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post #4 of (permalink) Old 22nd October 2006
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Most battery gauges are pretty much useless. I wouldn't trust the 50% reading for a minute, and I wouldn't be surprised if it means that the battery's nearly dead. As far as I'm aware, there are only two reliable ways of measuring the charge of a battery:

1) Measure the specific gravity of the electrolyte (you can buy a gizmo to do this from Halfords, though take care with the battery acid. Obviously this won't work with a sealed type battery.)
2) Perform a voltage test under load. Generally this is a big pair of metal probes with some heavy duty resistance wire and a voltmeter across them. The resistance wire puts a significant load across the battery, while the meter gives you a voltage.

Without load, a practically flat battery can give more or less the same voltage as a fully charged one. The figure is meaningless.
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