Battery bank

G,day all,

I'm wanting to make a 12 volt battery bank for my camp trailer. I have a AGM 120a/h currently & found on our recent trip to the Qld Outback this was not enough if there were a few cloudy days in a row with my 120w solar.I have the chance to pick up two more AGM 65a/h batteries at the right price. My question is can you mix different a/h batteries to make a battery bank? Any feed back on 12 volt battery banks would be appreciated.
Thanks,
Michael.
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Reply By: oldtrack123 - Saturday, May 28, 2011 at 09:49

Saturday, May 28, 2011 at 09:49
Hi Michael

No problems with mixing different amp hr capacity batteries
More important is they be same type AGM with AGM etc

While adding extra batteries will increase your reserve for cloudy days,the recharging may be a problem if your daily use is high

Peter
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Reply By: Member - John and Val - Saturday, May 28, 2011 at 10:15

Saturday, May 28, 2011 at 10:15
Michael,

Depends on your usage patterns and the amount of charging available from the vehicle, but I suspect that more storage is only a partial solution. More solar capacity, or more charging from the vehicle is probably also necessary. Our own experience suggests more storage is very desirable and I'd certainly add in those extra batteries (assuming same type, general condition and charging requirements) in parallel with the existing one. To fully charge them though, suggest either an extra 60W solar panel or a dc-dc charger so that the alternator can push them up faster and more fully. (Worth thinking about the solar controller too - if it's not an mppt type, you could pick up probably an extra 20% by fitting an mppt one, always provided the sun shines!)



Cheers

John
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Reply By: CSeaJay - Saturday, May 28, 2011 at 13:04

Saturday, May 28, 2011 at 13:04
Michael,

I am not knowledgable on this, and like you I posted the question on this and other forums before I did the same.

The feedback I received at the time indicated that it is best to have batteries of similar capacity. It further indicated that similar capacity is more important than similar type (Yes I know dissimilar types are not ideal and you should do this anyway, just adding to the importance of the capacity issue)

Sorry to throw a spanner in the wheel without offering a clear answer, just thought I'd share the feedback I got to assist in further discussion. Happy to be proved wrong

CJ
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Follow Up By: Member - Frank P (NSW) - Saturday, May 28, 2011 at 16:02

Saturday, May 28, 2011 at 16:02
CJ and Michael,

Different capacities in parallel are ok.

Different chemistries are not.

The ideal situation is same type, same brand, same capacity, same age all in parallel with same length + and - leads going to a common + post and a common - post. No-one does that, well, very few anyway, the world is not that perfect.

I think you'll be ok with your 3 AGMs in parallel.

I agree with John about improving your charging. A DC-DC charger with decent cabling throughout for when you're driving and more solar with an MPPT regulator for when you're camped.

You have 120 watts of solar - is that a fold-up unit? If so, consider doubling it with another fold-up unit. 'Course, I'm spending your money, so it's easy for me to say, but that's what I'd be trying to do

Only good while the sun shines, though!

Cheers

Frank

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Follow Up By: CSeaJay - Saturday, May 28, 2011 at 16:13

Saturday, May 28, 2011 at 16:13
Thanks Frank
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Follow Up By: Puma1 - Sunday, May 29, 2011 at 01:08

Sunday, May 29, 2011 at 01:08
Hi guys - I have a 120 amp hour (deep cycle) and a marine combined cca & deep 100 amp hour and I wanted to parallel them, but thought I better check to make sure it was OK to do this.

The outcome was interesting in that, - I could do it, but was advised it was not a good idea as the 120 amp hour could not get charged over the maximum capacity of the weaker battery (100amphr) in the link, ultimately causing the failure of the 120 amp hr battery "probably within 12 months" if I proceeded, and that would then cause the failure of the 100 amp hr battery too! I was advised to buy another 120 amp hr battery.

Anyway I bought another 120 amp hour (note: I was not sold a battery by the battery expert who gave me the advise, so he had no motive to bull me) so now have 2 x 120's (so equivalent to 1 x 240 amp hr now they are connected parallel), and I have not hooked up the weaker (100amp hr) battery.

Michael the point here is if you hook up 2 x 65's with 1 x 120 amp hr the 120 will effectively become a 65 amp hr battery (so you will have 195 amp hrs total and not the 250 you think you will get), and will ultimately fail fairly quickly which in turn will make the 2 x "new" 65's fail fairly quickly after that!

Is this information right or did the battery professional have s__t for brains?
If he was correct "and I think he was" then Michael "I didn't do what you are about to".
Cheers Nigel
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Follow Up By: CSeaJay - Sunday, May 29, 2011 at 09:15

Sunday, May 29, 2011 at 09:15
Nigel,

That is consistent with the advice I received. It is all coming clearer in my memory; Frank is correct above in saying iy doesn't matter to connect them in parralel and that is true in DISCHARGE situation if they are all fully charged
But what you are saying makes sense for CHARGING of the bank

CJ
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Follow Up By: Member - Frank P (NSW) - Sunday, May 29, 2011 at 10:17

Sunday, May 29, 2011 at 10:17
Yes, I've done a bit more research to refresh my memory and I must back off a bit. There are problems with mixing capacities, both in discharging and in charging.

It seems there's no easy or cheap solution. The best thing would be to buy a second battery that matches the first 130ah, or buy more of the small ones.

Either way, I suspect as does John, that upgraded charging is also needed.

Apologies for the bum steer.

Frank
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