XM-3500 BMS Circuit

While charging today, only one LED wasn´t lit up.
I charged that cell separately with 5A and it took only around 3 minutes for it reach 3.7V.
After a few minutes I connected the BMS and charged using the TS charger.
It only took a minute or so to get the LEDs to light up and the cell that sagged was just about to shimmer on when the charger stopped.
So now it works perfectly with all cells balanced!

Question to all the XM-3500 owners:

Does the charger still run if you place a diode between the charger and batteries?

Thanks, Jeff

Hi, I would like some feedback on my bms that I put together.I have a xm3500li and made a bms out of four commonsense rc balance pro cards. They are 7pin cards that can balance up to 6 cells. They are made for lithiun polymer batteries so the high/low voltage threshholds are different than lifepo4 but they balance the cells to within .05v. I just wired leads (with plugs to match the pins on the cards) to all the cells. So I just plug the cards in before and during charging. The only problem is it takes awhile after charging to get them all balanced. Is this because the cards can't shunt enough current to keep the high cells in line while charging?

I just checked the commonsense rc website and found that the balance pro board discharges high cells at a rate of 150mah. When used before and during charging the cells are staying within .05v of each other and when fully charged none are above 3.9v. So I think this system is cheap, easy to install (relatively) and versitile. You can balance up to 24 cells with 4 boards. The boards are 39.95 at commonsense rc. We added a lithium polymer pack to the stock system (4 10AH 22.2v cells wired parallel-22.2v@40AH)and are getting 60mph now and about 30mi at full speed with all stock equipment.(Don't worry Mountain chen if we blow anything we won't try to warranty it...we just want to see what she'll do.)The low voltage cutoff in the controller still keeps you from drawing the batts to low also. I now see that a better way to do this is just to buy 4 to 6 more stock lifepo4 cells and wire them in series with the pack and get the 87v 15amp charger from elite.com. We have to charge the lipos with a separate charger so it's hard if you want to bring the chargers with you.

PS It seems the balancers also turn off automaticly If they approach the low voltage threshold ( they turn off completly at about 2.58-2.55V ) so if you leave them attached all the time it would probably be fine.

Bill

To use any "current-bypass" to avoid overcharging a cell, one must reduce the charging current as soon as any cell becomes "full".
If the bypass current is only 150 ma, then continuing to charge at any current over that very, very low 150 ma level will continue to charge that "full" cell, in spite of the active current bypass on that cell.

If one has 40 Ah batteries, and normally charges at 10 amps (so the bulk of the charging would be done in 4 hours), one "bad" cell might appear "full" after 2 hours. Then, if the charger drops to perhaps 200 ma, the other cells will need am additional 100 hours !!! (20 / 0.2) to fill up.

How does this "balancer" tell the charger to reduce the charge current?

Does the XM-3500Li (or other stock) charger have such a "control" input?

Thanks, Gary

No the balancer does not reduce charge current (It doesn't communicate with the charger at all) it shunts current via LEDs when a cell gets more than .05v higher than the lowest cell an LED lights up that disapates current. It does'nt bypass current it dissapates it. This allows the other cells to "catch up" then the led on the high cell or cells goes off, allowing the pack to continue charging at the same rate. The charger just stops charging when it reaches the set voltage. As for the stock charger for the xm3500li I dont think it has any balancing feature but I'm not certain. I don't know how it could work as there are no individual wires going to each cell.

Bill-

No, the current "shunts" around the "full" cell, and several volts are "used up" in the process.

The stock charger certainly does not monitor each cell. However, it could have a "one-wire" input to force the charger into a low-current "trickle-charge" mode, which an external BMS could activate. Without that, the charger is not really compatible with a BMS.

But, a BMS could "add" resistance in series to trick the charger into thinking that all the cells are full.

Gary- Ok, first I have to say I am learning as I go. I am not sure how these commonsense rc balance pros work. I believe they just put a small load on the cells that get >.05v higher than the lowest of the group it is monitoring. Then the load is removed when voltage is within .01v or .02v of the lowest cell in the group. I know this wastes power and maybe you're right if it just bypassed some current around those cells that would be better. But I don't think that's how they work. What it seems to do is keep the cells balanced enough that when the pack reaches full charge,(I think the charger does go into a trickle charge at the end of the cycle) there are no stragglers (Low cells) left behind. Then as the balancers bring the high cells down, at some point the charger may turn on again allowing any low cells to continue to catch up. Then you just leave the balance cards attached till all the green lights go out. Each card gives you a digital voltage readout of each cell to within .01v. So you can sit there and view the voltage of every cell in the pack. Then unplug the cards and go. Or leave them on...I don't think it matters just a little draw from the monitor LEDs. Does this make any sense?
PS - each card is 1.75"x2.5"
Bill-

Well my homemade BMS seems to be inadequate. It's to slow. It makes monitoring all cells easy but discharges high cells way to slow to keep up with the charger. Sorry if I wasted anyones time. It does work but takes days to balance.

eped wrote (10/27/2008 - 18:26):
You're welcome henry42 - keep me posted on how it ends up working.

I have used your BMS for almost four months now. All of the problems I was having with the batteries are gone! THANK YOU!.

I especially noticed that the LEDs on circuits 1 to 4 light up well before the LEDs on circuits 5 to 24. I attribute the reason to be circuits 1 to 4 connect to the additional four batteries. I added the batteries using a post on how to add a 12 volt (I think Optima) battery to the motor and leave the rest of the circuitry intact. Without your BMS I would either have either burned out the batteries or be subjected to hours and hours of manually balancing the batteries.

Again THANK YOU!