AGM Battery Advice

[Vermonter]

My Dad is putting together a small PV system. He already has 250W of PVs, now he wants to spend around $1000 on batteries.

He doesn't want to have to water them, and wants to store them outside where it can get below freezing, so he's interested in AGMs. He wants to get 4 of the Diehard "Marine" ones, at $250 each for 100AH. That seems really expensive to me.

He's had a small 40W PV and has been using two Diehard 50lb lead acid "marine" batteries, one at a time charging the other being used. He swaps them by hand. He doesn't really understand about not running them down too far, and it seems like they only last him a few years. His inverter will shut down, but he also has some DC uses that he really drains them with. I've tried telling him to not do that, but he keeps doing it anyways. I think I've heard that AGMs are even more susceptible to damage from abuse? He'll hopefully be just using his inverter now that he has more PVs and won't run them down as bad, but you never know.

So, I guess I'm looking for a good price on AGMs, so I can get him to spend less, and I'm looking for advice on the longevity of AGMs, and perhaps some info I can use to convince him that quality lead-acids will really be just fine. Or perhaps LiFePO4 would be an option when compared to the expensive Diehard AGMs?

[Chuck]

Hi Vermonter.

I lived in Vermont for about 12 years, I recall -40F nights that usually happened every year just after Xmas. I don't recommend any kind of battery for going through that.

I use a large AGM battery bank (24v 1400AH) but the lowest my set has seen is -20F, and probably not that much as it's in a barn, unheated though it may be. As you mention, the key is to not draw them down too much (I never let it get below 80% of full charge). It's even more crucial when they are cold, as the capacity decreases with cold.

A possible source for these batteries is the telephone company, which uses large 48v banks to back up the phone system and cycles them every few years. They often have never been cycled, having sat at float for years. It's worth getting to know someone there as you might be able to pick up a lot of battery for not too much $.

[dnix71]

The best batteries are probably Optima's (sprial wound). The guys I know who build obscenely loud car stereo systems say nothing else can take the abuse.

http://www.1st-optima-batteries.com/

They are sealed, no maintenance and the deep cycle marine versions are made to discharge to 10.5 volts repeatedly without shortening their life.  They are not cheap, maybe double the cost of the average consumer grade battery.

[dnix71]

You definitely need some type of enclosure, even outside, to protect against the cold. Maybe build a box against a wall whose room stays heated all the time.

[electrak]

or maybe this is a good use of nickel iron batteries, or flooded nickel cadniums

[wooferhound]

I keep my batteries in an Ice Cooler

[Vermonter]

Sorry, I shouldn't have said "outside". They'll be in an uninsulated building.

Thanks for the tips.

[cylindar]

AGM's have a much shorter life cycle than flooded lead acid. Leaving batteries for stationary power outside in cold climates is a nono unless you are using low temp Hawkers but even they should be kept at a reasonable temperature. Batteries do not last long with that kind of treatment. AGM's have a higher acid content in the electrolyte but then the plates corrode faster and thus have a shorter life and thus are more expensive in the long run. In general AGM's are a bad choice for a renewable energy system.