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Salvage It! BY JULIAN EDGAR Build a $2 battery charger Battery chargers needn’t cost a fortune. Here’s one you can build for a couple of bucks – or even less if you’re a good scrounger! R ECENTLY, WE’VE described a very sophisticated lead-acid battery charger in SILICON CHIP – in fact, perhaps one of the most sophisticated DIY designs in the world. But hey, it might be good but it also costs a lot more than a few dollars. This design? Well, it’s right at the other end of the scale – the $2 trickle battery charger! Sure, it takes a bit of fiddling to initially set up but after that you’re laughing. The components This battery charger comprises just a plugpack, a resistor and a fuse. For charging a 12V battery, a plugpack with a nominal rating of around 13-14V and a current capacity of about one amp (1A) is fine. Low-cost plugpacks matching these specs can be found wherever consumer goods are being thrown away or being sold cheaply secondhand. Those with the required ratings were often used to power printers and older scanners. Make sure when selecting the plugpack it has a DC (direct current) output. Some higher-powered plugpacks have an AC (alternating current) output as they’re designed for garden lights and so don’t need to rectify the AC to DC. The plugpack shown here was sourced from a shop at the local tip – half a dozen of them for $5. The resistor needs to be a high power design (eg, 5W) with a resistance around 5Ω. Lots of junked goods use high power resistors so always keep an eye out for these components – they’re amongst the few individual electronic components always worth collecting. Chassis-mount fuseholders can be found in amplifiers and in-line fuseholders in the supply wiring to A 1A 13.5V plugpack makes an ideal foundation for a trickle battery charger. Plugpacks like this can be picked up for nearly nothing at the tip and where secondhand electrical goods are sold. 90  Silicon Chip A high-power resistor limits the charging current flow, especially when the battery is well down in voltage. Always be on the lookout for high power resistors – they’re amongst the few individual electronic components always worth salvaging. car radios and amplifiers. The fuses themselves? Well, they’re in nearly all goods – and like high power resistors, fuses are always worth salvaging, sorting and storing. Building it Fig.1 shows the trickle charger circuit. It’s really very simple. An unregulated 13-14V DC plugpack has a no-load output of up to 18V. Connect that straight to a flat battery and more current will flow than is healthy for the plugpack – hence the need for the resistor. To set up the system (a once-only event), you’ll need your multimeter, a fairly flat 12V battery, the plugpack and a resistor with a rating of around 5Ω 5W. Cut off the original low-voltage plug and use the multimeter to work out the polarity of the output – mark the positive lead. Then wire the plugpack to the battery as shown in Fig.2 – don’t forget to include the resistor! Switch on and use the multimeter to check the current flow. If for example the plugpack is rated at 1A (the rating will be written on siliconchip.com.au Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & Fig.1: the charger circuit is simplicity itself. The plugpack charges the battery through a high-power resistor, with a fuse providing additional protection. Fig.2: the required value of the resistor is found by measuring the current flow when charging a fairly flat battery. The measured current in this situation should be less than the rating of the plugpack. P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width it) and in this test condition the flat battery is drawing more than 1A, increase the value of the resistor. If the current is a lot less than the maximum the plugpack is capable of, reduce the value of the resistor. If you find that there’s no current flow, even with the resistor completely removed, you need a better plugpack. Once an appropriate amount if current is flowing, leave the system charging for a few minutes and then feel the temperature of the resistor. Careful: it may be very hot! If it is too hot to touch, increase the resistor’s power dissipation. For example, if you are using a 5Ω 5W resistor, you could try two 10Ω 5W resistors in parallel (resulting in a 5Ω 10W combination). Size the value of the fuse to match the peak current flow that you’ve measured – eg, a 1A fuse if you’ve measured a maximum of 1A. If you don’t have any fuses of that value, go smaller rather than larger. Using it In practice, the trickle charger works very well. Feeling the temperature of the resistor gives an immediate indication of battery voltage – if the resistor is warm, the battery’s voltage is still low and so charging can continue. If it is cold, measure the voltage of the battery to see how high it is. After a few charging events it’s easy to get a good feel (groan) for what’s happening SC in the system. 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 siliconchip.com.au 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 salvage 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! H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ February 2005  91