Car Electronics Questions? Ask an Electronics Expert for Answers ASAP
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Try disconnecting the battery - carefully and safely, at one electrode, preferably the ground, while the whole thing is Off.
Then try the battery charger on the battery.
Then measure the battery and see whether it has charged at all. If not, you have an internally shorted or otherwise defective and useless battery. If it has charged fully, on the other hand, you may have a shorted component in the vehicle's electrical system somewhere, which can be isolated by (still unhooked from the battery at one terminal, of course) measuring the total resistance in Ohms of the entire electrical system, then unhooking one component and measuring again, and seeing whether the Ohms of the entire system goes up suddenly when one component has been unhooked - and that would be the shorted component.