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10-14-2011, 04:56 AM   #1
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Load-testing Batteries

Does anyone know how to load-test D-SLR batteries? That is, how can I quickly determine whether a battery will hold its charge for a long time or become exhausted quickly. I have two Li-90 batteries and want to figure out which lasts longer.
Cheers

10-14-2011, 11:02 AM   #2
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Load testing may not give you a time frame just the voltage under load,probably not worth the effort to set up.
Jake
10-14-2011, 01:27 PM   #3
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I'm not an electrical engineer, so take this with a big grain of salt.

It would be fairly easy to rig up something with a resistor and an ammeter in series with the battery. I calculate that a 100 ohm, 1/2 watt resistor would result in a current flow of less than 100 milliamps, which should be safe. Just connect the battery, resistor and ammeter (not voltmeter) in series and monitor it to see when the current flow begins to drop off.

If my calculations are correct, a fully charged battery should last around 15-18 hours at 100 milliamps. This is long enough to allow a fairly accurate measurement.

What I'm not sure of is whether there would be any significant heat buildup in the battery under long-term constant current flow conditions. Even though the current flow would be small, these batteries aren't designed for a constant load. Even with constant shooting, the battery is drained in spurts, such as shutter actuation and flash charging. In between, the battery has time to cool.

Also, because this sort of test does not simulate actual shooting conditions, the results might not reflect real-world performance.

BTW, the formula is: e = ir, where e is the voltage of the fully charged battery, i is the current in amps. Pick a value for i that is safe (keep it under 500 milliamps or .5 amp). Solve for r and that's your resistance in ohms.
10-15-2011, 06:39 AM   #4
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Thanks for the advice. I guess there isn't an off-the-shelf device designed specifically for load-testing those kinds of batteries. I got the idea from the practice of automobile mechanics of load-testing automobile batteries when a customer brings a car in with a battery that has gone dead.
Cheers.

10-15-2011, 09:49 PM   #5
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I don't know of any off-the-shelf device to load test small batteries of any kind. There may be such devices for professional engineers, but it would probably be expensive.

Such a test would be interesting, from a scientific viewpoint, but I don't think it would make much difference in the real world. Load testing auto batteries is done, not to measure the life of a charge, but to determine if the battery can deliver the current needed. The starter motor on a V-8 can draw several hundred amps, even in good weather. In cold weather, it can climb. Lead-acid car batteries also drop in voltage quite a bit during cranking. Batteries, like any other electrical component, have internal resistance. A bad battery may show a full charge with no load, but drop to almost zero volts under load. A good battery should drop to about eight to ten volts during cranking. A fully charged battery should measure 13-14 volts, with no load.

I suspect that any difference you find between your batteries would not be that significant. I have a genuine Pentax battery for my K10D, plus a couple of third-party ones I bought for cheap on eBay. I haven't noticed any significant difference. I suggest that you just shoot with them and, as long as you're not running out of juice at a critical moment, forget about it.
10-16-2011, 02:12 PM   #6
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Load testing lead-acids is fairly straight forward, testing lithium's is a different matter. Unless you are looking for fireworks.
Any doubts about the batteries, just get new ones, cheaper and safer on the long run.
My 2c worth.

Last edited by Ex Finn.; 10-16-2011 at 03:35 PM.
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