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Old 11-27-2007, 12:08 PM   #317 (permalink)
MetroMPG
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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Last 3: 70.09 mpg (US)

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04-08-2007, 08:47 Pm

Fun with ammeters - making a load tester
Played a little this evening with the ammeter. Tried my hand at setting up a heavy-duty battery load tester.

One of the "professors" of the EVDL says you should be able to put a 75A load on a 6V battery and have it go for at least an hour before the batt drops below 5.25V (under load).

Hmm... Where to get a 75A load...

Some guys said: just dead short a very long piece of wire, like an extension cord. Someone else said: dead short a coat hanger, it's about the right resistance for its length.

So I dead shorted a coat hanger between jumper cables . Call me Sparky.

Just over 100A of current flowed at first, then as the hanger heated up, its resistance increased - and current dropped all the way down to 40A, where it seemed to stabilize (and the coat hanger gave off a toasty warm glow).

Repeat with the coat hanger submerged in a big pot of water, and current stabilized around 90A, with the test battery humming along steadily at 5.3V. I only let it do this for about 10 minutes.

So I'm thinking I'd have to make up some kind of test rig, and run each battery through it.

OR ... I was thinking maybe it would be easier to charge up the pack in the car, jack up the drive wheels, and spin the works in 5th gear - that's got to get be close to 75A. Then I could test & monitor a whole bunch of batteries at once.
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