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E- LED R P MB SEPCB S A Ideal for . . . • Cordless Power Tools • Model Racing Cars • Battery Appliances • Electric Planes • Just about anything with Nicad or NIMH cells INTELLIGENT CHARGER for Nicad and NiMH Batteries Cheap chargers supplied with the original equipment can – and often do – damage the battery. Proper chargers are usually expensive. This cheap and easy-to-build Nicad/NiMH Battery Charger is suitable for automatically charging a wide range of batteries. T his ‘intelligent’ charger, controlled by a microprocessor, was designed for high-current and rapid-charge charging applications such as required for cordless power tools and model racing cars. These battery packs are expensive and can be difficult to purchase (it’s usually cheaper to buy a new tool!). This charger uses the cell manufacturer’s recommended charge method to safely and quickly charge batteries. dreds of charges and could potentially last many years. The important part of that last statement is “properly treated.” Batteries can be ruined by just one incorrect charge. Unfortunately the battery packs are fairly expensive to replace, sometimes costing almost as much as the entire drill kit, if in fact you can purchase the batteries separately at all. Note that if you can solder, you can rebuild a pack with tagged cells – don’t solder directly to batteries though! Introduction (See SILICON CHIP, December 2006). If you’re really keen, Batteries for power tools and many other electrical prod- you can even upgrade the battery to the latest Lithium cells ucts range from 2.4V to 24V and usually consist of Sub-C (October 2013) but a special charger would be needed. Recently I found my 2-year-old 9.6V cordsize Nicad or NiMH cells. Properly treated, these battery packs should be good for hunby PETER HAYLES less drill battery wouldn’t perform to its rated 60  Silicon Chip siliconchip.com.au capacity after charging. I decided to repack the battery. In selecting replacement cells, I researched the manufacturer’s specifications on charging and discovered that the battery charger that came with the drill didn’t comply with these specifications. The supplied battery charger is a very simple device that applies constant current to the battery pack, with no cut-off, only a warning not to leave “on charge” for more than 14 hours. As we will see, even this is a recipe for disaster! There is no charge termination method used by the charger. During the recharging process, once a battery reach its full charge, the cells start to heat up and the internal pressure builds up, causing the battery to eventually 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. A better battery charger would require the charger to sense the condition of cells and charge accordingly. I soon realised that the simplest design would be a one-chip design. I selected a PIC controller as it was the smallest and cheapest device available at the time with a suitable analog-to-digital converter. Nicad/NiMH cell characteristics Even if you don’t want to build this charger, you still stand to gain something from this article by understanding how to get the most from your rechargeable batteries. A cell is defined as a single vessel containing electrodes and electrolyte for generating current. A battery consists of two or more cells, usually connected in series to obtain a higher voltage. Nicad/NiMH cells are nominally rated at 1.2V for design purposes although they normally develop about 1.25V. Under full charge they require about 1.5V to 1.6V. They can supply very high current and display a remarkably flat discharge characteristic, ie, they maintain a relatively consistent 1.2V throughout discharge. The voltage then drops quite suddenly, and they are almost completely discharged at 0.8V. This is called the “knee” characteristic because of the shape of the graph of voltage against time. Rechargeable battery capacity is rated in mAh (milliampere-hours). The total capacity of a battery is defined as “C”, that is it can supply C mA for 1 hour, or 2C for 30 minutes etc. Charge rates can vary – • from trickle chargers (to keep the battery ‘topped up’) of 3.3% of C to 5% of C • from a standard charger (a slow current charge) of 10% of C to 20% of C • or from a fast charger of 50% of C to 100% of C. • some ultra-fast chargers can go higher than 100% of C but these are normally designed for a specific type of battery. Slow charges are not meant to be continually applied, as they will eventually overcharge the battery. Since Nicad/ NiMH batteries are about 66% efficient, the slow charge siliconchip.com.au The heart of the charger is this pre-assembled PCB which makes construction a breeze! Basically, all you have to do is put it in a box, connect power . . . and connect your battery to be charged. time is normally about 8-15 hours. Fast charges, such as 100% of C, should be terminated after about 1.5 hours, providing the battery is flat to begin with. Once a battery is fully charged, it produces gas, creating a high internal pressure and a sudden rise in temperature. The charger should be switched to trickle charge at this point or the battery will begin to vent and release its electrolyte. My old battery was rated at C=1300mAh and my old charger was rated at 400mA (30% of C) so the charger should have been switched off after about 4 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 and once the a battery starts to deteriorate, I suspect this becomes a vicious cycle and the battery deteriorates rapidly due to more and more overcharging. The “Memory Effect” myth Possibly the biggest myth that exists particularly for Nicad cells is the “memory effect”. The myth is that cells have to be completely discharged - otherwise they develop a sort of memory, and can only hold a partial charge from there on. Like all good stories, this one has a grain of truth in it! The myth originated from the early days of satellites when they were using solar cells to charge batteries and because of the orbiting of the craft around the earth, the batteries were subjected to precise charge/discharge cycles many hundreds of times. The effect disappears when the battery cycle is suddenly varied, and it is extremely difficult to reproduce this effect even in a laboratory. In practice the “memory effect” is not a significant problem in home usage. While it may be OK to discharge individual cells to 0V, it is certainly not recommended to discharge an entire battery of cells. When the battery is discharged below 0.8V per cell, one of the cells is inevitably weaker than the others, and goes to zero first. Then this cell begins to be charged in reverse. This is easily observable on any battery pack. This creates a more common but less commonly known effect called “voltage depression”. The battery performance is greatly affected by the weakest cell, as the cells are all in series. One other thing – batteries don’t like getting too hot or cold; they do not take a full charge and they actually self discharge (even under no load) much faster when over 40° or below 0°. They can build up internal heat when working and this can also cause temperatures inside to increase. Particularly avoid leaving cordless tools inside a hot car for this reason. They also should be left to cool down for a while after discharge before placing them on charge. Nicad/NiMH July 2015  61 R1* 2.7W 1W REG2 LM317T 1 AC BR1 2 AC + REG1 7805CT KBL407* ~ 4 BAT– ADJ D GND ~ 4700mF CON1 63V G D +5V 100nF ICSP Vpp Vdd Vss PGD PGC 4 1 AN2/GP2 GP3/MCLR G 50V 1 Vdd BAT VOLTS 7 AN0/ GP0 4 6 5 IC1 PIC12F615 -I/SN GP4 CLKIN/GP5 GP1/AN1 D 3 G ADJ Q2 2N7002K 1k OUT IN ADJ Q3 2N7002K 180W A ADJ 180W A KBL407 2N7002 LEDS D S – G K A K +~ ~– S LM317T 7805 SC Ó 2015 NICAD/ N I MH BATTERY CHARGER ~ K G ~ l LED2 l Q4 2N7002K 1k + LED1 D R4* 2.7W 1W OUT IN K *RESISTORS R1– R4 MAY BE REPLACED WITH 1W 3W COMPONENTS TO INCREASE THE CURRENT RATING. BRIDGE BR1 AND DIODE D1 SHOULD ALSO BE INCREASED TO 8A DEVICES (EG GBU806 BRIDGE AND BY229 DIODE). 1k REG5 LM317T S D1 1N5404* A R3* 2.7W 1W REG4 LM317T 2 1k OUT IN 1k 5 Vss 8 CON2 R2* 2.7W 1W REG3 LM317T S 2 3 Q1 2N7002K S – 5.6k 1k OUT IN 3 BAT+ OUT IN 1N5404 A K GND IN GND OUT OUT ADJ OUT IN Four regulators share the load, which is the battery charging current. A single PIC microcontroller takes care of all the housekeeping, including monitoring the battery voltage to ensure it is not overcharged. batteries do self-discharge too, as a rule of thumb a battery will hold a full charge (with no load) for about a month or two, although when they get old or hot, they might only last a day. So therefore: • You should not discharge your battery before you recharge it, • Don’t flatten your battery below 0.8V per cell, • Don’t overcharge your battery beyond 100% of C, and • Nicad/NiMH don’t like to get too hot nor too cold (0° to 40°C is ideal) Nicad/NiMH charging Common values for C for cordless tools and racing cars are in the range from 500mAh to 3000mAh (mostly sub C cells and AA cells). The first step is to determine what C is for your cells. Inspect the cells or contact the manufacturer to determine the cell part number. In drills, the battery packs can often be easily disassembled. The value for C often forms some of the part number. For my new battery the value for C was 1700mAh. Note that the cell value for C is the same as the battery value for C. Usually the charge time required is as fast as possible, between 1 and 2 hours. This does not harm the cells, in fact they are designed for it. My battery was capable of taking a fast charge of 100% of C, which equates to 1.7A (Some can take up to a 2C rate). Each of the four regulators must These two shots show how the PCB is secured to the diecast box lid – it actually mounts on small threaded stand-offs with countersunk head screws used from the top (lid) side. The four regulators must be fitted with insulating washers and bushes to prevent them shorting to the lid. 62  Silicon Chip siliconchip.com.au Battery Charging Algorithm Start MODE LED display (FL – flashing) Test Conduct self test if no battery FL FL 0 Standby Wait for battery OFF OFF 1 Cool Wait for V to stabilise (1min) OFF ON 2 Soft 20%C for 4 mins 100%C until - DV or time out 3 Fast 4. Trickle 4%C 5 Error - Alternate flashing FL ON ON ON FL OFF FL FL If battery removed The algorithm flow-chart shows the steps the microcontroller goes through to charge the battery. LED codes are repeated on the front panel (see right) therefore supply around 450mA for a charge rate of 1C. This value should be good for most readers, and it doesn’t really matter if it is a bit less than 100% of C, because the charger will still detect a peak eventually anyway. However, some readers will want to increase the maximum current, and this is described a bit later on. 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 to shut off the charge. There is nothing wrong with doing this but battery packs do not always come with temperature sensors built in. Furthermore ones that do usually don’t sense all of the cells. The negative delta V system relies on the electrical characteristic that the Nicad/NiMH battery voltage peaks and drops about 20mV per cell when fully charged. This charger in its basic configuration will detect a peak of 40mV (per battery) from 10mm CSK M3 SCREW SILICONE INSULATING WASHER INSULATING BUSH M3 NUT 6mm CSK M3 SCREW DIECAST CASE LID PCB LM317 6mm M3 REGULATOR 6mm THREADED STANDOFF M3 SCREW (TO-220) Here’s how the four regulators are mounted using insulating washers and bushes; also how the PCB mounts to the case lid via four threaded stand-offs and screws. siliconchip.com.au (Above) same-size label which can also be used as a template to get the LEDs emerging in the right place, as seen below (before the label was fitted). 2V to 21.5V, thus will charge any battery pack in this range (ie, 2-20 cells or 2.4V to 24V). Another point to consider is the requirement to let a battery cool down. 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. Additionally, new batteries may show false peaks in the first four minutes of charge, as various cells synchronize their charge state. 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. Normal operation of the charger is fairly straightforward: the charger is switched on and both LEDs will flash once for self test. The charger uses a threshold of 2V (open circuit July 2015  63 voltage) to recognize that a battery has been connected. The charger will progressively start and peak the battery. The battery can be left on trickle charge indefinitely. Powering the charger The method of powering the charger depends on what you want to charge – that is, the voltage and current rating of the batteries. As specified, the charger is intended for low-voltage cordless power tools with batteries of, say, 9-18V. And as it has a bridge rectifier built in, you can power it with either AC or DC. Of course, the current rating of the transformer or DC supply needs to equal or exceed the required charging current. For batteries up to 7.2V (six cells) a 12VDC or 9VAC supply rated at 2A or so would be ideal (as you can see from our photos, we used a perfectly good 12V/3A supply from a perfectly bad laptop PC!). For higher voltage batteries, you’ll need a higher-voltage supply – say 24VDC or 15-16VAC for 12-14V batteries (again, look at surplus laptop supplies – there are plenty around with 16-18V output at 3-4A) but if you’re wanting to charge a 24V battery, you’re going to need something higher – say 30VDC or 24VAC. It is strongly suggested that a “plugpack” supply or transformer be used; these keep the “bitey bits” out of harm’s way, especially for beginners. If you must (and you know what you are doing) a trans- 12mm 12mm 48mm 45mm 5mm diam 14mm 65mm ALL UNMARKED HOLES: 3MM CSK 12.5 mm 18mm 21mm 12.5 mm 25mm Here’s a template to help you drill out the diecast case. It is intended for a standard 117 x 92 x 55mm case. former could be used and mounted inside a (much larger) case with the PCB. Operation In this “opened out” shot, the lid-mounted PCB is at the top with the output on the left and the socket for the AC or DC supply on the right. 64  Silicon Chip A constant-current supply is generated by several parallel linear regulators and pulse-width-modulated by a PIC12F615-I/SN microcontroller. The microcontroller senses the battery voltage and internally uses an analogto-digital converter to read the battery voltage. The microcontroller has its own 5V regulated supply (delivered by a 7805 regulator) and displays the current charging status on two LEDs. Four LM317T regulators connected in parallel will each maintain 1.25V between their OUT pin and ADJ pin. A 2.7Ω SMD resistor in the output limits the current to a constant 1.8A, or about 450mA per regulator. These resistors also help to ensure that the load current is spread evenly between the regulators. A power diode in series with the output makes sure that current can only flow in one direction; it will be reversebiased and therefore stop current if a charged battery is connected with the circuit not powered. A voltage divider, consisting of a 5.6kΩ and 1kΩ resistor monitor the battery voltage, while ensuring that even with a high-voltage battery (eg, 33V) the input to the PIC cannot exceed 5V (the input limit). From this point, virtually all circuit operation is controlled by the PIC. It monitors battery connection and if one is present, waits for one minute for the voltage to stabilise (which could be required if the battery is hot from hard siliconchip.com.au Increasing the charging current As supplied, each regulator has a 2.7Ω SMD resistor in its output. As well as limiting the charging current to ~450mA per regulator (or 1.8A total), these resistors help to ensure that the load is shared equally between the regulators. If you have a higher-rated battery (eg, ~5000mAh, which can handle higher charging currents), by lowering this to a 1Ω 3W SMD resistor (2512/6432 size), the total charging current will approach 5A. The bridge rectifier (BR1) would need to be changed to, say, an 8A GBU806 (same pinout), as would D1 – a BY229 is also rated at 8A. We haven’t tried these changes, by the way, but the output current is well within the regulators’ ratings. The only rider on this is that dissipation will also increase, so simple heatsinking to the case lid might not be enough. At a minimum, we’d also add some heatsinking compound to ensure as much heat as possible is removed. An alternative might be to use a “real” heatsink. use). It then provides a “soft” charge for four minutes, followed by a fast charge. The fast charge terminates when the microprocessor senses the “delta V” point of battery charge or in worst case, if the charge time is exceeded. It then enters a “trickle” charge state which is intended to maintain the battery at full charge until it is used. LEDs Two LEDs, driven by the microcontroller, give a visual indication of which mode the charger is in. These modes, with LEDs on, off or flashing, are shown on the front panel of the charger. Construction Because the PCB is supplied already built and tested (no need to solder those pesky SMDs!) the only construction required is to put the charger in a suitably drilled diecast box and connect input and output wiring. We used a diecast box not so much for its strength (though it certainly has that!) but because the diecast box provides heatsinking for the four LM317T regulators. The only fiddly bit about this is that the drilling for these, the two status LEDs and the four PCB mounting screws must be pretty good! Use our diagram as a guide (or even a template if your photocopier is accurate). We mounted the PCB upside-down in the case, so that it “hangs” from the lid rather than mounts on the bottom. This is to allow the two status LEDs to poke through the front panel. While using the lid does not give quite the same heatsinking as using the box itself, it’s more than adequate for the task. All screws through the lid were countersunk so the label could be glued flat on the panel. There’s not a lot of depth available in the case with the large electrolytic capacitor hanging down, so we used the shortest M3 countersunk screws we could find (6mm). These go into tapped 6mm stand-offs, with four more M3 screws (5mm pan or flat head, this time) securing the PCB to the stand-offs. If you find (as we did) that the stand-offs aren’t quite long enough to accommodate both screws, you could use 10mm types or place a 3mm inner diameter washer each side of the stand-offs to make them just that little bit longer. siliconchip.com.au And now for something completely different . . . Here’s something from the past that you will enjoy far into the future! Radio, TV & Hobbies April 1939-March 1965 Every article to enjoy once again on DVD-ROM ONLY $ 00 62 plus P&P Only available from SILICON CHIP Order online79 via See page siliconchip.com.au of this issue for a or call (02) 9939 3295 handy order form 9am-4pm Mon-Fri This remarkable archival collection spans nearly three decades of Australia’s own Radio & Hobbies and Radio, TV & Hobbies magazines. Every article is scanned into PDF format ready to read and re-read at your leisure on your home computer (obviously, a computer with a DVD-ROM is required, along with Acrobat Reader 6 or later (Acrobat Reader is a free download from Adobe). For history buffs, it’s worth its weight in gold. For anyone with even the vaguest interest in Australia’s radio and television history (and much more) what could be better? This is one DVD which you must have in your collection! The four LM317 regulators need to be insulated from the diecast box in the usual way – our diagram shows how the flat insulating washers and the round insulating bushes (one set per regulator) ensure there is no shorting to the case. Input and output leads depend a lot on your application. As we mentioned, we used a surplus laptop supply so simply drilled a hole in one end of the box and used a panel-mounting DC power socket which matched that on the supply. The output merely goes to some heavy-duty polarised figure-8 cable with spade connectors on the far end (again, because these suited our application). You might prefer to use crocodile clips or some other plug/socket arrangement. That’s up to you. Input and output leads all screw into the same PCBmounted terminal block. The PCB is clearly labeled so you shouldn’t be able to mix them up (did someone mention Murphy?). And that’s it! As we mentioned earlier, the PCB is tested when assembled so it should work straight away. SC Wheredyageddit? The pre-assembled and tested PH-00001 PCB comes from Shapely Electronics Design (www.shapely.asia) and sells for $50.00 inc GST, +P&H. All other components – the diecast box, DC power socket, standoffs, silicone insulators and grommets, etc, are commonly available from electronics retailers. Download the front panel from siliconchip.com.au July 2015  65