Third AGM battery

[shonky]

HI, all.

We've had 2x120amp AGMs for the past year, and they're fine, but on the principle that more is always better can one install a third unit safely, and,if so, how should it be set up? We have 550 watts of solar, so the charge capacity is adequate.

[T1 Terry]

With 360Ah of AGM it gives you a theoretical useable 180Ah, if you can fully charge them each time, 550w of solar in 5 peak sun hrs can theoretically produce 160Ah, so theoretically you should be in the ball park....... but you know my feels about theory don't you

[BruceS]

Well it would be best if all were the same size and age but as that's not going to happen ............
Do you have room alongside the 2 you've got to put the new one?
For now you could just connect pos to pos & neg to neg so you have 3 all hooked up.
I'd suggest you make sure the existing ones are fully charged as well as the new one BEFORE you hook them together.
If you're capable it may pay to test your existing 2 to get an idea of their performance. If they're half knackered then you may have problems.
Others will chime in.

[dapope]

go buy 200 ah lithium, and never worry again......

[oldtrack123]

Hi Shonky
You may find This link answers your questions

http://caravanersforum.com/viewtopic.php?f=2&t=41449


PeterQ

[shonky]

Hi, all.

Still thinking about a 3rd AGM, assuming we have adequate sunshine between 8AM and 4PM daily, my fridge draws 3amps which is 12hrs overnight = 36amps, and the Imac uses 6.5amps for around 4 hours each evening = 26 amps, giving a total usage of 62amps at least. Now with 2*120amp AGM's am I straining the battery's capacity?

[T1 Terry]

But there is 16hrs between 8 am and 4pm, that 3amp x 16 hrs is 48ah, plus the 26ah for the 'puter, then there's lights and either a tv or radio or movies and any other 12v things like phone chargers and camera chargers, more likely up around the 80Ah figure or higher. Now consider that AGM batteries won't fully charge in the length of time the sun shines each day, especially if you are using some out of them while they are charging, like when the sun nips behind a cloud to have a smoko or something, add that the batteries aren't new any more, say really you will get to 90% recharge capacity. That means you only have 48Ah useable before you dip below the 50% SOC safety line.
Now, you tell us the 2 x 120Ah AGM are fine, so the numbers don't add up, do they.
So either you don't use as much as you think, or you have tortured those battery well below their safe 50% SOC limit

[shonky]

Hi, all.

Might change the name of the thread to 3rd battery.
At this point I've been offered a large(used) deep cycle battery and if it tests out satisfactorily I'll be installing it as follows (1) move the fridge wires to it, in effect creating 2 storage banks with the AGM's supplying all other house power. (2) take a second set of leads from the solar controller to the new (old) battery, linked by an isolating switch.This will allow me to charge either bank, not both, and use the solar controller as a monitor for state of charge and amp flow, which I recently discovered to be most useful. Worse comes to worst, I can use mains power to charge the AGM's and the genset for the fridge battery. This strikes me as being the simplest and safest way of utilising an increased battery storage.

Who's going to be first adjudicator? BruceS you have 10 minutes...

[BruceS]

Only comment is that doing it that way will create a bit more manual interaction.
I'd try linking them all together as one bank unless you do suspect one as being 'suspect'.
Just make darn sure your existing ones AND the newly introduced one are all fully, totally charged up BEFORE you connect them together. (or all dead flat!)
Take the pos of one end & the neg off the other end.

PS. 9 minutes!! Just made it!

[T1 Terry]

There is no gain and all loss in separating the batteries, with AGM and any form of lead acid, the strength is in numbers. How to connect 3 batteries in parallel is the tricky bit.
Either:
all the positives and all the negatives linked, then the positive from one end and the negative from the other,
or
equal length leads from all the positives to one point, then equal length leads from all the negatives to one point, the positive and negative leads do not need to be the same length though. All charging and discharging is from these 2 points. This is the hardest to do but will most definitely be the most successful.