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To do a worthwhile check on a dry cell battery, you need to load it while you take a voltage measurement. The load can be a suitable resistor but there is a better way – use the green test strip which comes with Mallory Duracell “Copper TopTM” batter­ies. Build a simple battery tester for around $5 By JOHN CLARKE Most people have seen those green tester strips which are supplied with 9V and 1.5V battery packs. While you might think they are a marketing gimmick, they provide a far better means of determining the battery or cell condition than a simple multimet­er voltage measurement since they give a load test. In fact, they provide a more or less constant current load which presents a ideal battery test. But if you use these testers often, you will probably agree that they are fiddly to use. Of course, the battery manufacturer cannot be expected to produce a perfect tester for what amounts to a free addition to the battery pack. However, what they have supplied is a very good basis for making your own battery tester. All you need is a standard battery holder or clip lead suitable for the tester, short lengths of wire, a few screws, 84  Silicon Chip nuts, solder lugs and star washers, a bracket and a small plastic case. For 9V batteries, you have the choice of using a battery clip or the more expensive holder. The accompanying photos and diagrams show how we made a 9V tester. The same principle can be applied to single cell testers. The main thing to watch is that the tester strip is held away from any electrical or heat conductive surface. This means that it must be suspended in air to prevent the tester giving false results. Building it Fig.1 shows the assembly details. Begin the assembly by drilling and filing out the lid to accommodate the tester display and the battery clips. This done, drill small holes in the + and – contacts of the tester strip and line the strip up with the cutout in the PARTS LIST 1 battery tester strip 1 plastic case, 83 x 53 x 30mm 1 battery clip or holder and wire 1 bracket to support clip 1 3mm countersunk screw and nut 3 3mm x 10mm screws 4 3mm nuts 2 3mm flat washers 2 3mm star washers 2 solder lugs PVA glue lid. Mark out where these connector holes are and drill these holes in the lid. The tester strip can now be fastened to the lid – see Fig.1. Use star washers on the conduc­tive side and flat washers on the display side of the tester strip to ensure reliable contacts and Finally, the free end of the tester strip is held in place on the lid with a small dab of epoxy adhesive. Results Fig.1: here are the assembly details for the simple battery tester. Be sure to solder the wires to the lugs before bolting the assembly together and use star washers on the conduc­tive side of the tester strip, to ensure reliable contacts. We connected the 9V tester to a DC power supply in order to find out how the visible indications coincided with voltage and load conditions. For a start, the load current is about 100mA. Under this condition, a battery delivering more than about 8V shows as “good” and lights up all three segments of the display. Batteries delivering between 7V and 8V light up two seg­ments and are obviously marginal, according to the tester. Bat­teries delivering less than 7V will only light up one segment or none at all and, according to the tester, should be replaced. Whether you do replace a 9V battery delivering less than 7V is up to you. In some applications, 7V will be adequate; some applications will draw a lot less current than 100mA and so the battery will deliver correspondingly more voltage. For a single cell tester, the “good” indication comes on for battery volt­ ages greater than 1.25V, while the load current varies between 200mA at SC 1.25V to about 240mA at 1.5V. Silicon Chip Binders These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­ tinctive 2-tone green vinyl & will look great on your bookshelf. The “free” end of tester is secured to the lid using a dab of epoxy adhesive. Note the mounting technique for the battery clip. solder wires to the lugs before fitting the screws. We used a piece of clear plastic to cover and protect the tester strip from damage. You could also use the origi- nal plastic cover found with the battery pack and glue this to the underside of the lid. The battery clip is held in place under the lid using a suitable bracket, countersunk screw and nut. Price: $A11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. March 1996  85