Quote:
Originally Posted by Cjhstang
Thanks!
... i can't figure out what amperage rating i would need on an isolator. would i need to get an isolator with the same rating as my alternator OR would i need an isolator relevant to the expected amperage draw from the two starting batteries while charging?
... what is the maximum amperage draw that a starting battery can peak when charging?
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The maximum charging amperage a battery can draw is about the same as the maximum cranking amperage it can deliver. (not exactly, but close enough for this discussion) In practical terms, that means a fully-discharged battery can absorb everything the alternator can deliver.
But the starting-lighting-ignition battery in a vehicle is almost never fully discharged and it's not a steady-state condition when it is. It might accept 100 charging Amps the moment you start the the engine, but the charge current starts going down as its state-of-charge goes up.
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Installing a battery isolator with a capacity equal to the alternator capacity will always be adequate. But as you've already surmised, a smaller isolator might also be adequate if the alternator's seldom delivering its maximum rated output.
But if you weren't considering adding electrical demand, we wouldn't be having this discussion, and the price difference between an isolator rated for 100% of the alternator's rating and a smaller one isn't very much, especially after considering the installation cost.
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If you plan frequent deep discharges of the deep-cycle battery, either a relay isolator or a simple two-diode isolator may prevent the SLI batteries from being fully charged. When the deep-cycle battery is switched in, the alternator voltage will drop as it delivers its full output current.
