Can someone explain potential problems of connecting different 12V AGM batteries in parallel (same state of charge), if any?

mk1

Free Member

Messages
8
Hi all,

I never quite got my head around this one so an expert opinion is much appreciated!

Starter battery: Exide EP600 70Ah AGM
House battery: Pegasus SB12-105 100Ah AGM
VSR: Blue Sea PN 7600

Normally House & Starter batteries are only connected when driving van as you'd expect.

However, if I think I may use more than, say, 80Ah before the next drive it would be nice to draw from both batteries to prevent the voltage on the house battery dropping too low (by switching the VSR to 'always on').

I would reset the VSR before starting the engine (to avoid the starter overloading house battery wiring) & have a emergency booster battery which can start the engine if I get my sums wrong & wake up to 2 flat batteries lol

When I switch to 'always on' it would be after a drive when both battery voltages are the same.

Issues?
 

wildebus

Full Member

Messages
2,677
there is nothing wrong in 'stealing' a bit of power from the Starter Battery when need be :)

However, I would make a couple of comments ....
  1. If you think this is likely to be a common thing, it could be worth getting a jump pack in case one time you pull a bit too much from the Starter Battery (which you have already mentioned I see after I typed this comment!)
  2. You say ".... if I think I may use more than, say, 80Ah before the next drive". I don't know the specifics of the Pegasus AGM battery you have, but you are talking about using 80Ah out of a nominal 100Ah battery (so an 80% DoD) and that is probably quite a bit more than you should and that could be shortening the service life of the battery considerably if you do that too often. The typical Lead Acid battery should not be taken down below 50% DoD so with your setup, 50Ah is really the limit you should set yourself I think.
  3. Maybe you should consider either an additional or a larger replacement Leisure Battery.
 

mk1

Free Member

Messages
8
Thanks wildebus, yes I do have a lithium jump pack & the Pegasus is designed for going low if need be. OK cool well I'll crack on!
 

oppy

Full Member

Messages
760
I agree with wildebus. We now have 2 100 amp batteries (one under each seat) powered by both the engine whilst driving and by a solar panel whilst parked up. But I'm kinda neurotic and carry a combination unit that we got from Aldi a couple of years ago that will not only fire up the engine but also pump up the bike tyres
 

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