Smart dual battery isolators not so smart?

Submitted: Wednesday, Jun 10, 2009 at 17:09
ThreadID: 69687 Views:11671 Replies:13 FollowUps:19
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It seems there may be some misconceptions out there as to how a smart system (redarc, etc) actually charges the battery..
Firstly I am not a sparky but have now discussed this with a few..
A dual battery system promises to
1)charge the cranking battery first
2)then charge the aux battery
3)isolate the starter battery when ignition is off, power from au only

Now I understand item 3) above, it is items 1) and 2) that are grey. The 'promise' on the ads is that ALL the charge go to the starter battery, after which ALL the charge go to the aux. That also enables you to have dissimilar batteries as they are not in parallel

Right? Apparently not! Yes the charge go to the cranking battery, but when that is full the aux battery is simply 'hooked on' to parallel. There is no such animal as ALL the charge going to the aux battery only. As a non-sparky this actually makes sense as the isolator thingy is 'downstream' from the main battery, so surely it can't receive ALL the charge in isolation from the main? The chatge from the alternator goes by cable to the main then to the aux, there is no cable to the aux without going through the main first?

I hope I am wrong, as one of the main reasons I want to split my current factory fitted twin batteries was so I can install an AGM as aux and not worry about dissimilar batteries charging in parallel

Can someone please shed some light (provide a spark!) in plain English as I am confused

Cheers

CJ
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