Dangerous to connect up battery w unbalanced cells?

[Frakentrike]

Hi guys, is it dangerous not to charge a replacement cell group up to full charge before connecting it in a battery pack back together with other already fully charged cells?

I have just had 4 replacement cells soldered as a group of 4 cells into a 16s string of cells which is half of a new 48v 20Ah LiFePo pack but in the rush there wasn't a chance to charge the cell group up to 3.7v with the little singe cell charger included with the replacement cells, before closing up the battery package and heat shrinking the new shinkwrap around the battery.

I’m now worried about plugging it all together again in parallel with the other half of the battery pack and the BMS in case the out of balance cells cause damage or reduce the lifespan of the pack. Would it be safe for me to plug it back together and slowly charge the pack - perhaps for short periods of time and let the BMS balance the cells or should I cut off the new shrink wrap (drats!) and charge that cell group up separately with the single cell charger as per Ping's instructions?

The replacement cells had a charge of about 3.4v or 3.5v (if I recall correctly) in each of them (as per good practice for shipping I assume) and I'd previously charged up the entire 2x16s pack to the full 57-59v (3.7v per cell I assume) before pulling it apart so there is a bit of a voltage differential there.


(repost into a new thread)
Sorry for my noob questions!

 

[Punx0r]

As long as the cells with a different voltage (the new ones) have only been put in series with the old ones it will be fine - just will take a fair while for the BMS to balance. The danger is connecting cells of different voltages in parallel. In that scenario the higher voltage cells will charge the lower ones at a current rate only limited by the cell's internal resistance or of resistance of the parallel connection, which can result in damage or even fire.

 

[Hillhater]

Can you access the balance wires at the BMS ?
If so you could use them to single cell charge that new set to save balancing time.

 

[amberwolf]

I'd say that anytime you can raise low cells up to match full ones, rather than raising the whole set up and then bleeding excess off teh high ones, the full cells are better off (because they experience less time at higher voltages, and probably have a longer lifespan that way...but this is an educated guess, with no experimental data to test it).

 

[dogman dan]

Sounds to me like you are guessing at the voltage of those cells. 3.5 is full,, and I bet they were not shipped full.

Balance the thing, then connect with the other pack.

If you verify that the cells are nearly the same voltage, then it will be ok, But they have to be pretty close, like within .1v.

You don't have to cut the shrink, as long as you have access to the plug for the balance wires. Bring up low cells though the balance wires, at a rate less than 2 amps.

 

[Frakentrike]

Thanks for all the feedback guys. I chatted with Li Ping and he confirmed the replacement cells were charged to within 80-90% of full so indicated they should be close enough voltage to mean connecting it all back together and leaving it on a long BMS-balancing charge would do the trick. I charged for a short while this evening and then took the trike for a short run to bring all the cells down a little bit and now the trike is back on charge overnight.

I'll let you know how I go tomorrow commuting to work on the trike.

(Good to know you can charge up individual cell groups using the balance wires. Nice little trick!)

 

[dogman dan]

Cool you should be all ok then. I just assumed they would have been shipped 50-60% charged, 3.4v is so close to full, it won't matter.