Quote:
Originally Posted by HarryM1BYT
Unfortunately you fail to understand how batteries work...
Batteries are not buckets. They fill up very quickly with volts, then the current and charging stops. The difference between a flat battery unable to crank and engine and a fully charged one, can be as little as one volt. Two batteries in parallel will quickly equalise their voltages, with little actual current charge being transferred. The 12.7v of the small battery, will at most put maybe 2 to 3% back into the car battery; 13.8v will charge a battery to 90%; 14.5v will put a full charge in. So very different from two buckets.
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Two batteries of the same type and condition connected, will perfectly equalize their charges given time. They will behave exactly like two buckets of water connected with a pipe at the bottom.
It is physically impossible to have anything else.
Current will flow until the charge density is equal in both batteries, at which state the voltage will be the same.