12v/120ah UPS batts any good for off grid?

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12v/120ah UPS batts any good for off grid?

Postby jackforkstick » Thu May 07, 2015 1:42 am

are used 4yo 12v 120ah UPS batteries any good for use in a off grid solar system?i would series/parel them into a 48v 600 or 720ah bank so would trying to keep the cells equalised be too hard? would i be better off buying new real solar batteries ie 2v 650ah cells or even trojan 6v 225ah RE batteries as ill have to spend $1,200 on used UPS batteries in any case? reason i ask all this is ive won a power jack 8000 watt 24v inverter off ebay but they sent a email saying the 24v inverter is out of stock but they can supply a 12v or 48v model and as my 24v 650ah usa gel batteries are getting 15yo now i thought an upgrade to 48v might be the way to go,what do you off gridders think?

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Re: 12v/120ah UPS batts any good for off grid?

Postby Gordon-Loomberah » Thu May 07, 2015 8:58 am

Hi Matt, welcome to the Energy Matters Forums :)

Ideally it is best to avoid more than 2 parallel strings of batteries, so 48V is your least worst choice, but with 2nd hand batteries you really don't know what you are getting, so they may be more trouble than you expect.
"Real" solar batteries these days don't have any Lead in them- you'd be better off going for Lithium, which are much more efficient, meaning you'll have more available energy from your PV array. After some adjustments to the recommended charging parameters, I'm seeing better than 99% efficiency charge to discharge with my LiFePO4 battery, installed in 2012. There are quite a few threads about Lithium batteries on the forum that are worth reading, in particular:

Be careful with that inverter- it may use a lot more energy than you'd like, even when no or minimal loads are running. Some forum members have done modifications to greatly improve efficiency, but you'd need some electronics experience to be able to do that.
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Re: 12v/120ah UPS batts any good for off grid?

Postby offgridQLD » Thu May 21, 2015 9:53 am

Hi Matt

I agree with Gordon on that power jack inverter. In stock form the idle load can be over 200w (4.8kwh every 24hr's dong nothing if it's switched on 24-7) In comparison to 50w or less for other makes. If it's not to late them not having the 24v model in stock might be your ticket out to look at other options.

As for the used AGM cells . I once purchased a 2nd hand 200ah 12v AGM for about $60. The typical story it was replaced as part of a preventative mantanance program but (apparently wasn't very old and had plenty of live in it) It checked out ok from what little testing I could do on inspection. It lasted a few years in my 4wd as a dual battery system but never was as fresh as a nice new battery and only lasted a few years of moderate service (running a small fridge). For the price I guess it was ok but to be honest I wouldn't pay much more than scrap value (about 50c kg) for 2nd hand AGM batterys the risk of the unknown past life is to high.

If your going to bite the bullet and invest in something more long term (lithium cells - lifepo4) Is the modern choice with many benefits over lead acid batterys.

48v has a few advantages so if given the choice I would pick 48v over 24v. A bit more info on your current offgrid system and energy needs and we can offer more detailed suggestions on batterys and inverters.

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