[F16bmathis]

Just installed lithiums! Went 65 miles on first trip before the Mini BMS beeped at me, then it let me go another 5 miles without beeping. Charged the pack, but I have one battery I bought off of E-bay that shows fully charged before the other batteries I bought from ElitePowerSolutions.com.

So I'm really hoping I didn't get used lithium batteries. Really, how do you know? Thought the serial numbers would be in sequence, they are close though, and the mileage is awesome especially considering I drove it in cold weather that LA's would have given me about 20 miles.

So... How would I discharge the one battery so the rest can catch up? A ceramic resistor across the cell? Pack still together, or do I have to break it apart?

The Mini BMS is great, and I think it will let the rest catch up eventually...

 

[dexion]

I wouldnt worry about it yet. I had one cell that I thought was a runt but now after 1 year and 12000 miles, the runt moves around alot. I dont ever really get below 50% soc but at the top end (being 3.5Vpc) I have 2 or 3 that vie for filling up first depending on their mood.


It depends on how far they are out of ballance. Repeated charging until that runt sets of the hvc if they are close (bringing up the rest of the pack) or yes a resistor will work fine on the runt no need to break up the pack.

Dont stress much about them one cell will always get full/empty first no matter what just stay away from the ends (full/empty) and the cells will be happy.

 

[dimitri]

Actually, this shows that all Elite shipped batteries are better than the one from Ebay, since they are still accepting the charge while the one from Ebay is not taking any more charge. Since this one cell came from different production batch, its likely a smaller capacity, perhaps 3-5AH or so smaller than all others and might have slightly higher IR, so it gets to a cutoff voltage sooner than the rest.

Basically, you are limiting your pack capacity to this smaller cell, but in reality, you are getting pretty impressive range, so nothing to worry about.

I assume your final charger current is still high enough to overrun shunting and trigger BMS shutoff. If possible, reduce final current to about 1 Amp, so shunting can run longer. You might need to repeat the shunting phase a few times until all cells line up at the top. Shunting continues for a little bit even after the charger is off, so cycling charger on/off can be used for initial balancing. You might also need to reduce the max charger voltage by 1-2 volts to allow for longer shunting phase. Ideally you want the charger to stop on its own just when most cells start shunting, to minimize the waste of energy.

 

[EVfun]

You can put a resistor across the one to remove some amp hours. If you go this route you should measure the difference in state of charge, say remove 1 amp hour and then charge the pack for a bit. If it goes high first again try removing 1 more amp hour and then continuing the charge again. Repeat as needed until the rest of the pack comes up in voltage with this cell. The difference is likely small, and it does none of the cells good to continue pushing on them once they start coming up. Lead acid needs to be fully charged regularly, but I've seen no indication that Lithium iron cells benefit from it.

I occurs to me that you may only be seeing a difference in the state of charge between the cells. Voltage seems to be a decent indicator of SOC, but mostly at the very top and bottom of the charge. There is a range in the middle where 10 amp hours is only 1 or 2 hundredths of a volt per cell.

 

[tomofreno]

You don't explicitly say whether you charged the pack prior to the drive. Is it possible that the one Ebay cell was at a higher SOC than the others to begin with? It would then hit HVC before the others even if it has exactly the same capacity. You might need to compare voltages after charge and after discharge a couple times to see how the cells compare.

 

[F16bmathis]

Well, its not a runt, it fully charges before the rest, so I was worried the others might be really low. Turns out most of the others will start shunting if I turn the amps down to about 5. The E-bay one's shunt LED is really bright compared to the others. Drove it, and charged (full amps) and now (with amps down low after first charge) there are about half showing shunting before the one E-bay one shuts of the charger. But its always the one from e-bay that shuts down the charger. Looks like the MINI BMS is starting to balance the pack. I love them!

 

[F16bmathis]

Dimitri,

Didn't think of it that way, thought the E-Bay one was better (was cleaner looking), but yeah, that makes sense.

Since then, I have been turning the amps down to 5 then 2-3 and I'm getting about half the cells shunting before the E-Bay one kills the charger, so Its starting to balance.

Range is great at 65 miles while stopping and go traffic and stepping on that pedal!

Zilla is back, so I'll finish mounting it in a sealed box, then wire it back in.

Thanks Dimitri!

 

[F16bmathis]

You don't explicitly say whether you charged the pack prior to the drive. Is it possible that the one Ebay cell was at a higher SOC than the others to begin with? It would then hit HVC before the others even if it has exactly the same capacity. You might need to compare voltages after charge and after discharge a couple times to see how the cells compare.

I did connect the pack in parallel for a week and charged it with a hobby lithium charger. It never got them fully charged, but they read the same. I also charged the pack once in the truck once the MINI BMS was in. Didn't have the charger cut-off installed, but I monitored the MINI BMS for the green LED to go out and turned off the charger myself. Drove it 65 miles, charged, the one cell did shut it off, then low amp charged and they are all starting to shunt, so its getting better thanks to the MINI BMS.