Using SLA chargers for LiFeP04 batteries

[dogman dan]

There's the rub. Lifepo4 has a very flat discharge curve, so watching a simple voltmeter on the handlebars works really poorly. All is fine for a long time, then wham, you overdischarged it. News flash for you, 12v is pretty much fully discharged. So try not to let it go to 11v per pack.

That's why you see so little of lifepo4 being used without a bms. Other chemistries give you a few min warning before diving off the voltage cliff. So if you are watching, you can stop sooner. But with lifepo4, it hits like, WHAM. I think your controller lvc may be low enough to still allow damage before it trips.

The only solution short of a bms is to do careful testing, and find out what your capacity is with a wattmeter of some kind. Then, try to not use more than 80% of your field tested actual real world capacity.

No way can you let kids take it, with a warning to not over discharge. They will. It will be hard enough for you not to do it.

 

[Matt Gruber]

lifepo4 has no appeal to me. except as a brain teaser! so to dogmans excellent comments, i'll add a timer to estimate run time. a simple stopwatch along with the DVM, after an 80% dod is timed, could give a better chance at preventing overdischarge. as for dealing with kids, i'd hide a timer in the toy, set it to run for 70% dod, then it shuts off.
Go have fun, kids, when it stops, bring it back for a charge. Now and then i'd do a quick check of each cell, probably 1 or 2 will be known as weak after a few rides. They get individually charged to balance.

 

[hulagun]

Thanks again for all the info. I respect everyone's knowledge and appreciate their help.

Bioennopower (charger) and CleanRepublic (batteries) have both been communicative and helpful. I have two new batteries on the way, deeply discounted. I know this is making a couple people roll their eyes. :lol: I am putting the $$ outlay all down to learning experience. If the new batteries don't function satisfactorily, I can still return the charger, or sell it at a loss. Any bad batteries will be donated to a buddy who can recycle and use any cells that are still OK.

One lesson learned : if I had spent a little more $$, I could probably be using some sort of non-SLA battery pack, with a BMS, and without issues.

The scooter is not for commuting, it is intended for getting around a large pit area at a race track. Because it is likely that others (like the kid) will want to borrow it, having the scooter cut itself off at a "safe" voltage drop seems vital. This issue may be solvable, and I plan to pursue it. If I can find a 24v controller with a 23v cutoff, that will work with a Hall-effect handlebar throttle, it will be better than nothing.

I'll keep posting here - unless its time to start a new thread?