Third AGM battery

[shonky]

Hi, all.

Thanks for your feedback, gentlemen - greatly appreciated, but if the original two batteries are a bit suss (still to be tested) I feel any new additions are best kept separate. It's my understanding that if the current batteries are only 90% effective then any new ones will only charge to that level.

T.I.Terry - what's the downside in my hypothetical layout, apart from having to flip an isolator switch every day or so? Should have asked earlier - how's the neck treatment going?

[T1 Terry]

No more neck work till next Wed, so it's back to normal at the moment, well normal for me anyway.

If you try to run the load off one battery anything over a C20 load (the capacity divided by 20 and read as amps) will reduce the number of Ah available, i.e. 120Ah will supply 60 Ah as long as the load id never more than 6 amps, the time the load is greater than 6 amps you are using more of the battery capacity than you realise. If all 3 batteries are together that available draw increases to 18 amps, less likely to exceed the C20 limit so all 180Ah or so should be available.... theoretically

As far as the charging, the same charging voltage is used for all 3 batteries, if they are connected as described in the second option, so they will all fully charge, it's what remaining capacity they have is the problem, not the charging voltage, it's the discharge voltage that tells you that.
If, after fully charging each battery separately and test each battery separately, a 6 amp load causes any one of the batteries to drop below 12.6v after 5 mins, that battery is not suitable for connecting with a new battery, 12.6v under load for 5 mins would represent a loss of 20% capacity, 12.5v would represent 30% loss, that only leaves 20% useable so the new battery would be doing all the work for the first 30% or 36Ah, after a while all the batteries would be the same, all 30% stuffed and only have 20% useable left.

I bet none of that made sense did it

[shonky]

Hi, all.

T.I.Terry - so you agree with me then. That's great.

My understanding is that 2 new Fullriver AGMs at 120amps each should have a C20 load of 12 amps, and be able to run the Dometic 90ltr fridge. Similarly the older pair should, even with a lower C20 say 10 amps, should be able to run the Imac (6.5amps) and other domestic requirements, and the partially stuffed pair will not affect the new ones as far as charging is concerned.

So how's the neck now ???

[BruceS]

I agree with him too but it would be that proverbial 'pain in the neck' to operate.........
Perhaps test your batteries first? They may not be too bad & that way you'll save them & maybe get another couple of years out of them.
Better to be gentle on them than flog them?

[T1 Terry]

Hi, all.

T.I.Terry - so you agree with me then. That's great.

My understanding is that 2 new Fullriver AGMs at 120amps each should have a C20 load of 12 amps, and be able to run the Dometic 90ltr fridge. Similarly the older pair should, even with a lower C20 say 10 amps, should be able to run the Imac (6.5amps) and other domestic requirements, and the partially stuffed pair will not affect the new ones as far as charging is concerned.

So how's the neck now ???

Each battery has a C20 load of 6 amps, is the load of the fridge less than 6 amps? is the combied load of the Imac and other domestic loads less than 12 amps? With all 3 batteries combined the total load could be 18 amps and any over load is share 1/3rd between each battery, that keeps to C20 rating fairly close.

The pain in the neck is the minor bit compared to the headaches they cause

[shonky]

Hi, all.

FYI we've installed 2 more 120amp AGM batteries in parallel and they're working a treat. Thanks to those who submitted their valuable advice.