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Lionstrike
06 September 2013, 2212
Hello all,

So I am getting a real strong "NO!! NOT SLA!" vibe from all of the builders I've talked to. I don't like the various exotic lithium polymer compounds that I see for risk of fire or smoke hazard (I am building in a kitchen in my condo) but I am interested in lifepo4 (I've used the RC lifepo4 stuff with my ebikes).

Are there any brands that do not require a BMS? And since this stuff is expensive, how low an amp hour rating can I get away with for testing purposes? I am running a B&S Etek with a Curtis controller rated for 48v, 275 amps.

Thanks all.

podolefsky
07 September 2013, 0835
There's nothing wrong with SLA except that they're big, heavy, and don't last very long (like 300-500 cycles vs 1500-2000 for lithium). And they're not very glamorous. But they're cheap and you don't need a BMS. If you have room and are just doing a fun project, they're fine.

All brands of LiFePO4 are going to need some sort of monitoring and balancing. You can either get a dedicated BMS, or balance them manually periodically. The good thing about most LiFePO4 is that once they're balanced, they stay pretty well balanced for quite a while. But you still need to monitor them so you don't over discharge a cell. There are lots of ways to do that.

The exotic polymer batteries aren't as bad about fire as people make them out to be - but they're definitely not as safe as LiFePO4. The main advantage to LiFePO4 is cost and simplicity. They're generally easier to build up, only because you can get them as large bricks or cylinders that screw together. You can get polymer modules that are pre-built, but then you're talking $$$...unless you go with RC lipo, but that's another story.

I assume that's 275A peak, not continuous? You'll probably want 30Ah minimum, better to have 40Ah. If you just want to test it out, you could get 10Ah Headways and limit the current to 100A. Only problem is, I'm guessing your Curtis DC controller isn't programmable, so I'm not sure how you would limit the current other than using a meter and taking it easy on the throttle.

podolefsky
07 September 2013, 0836
Oh - and props for starting your build in the kitchen. That's where I started - then my wife made me buy a house.

Lionstrike
07 September 2013, 1601
Thanks for the response.

SLA has serious drawbacks and I was a huge supporter of it for a long time. I've mentioned elsewhere that I am a fan of building ebikes and I'd thought that the problem with SLA wasn't that it was somehow inherently evil, it was the weight distribution which needed to be kept low and balanced. Well, that clearly isn't it at all... the chemistry just has lousy characteristics which render it inappropriate for performance.

However... I've got 4000 EV commuter miles in on my Vectrix and it's not showing signs of being hurt at all. I just want a plan B.

Plan B in my mind needs to be shown as a functional concept before I dump $1500+ into lifepo4.

Thanks for not beating me up over trying this out with the SLA. Something about having 48-72v, 30ah lifepo4 in my kitchen doesn't quite sit well with me just yet. That's a lot of energy density sitting there idle, that I probably won't have to use for 2-4 years yet.

podolefsky
07 September 2013, 1739
I've got an R Martin scooter with SLA - it is also a few years old and has lost some of range, but I'm not sure how new the cells were when I bought it. They work fine for what they are...it's not exactly a performance machine.

There is absolutely nothing unsafe about having lifepo4 in your kitchen. Put another way, of all the things in your kitchen that could burn your house down, lifepo4 is the least likely to do it. You'd have to split one open with an axe and light the electrolyte on fire with a torch.

That said, if you're not going to use them for 2-4 years then there's no reason to buy now. They do degrade some over time, and besides in 2-4 years they'll be cheaper or something better will be the same price.

Hugues
07 September 2013, 2318
...

There is absolutely nothing unsafe about having lifepo4 in your kitchen. ....You'd have to split one open with an axe and light the electrolyte on fire with a torch.

..

i hope his wife didn't read this :)

Lionstrike
08 September 2013, 0149
Ah...

I am thinking of taking this clunky contraption to the local electric vehicle competition that's held up here in May, so I want something that can go... just not necessarily interested in "BEAST MODE" just yet. We do laps around the track at 35mph continuous for range. I'd say 48v 50ah of SLA and I could have an entry. (All of this is provided that it can be road worthy and pass inspection).

Now as far as practicality, I have a 13mile one way commute on the road with psychos from hell. 55 mph? Yeah... and by "55" a lot of those folks mean 70+. That, and there's some kind of objection to being behind a motorcycle for any reason EVER which promotes tailgating and RECKLESSNESS.

A 35mph ride for that kind of use is certainly weak sauce. Plus I am a big guy on top of that which makes for a bad equation to be sure. But as a concept... I think 35mph would be fine yeah?