what is acceptable difference in voltage between packs?

[jondoh]

Good afternoon fellow collective members,

i'm using my milwaukee packs and after 1500 miles or so, they are showing their age. part of the problem, i think, is that i didn't buy them all at the same time (4 packs bought 11/06, 2 packs bought 3/07, 2 packs bought 5/07). i've tried to even out the cycles but i'm getting some slight differences in pack voltage. maybe i'm over reacting a bit but i want to get an idea of what is an acceptable difference in voltage.

right now, 2 of my older packs are showing 28.5v while the rest are showing 28.8v. that's a difference of only 0.3v but since lithiums don't tend to drop much under load, it's more significant than SLA or nicad. the milwaukee batteries are 7 cells in series. i'm running 14s3p (6 packs in total). They're working fine but sometimes after a ride, when i test them, some show 3 bars while other show only 2 bars.

on my next ebike project, i plan to buy and use 12 packs at once so (hopefully) i won't run into this problem again.

anyway, is 0.3v something to be concerned about?

 

[Ypedal]

Good question.

My LiMn packs show an even greater difference, ( 1.xv between each pack ) and i run them in series, but i never go to BMS cutoff. Mine are NOT the E-moli cells tho.

0.3v difference sound fair to me, only way to know for sure is to check each cell on charge-discharge *( Got taps ? )

 

[xyster]

That's a .04v difference between cells -- less than 5%. My LiCo cells show that were bought at different times show about the same difference. 5% loss over 6 months doesn't sound bad if it continues at that rate.

 

[xyster]

Oh.... like all lithium chemistries, for longest life charge up just before a ride instead of just after. That way, the average voltage level is lower. Longevity is an inverse function of Time @ Voltage @ Temperature.

 

[GGoodrum]

With RC chargers and balancers, a pack is considered "grossly" out of balance if the cell voltage difference between the lowest and highest is above 0.2V. Most integrated charger/balancer combos will switch to a "balance charge" mode, where the charge current is reduced to .3A until the balancers can get the difference down to below .2v.

-- Gary

 

[jondoh]

I noticed something just now that i haven't noticed before. The connector blocks are supposed to click and lock into the pack. Well i guess when i bought my packs a year ago, they hadn't worked out the tolerances yet. some of my packs won't lock unless you slightly depress the front part of the release levers on the pack. While riding, the connector blocks vibrate out of the pack. This probably isn't good for the pack-- intermittent connections are not good for anything. i'm going to start to make sure the packs are locked in place and give it a few cycles and see what happens.

 

[Beagle123]

Another angle: minor differences could also be due to the chargers. Electronic components have variations of +- 5% to 10% so you probibly won't get exactly the same charge from each charger. Likewise, I doubt those little lights function exactly consistently. I think it sounds pretty good. I get slight variations in the individual v28 cells. +- 0.03 volts or so if I remember correctly. Again, I'm sure it was from the charger.

 

[erdurbin]

I would not worry about that at all. If you get up around 1.5-2 volts difference, then I would say it is time to retire. I rebuilt some 36 volt packs out of the 28 volts and I have many different age cells. They can take an overcharge up to around 4.6 volts, but go quickly back down to the standard 4.2 one you unplug the charger. I have never had any problems with overcharging an unbalanced pack. While one battery goes up to 4.6, you have another around 4.19. The 4.19v stays once you unplug and the 4.6 drops back down to 4.2. If your controller has a low voltage cutoff, there is nothing to worry about. My controller cuts out around 31.5v so I really never get the full potential of my batteries, but they are still good for about 10 miles. I have a 10s4p pack (40 cells, 12ah). The whole pack (inside a plastic toolbox) only weighs 13lbs.