LiFePo4 Charging practices w/out BMS?

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zarlor
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LiFePo4 Charging practices w/out BMS?

I've heard plenty of great reasons from folks here for using a BMS on the 20 cells in my XM-3500Li. But so far the BMS systems I've mainly been hearing about are home-builds. Suffice to say that my electronics skills are not nearly up to par as most folks here and finding the time to not only make something like that, but to learn enough to figure it all out as well is a tad more than I can handle at this moment.

So for those of us on the non-BMS side of things, what is the best way to use this Thundersky charger?

I'd seen a lot of discussion on the XM-2000 about making sure you unplugged the charger when it topped out the batteries. But this charger does that on its own... or it goes into some kind of "trickle charge" mode, whatever that may mean beyond what it seems to me it means. So, again remembering that I don't have a BMS to help me out on this, should I unplug the charger when it finally cuts off, or should I leave the charger connected whenever I'm charging up? Or should I just leave it connected for some extra amount of time after the initial cut-off?

MikeB
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Re: LiFePo4 Charging practices w/out BMS?

Good question. I'm waiting for sparc5 to get his greenBMS ready for testing, but in the mean time, I'm in the same place as you are.

I've got an outdoor rated electrical timer, and plugged the charger in to it. The timer turns on at 5am, and cuts off around 8am, in time for my morning commute. That's enough time to get the battery up to a full charge for me, as least with the riding I've done so far. I'm sure the charger has gone to green lights and turned itself off by the time the timer has cut power, because I've checked it before I eat breakfast.

Before I put the timer in, I just plugged the charger in and left it connected overnight. By morning, the charger showed two orange lights, rather than two green ones. Randall (from R.Martin) said it wasn't really a problem, but it still bothered me. The bike appeared to be getting a full charge each night, so I don't really know what was going on.

My electric vehicle: CuMoCo C130 scooter.

zarlor
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Re: LiFePo4 Charging practices w/out BMS?

I've never left it plugged in that long. I usually just plug it in when I get home and then unplug it whenever I notice that the light has gone green.

But considering that the charger is likely going to stop when the main battery (batteries?) it's looking at reaches whatever capacity it's set to cut out at, then that battery would (I think) slowly start to lose its charge out to the other batteries that are lower. So if we leave it on maybe the trickle charge just keeps adding a little bit of charge to that first battery as it passes its charge along, and maybe that's the better way to go. Or maybe that will just keep one batter too highly charged and be bad for it. Or maybe there's a middle ground where it should be allowed to trickle charge, but maybe only for a few hours instead of, say, overnight. I was hoping some of the pros around here might have some ideas on that for those of us still floundering our way into the EV waters.

Lenny Zimmermann
Metairie, LA

rossasaurus
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Re: LiFePo4 Charging practices w/out BMS?

Hi Lenny,
One time I fell asleep and left my charger on overnight.
When I realized, after about twelve hours, I panicked briefly, then checked voltages.
Six or seven of the cells were pretty high, around 4.2v, with one or two up to 4.3v and the remainder at 3.7v or 3.8v.
I immediately ran the motor for a few minutes on the stand and watched the voltages drop quickly...about 10mv/sec until the highest was down to 3.8v; this only took a couple minutes and made me feal that my cells weren't in much peril as they weren't very overcharged.
I only had to run the motor briefly, so not alot of energy involved there. I suppose it's the potential(voltage) not the energy(AH) that does the over-volt damage, so maybe I should have been concerned.
I find that when I remove my charger just when both LED's have gone Green, many of my cells are still at 3.4-3.6v, so not getting their fill perhaps; though that's where most of the energy lies.
I believe that once I get an over-volt protection circuit in place, I could leave the charger on indefinitely without problems, all the cells reaching optimum capacity, so that's my goal.
My cells seem to show imbalance only at their top end, where the voltage curve is steepest, and this makes sense as the variations between cells are more likely to appear here.
Once the cells drop into the 3.7v and under range, they track very well, usually within 10mv.

I guess to sum-up, I'd say run the charger to both Green LED's, then let it cycle on/off for another hour or two at most and call it good.
A timer is excellent, especially if you ride the same route and know how long the charger runs each time; mine seems very repeatable, always about 1.1-1.2Kwh in about three hours for my standard run to town; So I could set it for four or five hours and go peacefully to sleep, dreaming of a Vectrix ride through Yosemite :)

I'm still waiting for Xtreme to acknowledge receiving my bad cell after a month, so my info was mostly from before my cell died.
My scoot is gathering dust in the garage for the last month, :(

We've had the most fantastic Indian Summer, even an Indian Fall too, hot, clear, and I missed out on the warm evening rides due to the bad cell, as we're about to get some pounding rain, the first real storm this Fall.
I guess I'll have the Winter to tinker.

Oh, one other thing, you said...

"then that battery would (I think) slowly start to lose its charge out to the other batteries that are lower"

It doesn't work like that; that would be magical if they "just balanced" cell to cell; It'd be great and we wouldn't need active balancing, but no such luck.

Lenny, I didn't intend to rattle on so much, but there you go brother!

R

zarlor
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Re: LiFePo4 Charging practices w/out BMS?

It's always great hearing the experiences of others! I'm a bit luckier living in the deep south in that we get plenty of mild days during the winter, and right now we've got some pretty good weather already. So I should still be able to ride more than a few days before spring gets here (and then it becomes more of an issue with rain, especially in summer.)

I wasn;t sure how the batteries worked in the sense of charging and losing their charge. Too many of the "filling glasses with water" anologies regarding voltages that I probably took it all a little too simplisticly. ;)

Thanks for the feedback! I'll try it doing it with a timer from now on (well, until I can maybe get a BMS).

BTW, my experience with X-treme tech support matches up pretty much with yours. They are excruciatingly slow, annoyingly happy to skim your support request (and thereby miss important parts of what you need help with), and overall it's just not a very good experience dealing with them.

Lenny Zimmermann
Metairie, LA

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