 appears at pin 6 (RB0) and drives switching transistor Q1 via a 3.3kΩ resistor. Q1 in turn drives the ADJ terminal of REG2, an LM317K adjustable 3-terminal regula­tor. In operation, the LM317 maintains a constant 1.25V between its OUT pin and the ADJ pin. In this circuit, a 1Ω 5W resistor is connected between these two terminals and this ensures that a constant 1.25A is applied to the battery pack. If necessary, you can adjust this value to suit your appli­ cation. All you have to do is choose the charging current that you want and use Ohm’s Law (V = IR) to calculate the resistor value; ie, divide 1.25V by the current that is recommended for full charge. For example, a 0.68Ω resistor will provide a charg­ing current of about 1.7A, while 1.2Ω will provide 1A. The circuit works like this: when Q1 is biased on, it effec­tively pulls the ADJ pin of REG2 to ground and so the output of REG2 will only be at about 1.25V. However, very little current will flow in the output since D1 is reverse biased and there is a 1kΩ resistor in series between the 1Ω 5W resistor and the ADJ terminal. In fact Q1 is biased on by default, so that the unit is “fail-safe”. Conversely, when Q1 turns off due to the PWM waveform from IC1, REG2 behaves as a constant current source and it charges the battery pack via D1. Diode D1 ensures that the battery cannot discharge back into REG2 if the power is accidentally turned off! If the power is interrupted with a fully charged pack, D1 isolates the output circuit and the nicad battery will slowly discharge through the 3.3kΩ and 1kΩ voltage divider resistors. When power is subse­quently restored, the charger will detect the voltage peak again and return to trickle charge after just a few minutes. Built-in self-test A final feature of the software is that there is a “Built-In-Test” (BIT) on power up. This effectively tests all the components except the capacitors (ie, more than 80% of the com­ponents). During power up, if no battery is detected (ie, less than 2V on the output), the output is turned on for one second and the voltage checked. The output is then turned off. If the voltage does not reach at least 10V when high and go below 2V when low, then an error is detected. The LEDs are both Parts List 1 aluminium diecast case, 171 x 121 x 55 1 PC board, 77.5 x 85mm 1 front panel label 1 4MHz parallel cut crystal (X1) 4 2-pin PC-mount terminal blocks (4A, 0.2-inch pitch) 1 18-pin DIL IC socket 1 TO-3 insulating pad 2 TO3 insulating bushes 3 M3 x 12mm machine screws, nuts & washers 4 M4 x 12mm machine screws, nuts and washers 2 5mm LED bezels 1 5.5mm ID rubber grommet 1 2.5mm DC panel socket 1 2.5mm DC plug 1 4mm crimp lug 4 plastic cable ties 2 plastic cable tie mounts 3 300mm lengths heavy-duty multistrand cable (red) 1 180mm length heavy-duty multistrand cable (black) 1 200mm length heavy-duty multistrand cable (white) 1 600mm length heavy-duty figure-8 cable Semiconductors 1 PIC16C711-04/P programmed microcontroller (IC1) 1 BC548 transistor (Q1) 1 7805 3-terminal regulator (REG1) 1 LM317K adjustable regulator (REG2) 1 4A or 6A 400V single in-line bridge rectifier (BR1) (DSE Cat Z3310; Jaycar Cat ZR1360; Altronics Z0076) 1 1N5404 power diode (D1) 2 5mm red LEDs (LED1, LED2) Capacitors 1 4700µF 35VW electrolytic (36mm high) 1 0.1µF monolithic 2 27pF ceramic Resistors (0.25W, 1%) 2 3.3kΩ 3 1kΩ 2 100Ω 1 1Ω 5W wirewound Miscellaneous Thermal grease (see text), heatshrink sleeving, solder. April 2001  71 sinking for this device. It’s a good idea to mount the 5W resistor about 3mm proud of the board, as it gets quite warm during operation. This will allow the air to circulate beneath it for cooling. Unlike the other parts, the two LEDs are mounted from the copper side of the PC board. The top of each LED should be about 13mm above the board, so that they pass through matching holes drilled in the base of the case when the board is mounted in position. Note: the base of the case becomes the front panel. Mounting REG2 The connecting cable for the battery pack emerges from a grommetted hole in one end of the case. The adjacent socket is for the external AC plugpack supply. powered on simulta­neously during this BIT. If there is an error the LEDs then flash alternately. This mode can be verified by shorting the output on power up or plugging in a battery during the BIT. The error mode will also be invoked and the LEDs will flash if no peak is detected after three hours of main charge. The unit will then time out and switch off automatically. Construction All the parts except for REG2 are mounted on a PC board coded 14104011 and measuring 77.5 x 85mm. This board is mounted in a substantial metal diecast case, which is necessary to ensure adequate heatsinking for REG2. Fig.3 shows how the parts layout on the PC board. The board is easy to assemble but take care with the orientation of Q1, IC1, D1 and the 4700µF electrolytic capacitor. Pin 1 of IC1 is adjacent to a small dot in the body at one end of the device. Regulator REG1 is mounted flat against the PC board, with its leads bent at right angles to pass through the holes. It is secured to the board using an M3 screw and nut and the copper pad on the underside of the board provides all the necessary heat­ Fig.5: you can make your own PC board from this full-size etching pattern or buy a ready made board from RCS Radio. 72  Silicon Chip The LM317 (REG2) is mounted on the side of the aluminium diecast case using a standard TO-3 insulating kit to ensure electrical isolation. Fig.4 shows the mounting details. Use the insulating pad as a template to mark out the hole positions, then drill the holes and use an oversize drill to remove any metal swarf. Carefully inspect the mounting area to ensure that it is completely smooth and free of any swarf before mounting the device, as a sharp edge could “punch-through” the insulating pad and short the device to the case. The insulating pad can be either a mica washer or a silicone impregnated washer. If you use a mica washer be sure to smear all mating surfaces with thermal grease to aid heat transfer, before bolting the assembly down. Once the regulator is in position, use your multimeter to confirm that its metal body is indeed isolated from the diecast case. Note that the LM317 will dissipate about 12W when charging smaller batteries so don’t use a smaller case than the one speci­ fied, otherwise the heatsinking will be inadequate. If even higher power dissipation is required (eg, if you are fast-charging at more than 1.25A), then REG2 should be fitted to a substantial heatsink. Once the board assembly has been completed, it can be mount­ed inside the case. To do this, you will need to mark and drill out four 4mm holes for the mounting screws, plus two holes for the indicator LEDs. Another two holes are required in one end of the case to accept a small rubber grommet (8mm) and the power socket. The PC board is mounted on 10mm standoffs and secured using four M4 x 12mm countersunk screws, nuts and washers. Note that the screws must have countersunk heads, because they have to go under the label. The two LEDs should be pushed into matching holes in the case as the board is mounted, with their tops just flush with the case surface. The internal wiring is shown in Fig.3 and the photo. The external lead to the battery pack is run out via the 5.5mm ID grommet and is fitted with a 2.5mm DC power plug. The AC power leads are connected to an adjacent 2.5mm panel socket (note: choose a size that suits your AC plugpack supply). To ensure reliability, it’s a good idea to secure the wiring using four cable ties. Two of these cable ties pass through cable tie mounts, as shown in the photo. Finally, the front panel label can be affixed in position. The two LED can be dressed up by fitting plastic bezels if you wish. Testing & operation This unit requires a 24VAC input to charge 14.4V batteries, although only 16VAC is required to charge anything smaller. The AC power source must be rated at the chosen supply current or better – typically 1.5-2A. This can come from an external AC plugpack supply. The bridge rectifier and 4700µF filter capacitor should pro­duce about 1.4 times the AC RMS input. So if using a 16VAC sup­ply, the main rail should be about 22VDC. If using 24VAC, this rail should be about 30VDC. You should also check that the 5V rail is present at the output of REG1 and that there is at least 2.5V across the LM317, the 1Ω current sensing resistor and diode D1. For the connection to the battery, I used my existing charg­er pack after first removing the internal circuitry – which was no more than a transistor and LED to indicate that current was being delivered. For power connections, EIAJ DC voltage connec­tors and plugs are standard, with the positive usually being the centre pin. The front panel artwork includes a legend that explains all the possible states for the LED indicators. If both LEDs are flashing, it indicates that there has been an “error”. This simply means that the unit has failed to detect a peak voltage as the battery pack charged and has timed out (ie, Fig.6: the front panel artwork shows the mounting points for the PC board and indicator LEDs and also indicates the LED flash codes. after three hours) but this should rarely happen. Conclusion This unit has halved the charging time for my drill battery pack, from 3-4 hours to 1.5 hours maximum. It’s nice to know that I can now “throw” the battery pack on the charger and that it will be fully charged and the next time I want to use it – and that’s SC the way it should be. Where TO BUY PARTS A programmed PIC microcontroller for this project is available from the author for $A20 plus $A5 for post and packaging (in Australia). Payment may be made by bank cheque or money order. Contact: Peter Hayles at peterhayles<at>hotmail.com Note: copyright of the PIC software and PC board associated with this design is retained by the author. Individuals can make their own PC boards on a one-off basis or purchase a board from RCS Radio – phone (02) 9738 0330. April 2001  73