Battery Load Tester

From Fozzee.net

The Vision

I started out with this grand vision of building a battery load tester capable of ranging over a wide variety of resistances, having on-board volt and ohm meters and generally being awesome. I had, in my junk box, 3 50ohm 100w wirewound resistors. This would yield settings between 150 ohms (all series) and 16.6~ ohms (all parallel) ( 1/(1/50*3) ). Anyhow, it didn't quite work out that way, as I'd need a 3-pole 6-throw rotary switch to make it happen.

The Reality

I realized that the point of this thing was to be a "big" load, and that I'd likely never use any of the lower (higher resistance) settings. I also figured I'd usually have my Handy Dandy Tandy (Radioshack) volt-amp meter near by. So I went the easy way: Tie all 3 resistors in parallel and add huge freaking clamps (as I'm usually load testing batteries with this pack)

In Practice

The resistor pack is rated at 100w, per the component wirewound resistors. Per Ohm's Law, At 12 volts, 16 ohms will draw 0.75 amps. Multiply 12 volts by .75 amps to get 9 watts. Well within the limits. At 48 volts (like my UPS' battery pack) it's a bit dicier... 48v / 16ohm = 3 amps * 48v = 144 watts. In this application, the pack gets quite warm to the touch, in a hurry. I don't think there's any harm in this, for short periods, which is probably all you should use a device like this for anyhow. In case you were wondering, 36 volt systems (like an electric scooter's pack) would be 81 watts.

Disclaimer: All of these calculations are rounded somewhat, to make it look nice on the page. These resistors have tolerance of 10%, so my rating might not be quite 16 ohms. As always, YMMV.

Ultimate Recycling Note: The clamps came off a pair of cheap (12ga) jumper cables in one of those KMart "Auto Emergency Kits". The power wire is 10ga, retrieved from a local dumpster. Ironically, my implementation of these jumper clamps is beefier than the original application.