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Salvage It! BY JULIAN EDGAR A high-current car battery charger for almost nothing Want a high-current 12V battery charger but don’t want to pay big dollars? It’s easy – just scrounge a salt-water pool chlorinator and modify it. H IGH-CURRENT battery chargers are expensive. Those that can deliver a genuine 15A or 20A, rather than just say 3A, can cost hundreds of dollars, which most of us find hard to justify. Well, forget about high costs and instead find a salt-water pool chlorinator someone is throwing away. It can be easily adapted to make a high-current battery charger, as we shall see in this article. Salt-water chlorinators Salt-water chlorinators are used with some swimming pools, whereby salt – rather than chlorine – is added to the pool to provide disinfection. In operation, a high-current, low-voltage DC power supply is connected to an electrolytic cell through which the salty pool water is pumped. This process then breaks the salt down into sodium hypochlorite. Inside a typical salt-water chlorinator control box you’ll find a big mains power transformer, a bridge rectifier (or alternatively, two stud diodes) and a pressure switch. In addition, there will often be a front-panel ammeter (occasionally marked in odd units relating to chlorine), a fuse, an on/off switch, a pilot lamp and sometimes a high/low (summer/winter) switch. Some of the fancier units may also have an electronic timer. They may also be able to monitor chlorine levels and include automatic polarity reversal circuitry to periodically clean the electrolytic cell. All these latter bits can be discarded for this project. The current and voltage specifications vary from unit to unit. For example, the three units I recently picked up ($30 total!) have ratings of 25A at 8.6V, 12A at 4.6V, and 25A at 7V. Ignoring for the moment the added bits and pieces like timers, the design of the power supply also varies. Fig.1 shows a mains transformer using a centre-tapped secondary, with two diodes used for the rectification. Fig.2 shows another approach – in this case, a mains power transformer connected to a bridge rectifier. Increasing the voltage These salt water chlorinators look like old junk but here is nearly all that you need to build a high-current car battery charger. Chlorinators often appear the worse for wear because of their exposure to salt but inside, the important components usually still work fine. 64  Silicon Chip So how do you increase the voltage output of these devices? After all, 4.6V, 7V and 8.6V outputs are all too low to charge a 12V battery – for that you need at least 15V and preferably 16-18V. The approach you take depends on the internal design of the chlorinator. If the chlorinator uses Fig.2’s approach, you’ll need two such units. siliconchip.com.au You then wire the secondaries of their transformers in series (and in phase, so that their combined AC output voltage is added) and use a bridge rectifier on the output. By taking this approach, with the voltage outputs of the two transformers effectively added together, the maximum current output is dictated by the transformer with the lowest rating. For example, lets say you have two chlorinators – one with 25A output at 8.6V and another with 12A output at 7V. In this case, they can be combined to produce an output voltage of about 16V at a maximum current of 12A. Alternatively, if your chlorinator uses the approach shown in Fig.1, it’s even easier. In this case, twice the nominal output voltage can be gained (at half the current) by discarding the existing diodes and connecting the transformer’s secondary output to the AC inputs of a bridge rectifier and heatsink instead (note: the centre tap wire is no longer used). This approach is easier because you don’t need to fit a second transformer inside the one box – instead, all you have to do is make some internal wiring changes and add the bridge rectifier and heatsink. In addition to a source of high current DC, you’ll also need a resistor. This resistor is used to limit the maximum current that can flow when you connect the charger to a flat battery, to prevent damage to both. Although the value of this resistor can be calculated, it’s much easier in practice to try different resistors and make a few measurements. We’ll show you how to do that shortly and describe how to make a suitable high-power, low-resistance, adjustable resistor. Finally, the charger should automatically disconnect when the battery is fully charged. This is achieved by using a modified “Simple Voltage Switch” kit, as originally described in SILICON CHIP’s “High Performance Electronics for Cars” book. The obligatory warning! OK, the theory is pretty straightforward so now let’s do it! But first, a word of warning. Unlike some of the other projects covered in this column, this definitely isn’t a 5-minute job. By the time you repaint the metal box, make a current limiting resistor, build the “Simple Voltage Switch” and put it all together, siliconchip.com.au Fig.1: salt-water chlorinators commonly use a transformer with a centretapped secondary and two diodes for rectification. If the diodes are replaced with a bridge rectifier and the centre tap no longer used (see Fig.2), the output voltage is doubled while the current rating is halved (although still very high). This makes for a very effective high-current battery charger. Fig.2: another common approach is to use a bridge rectifier with a noncentre tapped transformer. To increase the output voltage, a second transformer needs to be added, with the secondaries connected in series and in phase. The voltage outputs of the transformers are then added together, with the current output capability dictated by the transformer with the lowest rating. it’s probably a full day’s work. And it’s not a project for the inexperienced. On the other hand, it’s a lot of fun, you’re guaranteed to learn something and you won’t need to reach very deeply into your pocket. Best of all, you’ll end up with a high-current battery charger that should prove really useful from time to time. Picking the donor Salt water chlorinators commonly appear anywhere junk is being discarded – especially in areas where there are lots of swimming pools! Garage sales, the shops associated with municipal tips, household goods auctions and secondhand stores are all good places to look. Of course, like many of these things, if you specifically go looking for them, you’ll never see any, so it’s best to keep a look out over some time. The chlorinators ideally suited for battery charger conversion have a transformer with a centre-tapped secondary. You’ll have to open it up Main Features • • • • • • • High current charging Automatic switch-off when battery charged Ammeter to indicate charging rate Fan cooling – essential! Over-temperature shut-down Charge-finished indicator Very low cost to check this out – look for three wires coming from the secondary (low-voltage) side of the transformer and their associated large diodes. In addition, it should have an ammeter, a high current rating and a voltage output that can be doubled to 16-20V to make it suitable for battery charging (ie, an initial DC output of 8-10V). A high/low setting will also give your completed charger greater versatility. June 2006  65 heatsink from one, the power switch from another, and so on. Making the modifications ➋ ➌ ➊ ➍ ➎ ❼ ❽ ❾ ➏ This is what a typical chlorinator with a centre-tapped transformer secondary looks like inside: (1) transformer; (2) pressure switch; (3) one of the two diodes (the other is closer to the camera but hidden); (4) ammeter; (5) DC output connector; (6) DC output pilot lamp; (7) AC fuse; (8) winter/summer switch; (9) power switch. This type of design is easily modified to produce double the original output voltage at half the current, making it suitable for car battery charging. Obviously, you also want the transformer to be working but this can be difficult to assess when looking at a discarded unit – the fuse, internal pressure switch and pilot light may all be broken, so it can be difficult to tell! However, if it’s cheap enough, buy it anyway – in most cases, the transformer is fine. In fact, if you can buy two or three low-cost chlorinators, do so – you may be able to take the bridge rectifier and Check The Mains Wiring Before making any modifications, it’s important to carefully check the original mains wiring, to make sure it is safe. First, the Earth lead from the mains cord should make good contact to the case. Use your multimeter to check for continuity between the Earth pin of the mains plug and the case – you should get a reading of zero ohms. Next, check that the mains cord is in good condition (no nicks or cuts) and that it is securely clamped. The Active and Neutral wiring should have insulation that is in good condition and the leads must be correctly terminated. Check also that the mains plug is wired correctly. It may have been replaced at some stage and someone might have made a wiring mistake! Finally, it’s a good idea to insulate any exposed mains connections that might be present (eg, at fuseholders and switches), to avoid the possibility of receiving a severe (possibly fatal) shock. Do not attempt any work unless you know what you are doing. 66  Silicon Chip Now let’s modify the salvaged chlorinator. The first job is to electrically bypass the pressure switch (in the original application, this switch is used to detect water flow). That done, check the fuse (replace it if necessary) and reinstall the cover. Next, connect the unit to mains power and use your multimeter to check the DC output voltage. If this is present, place a load across this output (eg, a 50W car headlight bulb) and check that the ammeter (if fitted) reads correctly. If there is no DC output, switch off immediately and pull out the mains plug. Remove the cover, then measure the resistance of both the primary and secondary windings of the transformer. In each case, the measured resistance should be very low – a few ohms or less. If it is infinite, the coil winding is open circuit and the transformer is faulty. OK, let’s assume that you have a unit with a working transformer. We’ll also assume that the transformer has a centre-tapped secondary and that the unit uses two rectifier diodes (ie, it uses the configuration shown in Fig.1). The modification procedure is as follows: (1) Check that the mains plug has been pulled out of the wall socket, then remove the diodes and the associated heatsink. (2) Cut and insulate the centre-tap lead (ie, the wire on the secondary side of the transformer that went straight to the negative output terminal). (3) Connect the transformer’s two secondary leads to the AC (~) terminals on a bridge rectifier. Assuming you have salvaged the bridge rectifier from another chlorinator, make sure that it has a current rating at least as high as the rating of the modified unit. This bridge rectifier should be mounted on a heatsink. (4) Connect cables to the plus (+) and minus (-) terminals of the bridge rectifier and temporarily run them out of the case. These form the DC output leads (use red for positive and black for negative). (5) Reinstall the cover, connect the chlorinator to mains power and switch on. You should now be able to measure twice the original DC output voltage, siliconchip.com.au while the maximum available output current will be halved. (6) Switch off and install a fuseholder in the positive line. A fuseholder can be salvaged from other equipment or you can use an in-line fuseholder that takes an automotive-type blade fuse. Match the fuse rating to the new current rating of the power supply (remember, if you double the voltage, you halve the available current). In the author’s unit, a square hole had to be cut in the rear panel in order to install the bridge rectifier and its associated large heatsink. This heatsink had been held in place in another chlorinator by means of pop rivets and so rivets were also used to secure it in its new location. However, before doing this, the metal box was stripped of all components and painted inside and out with rust-proof paint. Incidentally, if you want a really impressive visual result, get the cabinet sand-blasted and powder-coated – it will look like new. Making a resistor The next step is to organise the large current-limiting resistor. After trying a number of approaches, including commercially-available resistors and light bulbs, the following method was adopted: (1) Buy a small reel of 0.9mm galvanised steel wire from a hardware store ($5). (2) Stretch out 3m of wire, then double it back on itself and twist the two lengths together using a bench vice and pliers. (3) Wind the twisted wire tightly around a pair of insulating posts spaced about 100mm apart and mounted on an aluminium bracket. The prototype used a couple on porcelain insulators that were scrounged from the local tip (see photo) but you could also use the formers from jug elements. The beauty of this scheme is that most of the resistance wire is exposed to cooling air. Alternatively, you could also wind the wire tightly around a long narrow mirror or a glass jar (the wire is stiff enough to keep its shape and position). Take care to ensure that the windings do not touch each other. That’s it – your high-power resistor is complete! Hint: one way of tightly winding a coil on a glass jar is to first wind it on a former with a slightly smaller dia­ siliconchip.com.au Salt-water chlorinators with non-centre tapped transformers use a bridge rectifier (arrowed) rather than two diodes. The bridge rectifier can either be removed and used to double the ouput voltage from an existing centre-tap design (see text) or, alternatively, a second transformer can be added to increase the available voltage. Either way, it makes sense to collect a few salt-water chlorinators when you’re buying. Galvanised steel fencing wire is used to wind the resistor that limits the charging rate. Here it has been wound between two ceramic insulators but it can also be wound on a narrow glass mirror salvaged from a scanner or photocopier. Directly above the resistor is the adjustable temperature switch (another salvaged part) that turns off the charger should the cooling fan fail. meter. The completed coil can then be slipped over the jar and the lid used to mount the terminals. The beauty of making your own is that its value can be easily adjusted. If less resistance is needed, just shorten the wire. Conversely, if more resistance is needed, use a longer wire! The steel wire has a far higher resistance than copper (so a much shorter length can be used) and is rugged. Note that in this application, the resistance June 2006  67 When the resistance value is correct, the completed resistor can be installed inside the box. When picking the mounting location of the resistor, remember that it will get very hot – don’t place it too close to other components and make sure it has plenty of ventilation. In fact, we strongly suggest that you install a cooling fan inside the box. Suitable 12V fans can be easily salvaged from old PCs, printers and photocopiers, to name just a few sources. Locate the fan so that it draws air out of the box – most chlorinators already have plenty of inlet vents built into them. Fig.4 (covered in detail later) shows how to wire the fan into circuit. The chlorinator modified by the author has two “power” levels, controlled by a front-panel switch that selects between two primary windings on the power transformer. This gives a charging current of either 18A or 9A when matched with a 1.5-metre length of resistance wire. At this stage, you effectively have a working charger. However, it’s much too risky to rely on manual control, as this could lead to serious over-charging and irreversible battery damage. It’s much safer and more convenient to have an automatic switch that turns the charger off when the battery reaches its fully charged state. An over-temperature cut-out adds another worthwhile safety element. Fig.3: the Simple Voltage Switch needs a number of modifications to perform in its new role. These include components that are deleted or changed, two tracks that are cut and some added underboard wiring. wire will get very hot, so be sure to use ceramic formers or a glass jar, rather than a wooden dowel that would char and perhaps catch fire. Using the resistor So what do we do with the resistor? First, you’ll need to have modified the chlorinator as described above (ie, output voltage doubled and an in-line DC fuse). The unit should also have a working ammeter – if not, you can use your multimeter if its current rating goes high enough. You’ll also need a “flat” lead-acid car battery – ie, one at about 11V (leaving car headlights on is a good way to flatten a battery). Next, make sure the cover is back on 68  Silicon Chip the chlorinator box, then connect your home-made resistor in series between the charger and the battery. Switch on and closely watch the ammeter. If the current flow is less than the new maximum that can be drawn, switch off and shorten the resistance windings (they will be hot, so give them time to cool). Conversely, if the current flow is too high, increase the length of the resistance winding or use only one strand rather than two. Note: the 1.5-metre resistor length (ie, a 3m-length of wire doubled over) is based on a measured DC output voltage of about 16V. If the no-load output voltage is higher than this, start off with a 3m length of doubled wire for the resistor. Voltage switch Apart from incidentals like cable ties and nuts and bolts, the automatic voltage switch is the only part of the system that you should need to buy new. In this case, we’re using the Simple Voltage Switch (Jaycar Cat. KC-5377) and as the name suggests, it switches a relay on the basis of monitored voltage. This particular project was originally designed for use in cars (where it can monitor engine management sensor outputs, switching fans and warning lights, etc) but in this application, we use it to switch off the battery charging current when the battery voltage rises above a preset level. The circuit is easy to build and features an adjustable trip-point, adjustable hysteresis (the difference between the switch-off and switch-on voltages) and an onboard 5A double-pole, double-throw (DPDT) relay. siliconchip.com.au However, the circuit does require a few simple modifications for use here. The first problem is that it was designed for use with car voltages. This means that it could easily be damaged if the battery charger has a no-load output of 18-20V and was switched on without the battery connected. Second, the hysteresis also requires some changes and a reset pushbutton needs to be added. And finally, because we want the regulator to drive a second high-current relay and a “Charge Finished” pilot light, some alterations need to be made to the power supply. Fortunately, the modifications are straightforward (see Fig.3): • Change the 8V 7808 regulator to a 12V 7812 type and fit it with a heat­ sink. • Delete zener diode ZD1. • Replace the 10W resistor with a wire link. • Replace the 10kW resistor next to D3 with a 1kW resistor. • Change the 100mF 16V electrolytic capacitor (the one below ZD1) to 470mF 63V (this will be a tight fit and the capacitor will need to be mounted a little off the PC board). • Delete the 100nF capacitor and wire two flying leads to its solder pads (these go to the Reset button). • Cut the PC board track that goes to pin 8 of IC1 and connect pin 8 directly to the output of the regulator. • Cut the track that supplies power to the relay above the 100mF capacitor and to the left of LED1 – see Fig.3. • Connect a flying lead between the regulator output and the positive terminal of the 100mF capacitor. Incidentally, the PC tracks are easily cut by using a sharp drill-bit rotated by hand. The Simple Voltage Switch needs to be built in the “Low-to-High” switching configuration – ie, we want the switch to activate as the battery voltage rises to the preset level. To achieve this, the 1N4148 diode needs to be mounted with its band nearest the top of the PC board and the wire link (LK1) placed in the righthand position (the original project article – included with the kit – covers these points in more detail). The on-board relay used with the Simple Voltage Switch doesn’t have sufficient current capability for the battery charger, so we need to add a high-current automotive relay. Again, siliconchip.com.au The charger uses the Simple Voltage Switch kit to disconnect the charger when the battery voltage reaches a preset level. Some modifications need to be made to the kit to allow it to perform satisfactorily in its new role. Blobs of silicone have been used to help secure the regulator heatsink and a new large capacitor. it’s available for nothing – every wrecked car less than 20 years old has half a dozen! As shown in Fig.4, a 1N4004 diode is wired in parallel with the relay’s coil, with its cathode (banded) end to positive, to protect the voltage regulator from spikes as the relay switches off. In addition to triggering this relay, we also use the on-board relay to turn on a “Charge Finished” 12V pilot lamp. Referring to Fig.4, the common (COM) terminal of the on-board relay is connected to earth, while the NC (normally closed) relay contact goes directly to one side of the external relay’s coil. The other side of this relay coil is connected the +12V regulator output via a thermostatic protection switch (which is detailed in a moment). That way, the high-current relay is normally activated and so the battery charges via the current-limiting resistor (made earlier) until a preset voltage is reached. At that point, the relay on the Simple Voltage Switch clicks over and disconnects the charger’s output from the battery by breaking the ground connection for the external relay – ie, the external relay turns off and its NO contacts open. At the same time, the on-board relay connects one side of the “Charge Finished” lamp to ground. The other side of this lamp is supplied with +12V Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you have a use for the highquality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! June 2006  69 Fig.4: the output of the transformer is rectified using a bridge rectifier. It then passes through a heavyduty relay, a custom-made current-limiting resistor, a fuse and an ammeter, before reaching the charging output. The battery is subsequently automatically disconnected when fully charged by a modified Simple Voltage Switch (which monitors the battery voltage), while a thermostatic switch protects the charger from overheating if the in-case temperature exceeds a preset point. and so the lamp lights to indicate that charging is complete. Note that there’s no “Charger On” indicator light. This was deemed unnecessary for two reasons: (1) the fan runs (and is audible) when ever the charger is switched on; and (2) the ammeter shows if any charging is occurring. Fig.4 also includes the temperature protection switch. Since we have a heavy-duty relay controlling the charger current, this switch can be incorporated in the relay coil’s supply. A bi-metallic thermostat from an oilfilled space heater is an ideal candidate here, although a variety of adjustable temperature switches (or thermostats) can be used (see the “Salvage It!” 70  Silicon Chip siliconchip.com.au ➌ ➎ ➏ ➋ ❼ ➍ ➊ ❽ ❿ ❾ An inside view of the charger: (1) heatsink for bridge rectifier; (2) high-current relay; (3) voltage switch; (4) transformer; (5) high/low charge switch; (6) on/off switch; (7) mains fuse; (8) DC fuse; (9) one of the two insulator supports for the resistor wire; (10) adjustable temperature switch. Note the uninsulated terminals on the mains fuseholder, the on/off switch and the high/low charge switch – these should all be insulated to avoid possible contact. for July 2005). These switches are normally closed (ie, they open when the set-point temperature is reached), so if one is wired in series with the feed to the high-current relay’s coil, it will switch off the charger if the temperature inside the case exceeds its set-point. Finally, the buzzer and diode across the output provide a warning if the battery is incorrectly connected. Normally, the diode is reversed biased and so the buzzer if off. However, if the battery is connected the wrong way around, the diode will be forward biased and so the buzzer will sound. No damage to the circuit will result siliconchip.com.au if the battery is incorrectly connected, provided that the charger itself is switched off. If the charger is on, then the DC fuse will probably blow. Setting the Voltage Switch At what voltage should the charger switch off? The near-new car battery that I used as the “guinea pig” in the development of this charger has written on it: “Maximum charging voltage: 14.8”. However, this is a very high cut-off point – normally, the voltage is set between 13.8V and 14.4V. To set the cut-off level, temporarily mount the voltage switch outside the box so it’s easily accessible, without exposing you to any danger of electrocution from high voltages inside the unit. Now charge the battery and monitor its voltage with a multimeter. Then, when the battery voltage reaches the desired level, very slowly rotate the multi-turn trimpot (VR1) on the Simple Voltage Switch until the main relay clicks off (the “Charge Finished” light should illuminate). The hysteresis pot (VR2) should be set fairly high (eg, 80% clockwise) otherwise as soon as the charger disconnects from the battery, the battery voltage will fall sufficiently to immediately reconnect it. When the “Charge Finished” lamp June 2006  71 OK, so it’s not exactly the best-looking charger you’ve ever seen but it cost very little to make. The heatsink and bridge rectifier used were taken from another chlorinator unit, while the fan and its grille were also salvaged. The sticker came from the shop at the local dump (there was a whole bag of ’em!). turns on, press the Reset button to reconnect the charger to the battery. The charger should immediately disconnect again and the “Charge Finished” lamp again illuminate (this is because the battery voltage will still be above the lower hysteresis level). Now turn on the high-beam headlights of the car for a few minutes. This time, after Reset is again pressed, the charger should spring into action, staying on until the cut-off voltage is again reached. This is also a good way of double-checking the cut-off voltage setting. Note: the LED on the Voltage Switch acts as a relay-tripped indicator. It will be off while the battery is charging but Follow These Precautions! (1) Hydrogen gas (which is explosive) is generated by lead-acid batteries during charging. Always charge batteries in a well-ventilated area. (2) Make sure the charger is switched off before connecting it to the battery, to prevent arcing at the terminals (a spark could cause the battery to explode!). Similarly, switch the charger off before disconnecting the battery. (3) The electrolyte inside lead-acid batteries is corrosive. Wear safety goggles when making connections to the terminals, removing vent caps or otherwise dealing with the battery (note: the vent caps can normally be left in place for charging). (4) Make sure that the battery is correctly filled with fluid before charging. (5) Make sure that all battery connections are clean and tight before connecting the charger for in-car charging. (6) Disconnecting a battery from a car will require you to re-enter the PIN security code for the radio. Other memory settings may be lost as well, including the memory for an adaptive transmission. (7) Do not operate this unit unattended. If the voltage cutout fails to operate due to a fault, the electrolyte in the battery could boil dry and seriously damage the battery – and perhaps cause other damage as well. 72  Silicon Chip The completed unit can charge at a continuous 18A and is nothing like those cheap units you can pick up for $30. will light when the Voltage Switch trips and the external relay turns off and disconnects the battery. You could of course mount this LED on the front panel but we chose to use the separate (and much brighter) 12V pilot lamp instead. Setting the thermostat The easiest way of setting the temperature switch is to temporarily disable the fan and then charge a battery for a few minutes on a hot day (if it’s a cold day, direct a hairdryer through the box vents). If the charger has a switchable “high” charge rate, set it to this mode. After a few minutes, the currentlimiting resistor should be hot and the inside of the case should be quite warm – so switch off mains power, pull the plug and open up the cover. Now turn the temperature switch until it audibly clicks off and then turn it back the other way a little. Set like this, siliconchip.com.au Over-Rating The Charger The charger that I built had an original rating in salt-water chlorinator form of 25A at 8.6V. After re-wiring the centre-tapped secondary into bridge rectifier configuration (the bridge rectifier complete with a large heatsink was taken from another unit rated at 25A at 7V), the charger would have had a nominal rating of, say, 10A after allowing for the 100Hz pulse current waveform in its battery charger role. However, I used a resistor that allowed a peak current flow with a flat battery of 18A – substantially higher than the transformer’s nominal rating. But isn’t this dangerous – won’t the transformer get very hot? The answer to that is “no”. After a few hours of continuous use in 30°C ambient conditions, my trusty infrared thermometer showed a rectifier temperature of 60°C, a transformer temperature of 55°C, and a resistor temperature of 85°C. The reason for these relatively low temperatures (the resistor is supposed to be hot – it’s dissipating about 100W!) is the very strong fan-forced cooling that I had added. As mentioned in the main text, fans are free when salvaged from innumerable consumer goods and can easily be driven by the battery charger. In addition to keeping the resistor cool, the fan also circulates a huge amount of air past the transformer, effectively boosting its power rating. If you’re pushing the boundaries of power ratings, monitor things very carefully, but with careful thermal and electrical design, it’s possible to get some very hefty power outputs – all at a very low cost. However, don’t expect the transformer to last as long as it would if rated much lower – if you are using the charger continuously (for example, to maintain the charge of a bank of lead-acid batteries), always respect the original power rating. the fan would have to stop working for several minutes before the temperature switch activated. If the temperature switch triggers too early in use, set it a little higher. Odds and ends You’ll need heavy duty cable and a pair of large alligator clips to connect the charger to the battery. In my case, the battery clips were one of the very few items I bought new. The heavy cable came from one of the chlorinators I’d salvaged. As mentioned previously, a DC fuse needs to be installed and every chlorinator already has an AC mains fuse. Make sure the values of the fuses match their new applications. Using the charger Before connecting the battery, make sure that the charger is switched off or that an isolating switch (if fitted) is off (see panel). It’s then just a matter siliconchip.com.au of connecting the charger leads to the battery (making sure the polarity is correct) and then switching the charger (or isolating switch) on. If you do get the polarity wrong, the warning buzzer will sound. In that case, disconnect the leads and reconnect the battery correctly. Assuming all is correct, the ammeter should immediately indicate that charging is occurring. If the “Charge Finished” lamp immediately comes on and the charge rate drops to zero, press the Reset button. If the charger again immediately reverts to its “Charge Finished” mode, the battery voltage may already be high – ie, it doesn’t need to be charged! Alternatively, if the “Charge Finished” lamp comes on and the charger has a “High/Low” charge-rate switch, try setting the switch to the lower rate. In fact, it’s best to charge the battery on the high level until the “Charge Finished” lamp comes on, then press the Reset button and continue charging on the lower of the two settings until the “Charge Finished” lamp again illuminates (charging is then finished). If the ammeter shows no charge occurring but the “Charge Finished” lamp is off, switch off mains power, pull the plug and check that the tem- perature switch hasn’t tripped. After that, check the fuses. Conclusion There’s a battery charger I’ve had for years. It has “Four Amps RMS” written on the front panel and I’ve never had any reason to check its charging current with an ammeter. But after spending hours testing the charger described here – and being amazed at how quickly it can bring up battery voltage – I decided to test “ol’ faithful”. I connected it to a battery which had a reasonably healthy voltage of 12.6V and measured the charging current. Would you believe it – just 0.25A! By contrast, the salvaged charger can pump in no less than 18A at the same battery voltage! No wonder the voltage was coming up faster than I expected – the dirtcheap salvage charger under the same conditions delivers about 70 times the current of the commercial unit! So as you can see, there are chargers and there are chargers. And the one SC shown here? It’s a charger! Acknowledgement: thanks to John Clarke and Robert Edgar who contributed to the design of this unit. June 2006  73