DIfferent Battery Sizes ?

Submitted: Sunday, Feb 03, 2008 at 08:50
ThreadID: 54165 Views:4198 Replies:2 FollowUps:5
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Hi,
I've been a "silent watcher" of this forum for some time and have learnt a great deal from all of the posts - thanks very much - it's great.
I would like to know if it's is acceptable practice to parallel two AGM batteries of different sizes e.g one 90 A/hr and one 120 A/hr , for the purpose of charging and or use. I have one (120) permanently mounted in the back of my Cruiser isolted via a Redarc which runs my fridge and have another (90) in a battery box.

Currently I charge the 90 with a Smart Charger in the shed every month but am wondering if I connected it via Anderson Plugs to the 120 if it would do any harm?

Any advice please and thanks in advance

Bigbones
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Reply By: Member - Mike DID - Sunday, Feb 03, 2008 at 11:00

Sunday, Feb 03, 2008 at 11:00
1. Both batteries are AGM so they will have simialr charge discharge behaviour, so no problem.

2. 90 and 120 amphour are close enough to be insignficant.

3. Unless you have a charger that puts out more than 25 amps there is no problem charging them in parallel.

4. If one cell were to short in one battery, it would cause the other battery to be overcharged eventually.
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Follow Up By: Sand Man (SA) - Sunday, Feb 03, 2008 at 13:01

Sunday, Feb 03, 2008 at 13:01
Yep, correct Mike.

And if one or more cells fail in one battery, it would cause the other battery to drain down (flatten) as well.

I prefer to have separate controllers for remote batteries.

My auxiliary battery is isolated from the starting battery by the Piranha controller.
My portable battery system is connected to the auxiliary battery, but through a separate in-car charge controller so it can be charged along with the auxiliary battery via the alternator, but remain isolated from the other batteries.

Bill


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Follow Up By: Mainey (wa) - Sunday, Feb 03, 2008 at 18:07

Sunday, Feb 03, 2008 at 18:07
There are 6 cells in a 12v 'wetcell' battery

Question:
How many 'cells' are there in a 12v AGM battery ?

Mainey . . .
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Follow Up By: Farrelly - Sunday, Feb 03, 2008 at 22:52

Sunday, Feb 03, 2008 at 22:52
Same 6 x 2.2 volt cells
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Reply By: Bigbones - Sunday, Feb 03, 2008 at 13:33

Sunday, Feb 03, 2008 at 13:33
Thanks for the reply Mike. I'm thinking that's what I'll do

Sandman thanks also - what kind of charge controller do you use ?

I'd like for the fridge to be able to draw off both batteries if necessary.

Does the charge controller allow that ? Or would I have to change over at some point ?

Regards

Bigbones
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Follow Up By: Sand Man (SA) - Monday, Feb 04, 2008 at 03:12

Monday, Feb 04, 2008 at 03:12
I use an in-car charging kit rated at 30 amps, purchased from Blue Apple (Home of 12 Volt).

This heavy duty charging kit incorporates an isolator in its makeup.

I don't understand why you believe you need to parallel both batteries together to run the fridge. Apart from changing from one battery to the other as one drains down, you would get the same elapsed running time.

In my setup, I use the auxiliary battery (80Ah) whilst travelling, then swap over to the 75Ah "Thumper" as I generally take the fridge out of the vehicle. Between the two I have sufficient capacity for 4-5 days minimum and supplement the Thumper's available capacity by "charging while using" via a solar panel.

The 30 amp in-car charging unit will restore the Thumper to a full charge in 2-3 hours or so of driving, depending on it's state of charge. In the meantime the auxiliary battery should be at, or near full charge, should I require it.



Bill


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Follow Up By: Mainey (wa) - Monday, Feb 04, 2008 at 09:13

Monday, Feb 04, 2008 at 09:13
""I don't understand why you believe you need to parallel both batteries together to run the fridge. Apart from changing from one battery to the other as one drains down, you would get the same elapsed running time""

Sand Man-> An excellent reason for running a fridge from 'both' batteries is you only discharge 'both' batteries very lightly, and then recharging is very fast and far more efficient.
I'm sure you will also find the 'elapsed time' will be actually increased, due to the much higher battery capacity, because you will find the (quality) AGM battery will partly 'regenerate' itself during the fridge off cycle due to it's chemical make-up, this can be verified by my own AGM battery system still being 12.7v first thing in the morning after running a fridge and lights during the night.

By using only (1) battery it will cause this battery to be discharged much deeper, and yes most probably to the point of the "low voltage disconnect device" disconnecting the fridge from the battery, and when this happens while your away from the vehicle and therefore don't notice, then you will definitely have warm beer and an upset cook too.
If there is no 'low voltage disconnect device' then the single battery will be fully discharged beyond the point of being useful, if fridge is not changed over to the other battery at the correct time and the original battery recharged again.
If charged by solar it would not be a problem :-)

Mainey . . .
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