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Can anyone tell me why I would pick a battery combiner instead of a battery isolator,they seem to have the same function
 

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They are actually entirely different devices, though both can be wired to be able to charge multiple battery banks. Check out Calder''s book to get details.
On the combiner, whenever the combiner senses charging voltage, it combines two banks (say the engine-start battery and the house bank) and charges both.
On the isolator, it is really a set of diodes (like a one-way valve) that allow the current to flow to both banks without the banks discharging into each other.
The isolator disadvantage is complex, but the diode has about a 0.7V voltage drop in it, which causes the charging source to rev up to higher voltage to achieve the same result. And the sense wire on a multi-stage charge regulator needs to see the higher voltage, so should also be wired through an equivalent diode, or else the battery banks will be perennially undercharged. Advantage is that charging is automatic, and if one bank dies (like an open cell), it does not hurt the other bank.
Disadvantage of the combiner is that it is automatic that the two banks are joined. Personally I don''t like the automatic nature of it.

I chose the isolator route for my two house banks and one engine bank, wired per Calder. However, in retrospect, I would probably have given the edge to having a two-output alternator, combiner for the engine charge bank and manually-switched sense wire. But you pays your money and takes your choice........
 

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I configured a charger and alternator to work like this: I use a charger that has isolated outputs for three banks, so each bank can be given what it needs. This is fine when shore power is available. I have a combiner that I wired with a disconnect switch so I can defeat it''s "automatic" nature when I am on shore power. When on engine/alternator charging, I enable the combiner so I can charge all banks at once. The caveat here is that you always have to remember the switch, no big deal, but one more thing to remember.
 
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