Pack Balancing: How Close before you consider them balanced?

[RLT]

I'm using 'single cell' charging to try to keep my battery packs balanced., due at first to a bad experience with a crappy BMS, and now because I'm running an 'in between' voltage (44V, nominal) on my 2WD project

How close do you guys think that the cells (or, rather, parallel groups of cells as I'm using) should be before you can reasonably call them balanced?

For instance, my Voltphreaks LiFe chargers cutoff vary from 3.83V to 3.91V, and after most of the surface charge runs out, after a couple of hours, the resting charge of the individual cells will vary from 3.63V to 3.80V.

And the single cell LiIon chargers cutoff range is anywhere between 4.29V to 4.24V... And unlike the Headways I'm working with, My 32650 LiCos only SEEM to lose only .04 to .08V off the surface charge even if they sit for a few days.... (I haven't actually documented that, but casual observation gives me that idea.)

I imagine that once I get my big 10A to 200A charging system setup and all the batteries are getting the same voltage at the same time, things will be closer, but I'm kind of curious to know when 'close enough is close enough'.

 

[stew007]

the charger I use balances the cells in my lipo/a123/lifepo4 packs to .002v on each charge, so that keeps the packs perfectly tuned and ready to go, an out of balance pack probably wont last long.

 

[slayer]

i think that anything over 3.40 volts does not hold any ah ...and that the reason they choose 3.65 volts is because it is stilll in the safe zone and can be match with already existing sla chargers ...in other words anything over 3.4 volts is a battery that is full but some of them have ability to hold that overcharge ...if you try putting a small load on them for 1 minutes you should see the real voltage they are ...where they can be imbalanced is mostly in the ah they deliver

 

[bquick]

I don't think it really matters how closely they are balanced. The idea is that you don't want one cell's voltage to get way off over time such that that cell goes under critical voltage when the pack is drained. As long as you do any sort of balancing, you should be OK.

- Brad

 

[slayer]

yes over times discharging the pack ah to your weakest cell is in ah is the real danger and where it can cause them to go imbalanced but if you recharge with single cell charger you should be fine ...but you would still have to worry about the low voltage or make sure you don t discharge the pack too low say if you have lots of ah to spare .

 

[johnrobholmes]

With LiFe, I shoot for 0.05 v, but will be satisfied with .1v if there is a bad cell showing up. Depending on the quality of the cells and your voltage cutoff profile, you can get away with pretty large voltage imbalances with life. Lipo, I would not allow more than .1v difference before a cell gets scrapped. After balance Lipo should be within 0.01v charged, then no more than 0.1v discharged. You can find the weak cells after a balancing charge this way. The ones with lower voltage have less capacity.


I have found that when LiFe cells start to go bad, they won't charge up fully (voltage). So you work with a cell that is starting from a lower voltage and corresponding capacity. With Lipo the voltage will still hit 4.2v charged, but the voltage falls fast under load and capacity suffers likewise.

 

[Link]

My multimeter isn't able to go low enough to see how close mine are balanced. They're all at 3.33V. I'd say as long as you're getting near full capacity out of the pack and the cells are within 0.2V of each other, you're probably okay.

 

[RLT]

Thanks Guys,
(Especially Link and John who actually figured out what I was TRYING to ask.)
Guess I wasn't too far off in my expectations.

 

[Patrick]

Kind of a tricky question. I think the best answer is "the closer the better".

I'm sure it depends on both the chemistry and the manufacturing of the cells. Unfortunately, there is no way to know the long term results as there isn't any long term with these cells just yet. You can look to the vendors for some answers. Naturally, being vendors, the ones who sell batteries say one thing, and the ones who sell chargers another.

ThunderSky says +/- 100 mV is OK for checking batteries. This is not specific to charging, just "general" info in their battery spec. I personally think this is a little too generous, and like to see +/- 50 mV; this is a resting voltage 24 hours after charge. And this is for cells charged in parallel.

The battery charge folks want to sell product, so they stress the specifics of their solution. You can find an interesting product at: 1powersolutions.com ; they balance to 2 mV. What I think is most interesting is that they charge to 4 V/cell, rather than the 3.7 that is more common. There are some discussions going on about proper charge termination, and no answers yet.

Does that help?