I'd like more energy density, please.
July 31, 2012 5:52 PM   Subscribe

What are the best resources (forums, articles, etc.) for assembling lithium ion (specifically LiFePO4) battery systems for small electric vehicles (read: 12V)? My Google searches seem to only turn up shady battery company websites.

I'm looking to replace an existing 12V SLA battery system. It's for an electric wheelchair, so range, charge cycles, and the ability to go into deep discharge on a regular basis are important. We ruined the SLA in about 3 months. I'm thinking 60Ah would be the sweet spot.

The chair has a built-in charger and controller, so I'm guessing that the solution is more difficult than just connecting 4 3.3V battery packs. But I can't find decent articles online that aren't poorly translated sales websites.
posted by hwyengr to Technology (5 answers total) 3 users marked this as a favorite
I can't tell you much, but I can tell you that trying to charge lithium batteries with an SLA charger will not end well. So first, you need a charger that can stop charging before it causes the batteries to vent with flame. Most SLA battery chargers just trickle charge indefinitely because it doesn't hurt them much if they're in fairly regular use. Also, the charge meter will be wonky if it estimates charge based on voltage. The voltage curve on a lithum battery is nothing like that of an SLA.

It's not going to be a drop in replacement by any means.
posted by wierdo at 6:20 PM on July 31, 2012

Forgive me if I'm wrong, but it sounds like you don't really know what you're doing. There's no way you should have ruined a SLA in about 3 months - unless it was the wrong sort to start with (e.g. stationary vs traction use), poorly treated (e.g. deep discharges beyond the point of no return), poorly sized (i.e. under-rated, leading to too-rapid discharge over prolonged periods of time), or poorly charged (e.g. charged too quickly, repeatedly undercharged / overcharged, etc.)

And, as weirdo's comment suggests, all types of lithium cells are ultra-picky about charge, discharge, & general care & feeding, & all more likely than not to fail in interesting but violent ways. Especially & doubly so if subjected to the same sorts of conditions that lead to SLAs failing in 3 months…

So, before you go down that road, you should really ask the advice of a battery &/or electric traction expert. Or, at the very least, give more detail here - what type of battery was the chair designed for, what type of SLA did you use, what was its rating, how sophisticated are the charger & controller circuits, etc.
posted by Pinback at 6:58 PM on July 31, 2012

Best answer: First, if you just want something that works, buy a Valence battery.

If you want to play around, start reading:
posted by flimflam at 8:03 PM on July 31, 2012

Response by poster: poorly treated (e.g. deep discharges beyond the point of no return)

Bingo. There were a couple of incidents where the circumstances resulted an almost 100% drain. And really, I would like to get more range than the safe 50% discharge for an SLA allows anyway, hence the interest in LiFePO4. The chair was spec'd by the manufacturer for 2-12V 36Ah SLAs, which we installed. By upping the capacity to 60+Ah, I don't see a circumstance where we'd end up discharging the battery to a damaging level again.

I've got 3 semesters of EE under my belt, so I'm not completely in the dark here. But there was a reason why I changed majors...
posted by hwyengr at 8:58 PM on July 31, 2012

Ah, fair enough. Your best bet then is probably to talk directly to one of the manufacturers / distributors specialising in traction use (e.g. in Aus, I'd go see EV Power; maybe Lithium Storage Inc or someone similar in the US?). There's some battery specs & guides there; if you're looking to build battery management or charging circuits then I'd suggest the usual suspects like Microchip etc.

Again, apologies if I'm underestimating your knowledge, but guessing at the overall capacity required doesn't really cut it when using lithium cells - none of them, regardless of chemistry, are anywhere near as as forgiving as flooded or sealed lead-acid cells in either tolerating or responding to abuse, so you need to pay attention to the rest of the specs too. Particularly, you'll want to know the peak start & nominal running current requirements of the chair so you don't exceed those battery specs too much &/or too often.

Additionally, be prepared at least to add a battery / cell management controller & also totally rejig / replace the charging circuit. Ideally, you'd be replacing / reprogramming the chair controller too.
posted by Pinback at 10:26 PM on July 31, 2012

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