Quote:
Originally Posted by kaiser
There is a direct correlation between the voltage measured and the charge of the battery. Two batteries of the same type and the same condition, yet one charged and one not, would display two different voltages.
If you would care to go and verify that from any graph depicting charge vs voltage, you could perhaps go and look at the link in the previous post?
It is perfectly sensible to expect two batteries connected together to equalize their charges, and that is indeed what happens.
What would be unreasonable, is to expect the one battery to charge the other fully, and I have never claimed that. What I have claimed, and what is indeed true, is that if you connect another battery, that is charged to your discharged car battery, that a flow will go from the charged battery into the uncharged battery and that will persist, until the voltage difference is nil.
So the case is, in this story, it will help to connect any other battery with a higher charge to your car battery.
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It just doesn't work like that. The flat battery rapidly develops a surface voltage, which prevents any flow of current. If you then disconnect them, the surface charge voltage will rapidly decline again from the flat battery.
The charge versus volts charts can only be relied upon if that surface charge has dissipated.
The only real way to charge one battery from another, of the same voltage is to boost the voltage output via an inverter.
I do suggest you try it and let us know how you get on, because it would be very useful if batteries could be charged that way - but we all know they cannot :-)