Which battery?
AGM + AGM without isolator, switch to disconnect.
AGM + standard wet cell, use an isolator.

Those red and yellow topped batteries are AGMs and barely vary in internal design.

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I don't run an isolator. You basically run that or a solenoid (or switch). Never had any trouble connecting a dead battery to a fully charged one. Just be sure to use large enough cables to handle the charging current. I use 1/0 welding cable in mine.


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Isolators work by adding large diodes to each battery to keep them separate under discharge but still let them charge together.


Depends on the isolator. Some use FETs. Some relays. I'd go for a heavy duty relay based one, but I bet most short on the size of the relay. If I was making one I'd try to find a 300+ Amp DC relay. Note, they are not cheap. I guess another design may work well. That is to use large diodes for separation when the voltage difference is high, then close a relay when they get close in voltage. A pair of 200+ amp low voltage drop diodes in half-pack cases should only cost $50 plus heat sink (needs to be isolated), and misc parts. For a hundred bucks a robust diode based isolator could be made.

-- Bryan