:2 100ah 12v batteries that are used independently of each other. To power :the invererter for short period with 280w on AC needs the batteries to be :paralled. With the batteries at differing states of charge dont want 1 :battery attempting to charge battery 2. Realise that one battery will supply :more power till they equalise, but generally there is not much difference :between the batteries. However the batteries cannot be directly connected. : :Since found this article :
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: :Going on that article I would need a P-channel Mosfet as the negative cannot :be isolated. In the Jaycar catalogue there is only a IRF9540N 100v 23A :
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:can these be placed in parallel to increase the current rating? :is there a higher powered P-channel Mosfet easily obtainable? :will the Mosfet need a heat sink? : :Take it the Gate is the negative power for switching, the Source is the 12v :battery supply and the Drain is the power to the inverter. Is that correct? : :Many thanks to those offering their help and expertise. :
There is a little more to using mosfets to isolate two heavy duty 12V batteries than you seem to imagine. Usually, these devices are incorporated into what are sometimes known as "Smart" dual-battery isolators, to distinguish them from the "dumb" diode-only type of isolator.
For example, here is a design by Dave Johnson which incorporates paralleled P-channel mosfets to give only 50mV drop, driven by a dedicated control IC.
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Note that the mosfets must be on a decent heatsink to handle above 20A.