Originally posted by Sage Oldmann
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I fault the smartgauge site for showing how to put batteries in parallel, without explaining why it is such a poor design choice. Parallel batteries are inherently unstable. That means that if they are in equilibrium, it is an unstable equilibrium. It's sort of like trying to stand a pencil on its point. Mathematically, a pencil on its point is in equilibrium. In the real world there will be tiny perturbations from equilibrium. When the pencil leans one way, the forces on it cause it to lean further in the same direction. That's positive feedback.
Batteries in parallel are much the same way. Suppose you had 6 impossibly identical batteries and the wiring was impossibly perfect. The batteries (to start out) would be in equilibrium (but an unstable equilibrium). Consider, for an example, the effects of temperature. If just one battery is a tiny bit warmer than the other, then when charging the battery bank the warmer battery will get more of the current. Because it gets more current, it warms up more than the other batteries. Because its warmer it gets more current. Because it gets more current it gets warmer. Because its warmer it gets more current. Because it gets more current it gets warmer. That's positive feedback... thermal runaway.
Parallel batteries (for the above mentioned reason and several other reasons) should have a fuse in each string. Of course all the extra wiring for fuses adds more resistance and potential points of failure to the system.
Getting a DC clamp ammeter (as has been suggested by others) will be an eye opener for you. You may find out that when the battery charging stops, some batteries will be discharging into other batteries.
--mapmaker
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