Bill Collins via EV wrote:
The resting voltage of a lead acid battery is significantly lower than the voltage required to charge it. For example: a fully charged 12 volt battery has a resting voltage of 12.6, but will not draw much current unless you apply 13.5 volts or so. This means that when you connect two batteries with different states of charge, less current flows than many people think.
The steady-state current is indeed fairly low. But the *peak* current when you first connect them can be very high. They also act like huge many-farad capacitors. The peak current is approximately their voltage difference (V1-V2) divided by their equivalent internal resistance (R1+R2). Since the voltage difference can easily exceed 1 volt, and the internal resistance is as low as a few milliohms for AGMs, I've measured well over 100 amps on several occasions.
Usually, the wire you use to connect the two batteries has significant resistance; this limits the peak current. But if you have big wires and are closing the circuit with a good switch or contactor, the peak current can damage its contacts.
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