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@nanomonkey

Lets talk batteries.

Does anyone have a favorite battery chemistry that they are using for their solar setup? In the past I've purchase AGM lead acid, only to have them go dead when I forgot to charge them up before storing them. And because they were AGM, I couldn't replace the acid or break up the sulfates. Plus they are heavy and if I'm not mistaken you should only really run them down 20%, which means there is a lot less useful energy then what I'm banking on.

Lately I've been using a variety of Lithium-ion. I have a variety of portable tools that use 36v Lithium Nanophosphate batteries made by A123. These are great...unfortunately they are hard to come by as far as I can see A123 went out of business.

Next I tried out Philips 18650's. I have a few 75watt battery banks that I've put together. So far so good, but I have this lingering fear that a poorly made battery management circuit is going to cause a fire. I do like that I can discharge them to nearly zero without damaging the batteries.

I was briefly stoked on Nickel Iron batteries, as they are supposed to last 30-50 years...but they don't appear to be that efficient, and they self discharge pretty quickly.

I'd like to take the plunge and build a 1kW battery bank, but I'm not sold on any one technology as of yet. Anyone have experience they'd like to share, or a well thought out opinion? Any open source battery management systems for Lithium ion?

@tiago
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@nanomonkey

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@nanomonkey

From the sounds of it I should probably look into LiFePO4.

@Dominic

I like the sound of LiFePO4 but I can't really comment because I've only owned any for 4 days.

Either way, if you wanna store electricity in chemical form you gotta make a deal with the devil. I'd rather have something I could understand like raising a weight up really high or compressing a gas, but those sorts of solutions aren't commercially available.

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@nanomonkey

LiFePO4 always reads like leet speak for Life Power, then I think of skeksis draining life from pod people in The Dark Chrystal.

.LifePower.gif

@Kodo

:thumbsup: for dark crystal reference

@nanomonkey

:-1: for misspelling Crystal

@keks

Considering that LiFePO4 is significantly (around factor 4.5) more expensive than a regular car battery of comparable capacity - what is the big advantage of LeFePO4? I mean yeah, it will not break as fast, I think it's a bit lighter and the energy density is higher, but it seems those don't really justify the price. Paying a thousand bucks for a 12V 200Ah battery seems kind of much when you could also pay two hundret.

@nanomonkey

Lead-acid batteries aren't meant to be run down much, so when one buys a 200Ah lead acid deep cycle battery, you only really have 60 Ah (30% of 200Ah) to play with before you start doing damage to the battery. Even at 30% depth of discharge (DOD) a deep cycle battery only has a thousand cycles. From my experience, if you don't keep a lead acid battery fully charged while it is sitting, it will sulfate and die. Starter batteries, like what is in your car are even worse. They will die after 150 30% discharges, and only 15 full discharges.

A LiFePO4 has 1,500-2,000 80% DOD cycles giving and prefer to be stored at a semi charge state. In fact the best way to kill your laptop battery is to leave it plugged in all the time.

@Dominic

@keks I paid $340 NZD for 200ah@12v, we'll see how it goes though.

@ezdiy

@nanomonkey. Not quite, LA doesn't necessarily mean high current, car starter battery (which indeed won't last long with deep cycles).

The standard VRLA battery used in PV/large scale mains backup is 800Ah, 2V, 50Kg tube. 2000 cycles at 85% DOD. One tube costs about $150 when bought in bulk.

lifepo4 has almost exact same characteristic - 2k cycles at 80-90% discharge.

Both battery types have optimal cycle count (around 5-10k) around 50% DoD, however.

@nanomonkey

@Sybil I was responding to a question about why someone would go for a LiFePO4 over the much cheaper lead acid, that's why I mentioned the two most common variety of lead acid and their general problems with deep discharges.

@Sybil I've heard of those, or at least the tubular ones that the telcos use. Could you point me to a commercial seller? They are behemoths though, six cells (12v) would weigh 300 kg, where as the equivalent LiFePO4 would weigh 100 kg.

My experience with gel cels and AGM (which are also VRLA) show them to be quite picky. They charge up well to 70% charge, but then need to be trickle charged to 100%. And because of their construction, they don't do well with overcharging as you cannot top them off with more fluid. In fact there is such a thing as a thermal runaway with gel cells.

As I mentioned before, they also don't do well with being partially full for long, something like 5 hours below 80% is enough to start sulfating.

@nanomonkey

@Dominic I thought you meant you paid $340 for each cell...you paid $340 for all 6 cells (12V)?!! That seems like a steal. Around here, the same size is $1,400 usd (1981 NZD).

@ezdiy

@nanomonkey These are called OPzS and OPzV (gel vs flooded. difference is usually only in electrolyte and valves vs seals, but the plates stay pretty much the same otherwise.

These are "kinda" AGM (there's usually a fiberglass separator), but key difference is more rugged plates (braided tubes or somesuch, not pasted grid) - compared to "small" VRLA you normally see. OPz cells have power to weight ratio worse than the usual paste pressed lead acid, as surface to weight ratio of electrodes suffers a bit.

The technology is patentented which is why brandname ones are fairly expensive. Obviously chinese fabs were never that good at paying attention to patents.

Albeit much reduced compared to smaller batteries, sulfation is still a problem, the advertised duty cycle is rated for PV daycycle - including year-round variation in day lengths (meaning it has to cope with 16h discharge / 8h charge for half a year and vice versa). They can last 5-7 years in PV application, and 15-20 years for backup power.

But if you prolong the daycycle as such, the lifetime will probably suffer accordingly. It's difficult to say how much because I don't know if anyone ever measured it.

@Dominic

@nanomonkey no it's 4 times 3.25v cells = 13v, aka 12v. Yes, 340 for 4 cells. I didn't have to pay shipping at all because @neftaly's mother brings a whole container over semi regularily. It did take several months before they got around to sending the container though! I expect this saved me quite a lot.

@neftaly

@Dominic If you fell especially well-dealed, I'm sure she'd appreciate some Whittakers or flowers or something :)

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