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How unbalanced
#1
My packs are out of balance, just not sure what's too bad.
My cheap bms does top end balancing, but I don't charge them enough to get that benefit.

Two strings are around 40-50mv out of balance, idk but I figure that's not too bad.
But the third is currently 70mv apart. I noticed yesterday that it was 100mv different. So I hooked up an imax to just that pack and get it down to like 50mv.
After it rebounded and recharged I'm sitting at .07v difference in that string.

I am about to manually balance it a bit more and see where it's at tomorrow.

But how unbalanced is too much?
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#2
Hi, if the packs are recently build may happens, after test them and let rest for some weeks not all cells stay at 4.15 or more. I use the cells that discharge no more than 4.0. So maybe after build the packs may be unbalanced.
Charge the third pack until reachs the others and check after some days, if this pack goes unbalanced again some cells in this pack must be bad and they selfdischarge.
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#3
70mV is not too serious yet but starting to climb.

You may need quite a long charge, holding the pack at the top to let a cheap balancer get the cells equal.
Cheap balances only have a small balance current, eg 50mA, so if some cells are low, they only get about 50mA while balancing.
This 50mA might take many hours or even a day or two to balance.

Manual balancing would be faster. Remember to hold the cell at the full voltage for a while until the current has dropped away, that means the cell really is full.

Like jesusangel says above, if some cells drop in voltage much more than others, you might have some bad cells, either self discharger (if voltas drop just sitting) or maybe lower capacity (if pack is used).
Running off solar, DIY & electronics fan :-)
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#4
Good to hear.
I would guess that pack has higher capacity than the rest. As the lower packs have a voltage closer to the packs in other strings.

I have it manually balanced now by discharging it, not by giving it a full a balance charge.
we'll see what it looks like tomorrow.
I also have those cheap buzzer voltage monitors hooked up now, so I can keep a closer eye on it.
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#5
It's not about how much it is unbalanced, but more about how fast it goes unbalanced. I've been without my BMS for about 2 months now (just lazy to put my replacement in since it's cold) but I check on it once in a while. It's off by 50mV now. When I was on the BMS it was off by no more than 25mV.

You need to start with a good set of battery ranges. If you mix 1800 with 2800, you will find they just don't work well as one drains faster than the other then it throws the other's out of whack. I stick mine to a solid range of no more than 300mah difference between every cell. Repackr is a waste of time, tedious output and repacking for something that will eventually deviate if you have a wide range to begin with.
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#6
Agree with not2beme, you also need to be watching for drift/self discharge.
That said, if the pack is recently built, you also have to get in into being balanced at first too.
Running off solar, DIY & electronics fan :-)
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#7
(01-23-2019, 09:14 PM)Dmcbudman Wrote: But how unbalanced is too much?
Basically, you loose capacity.  (by unbalancing)

So your question reconfigures to:
How much capacity can you afford to be lost ?
DCkiwi likes this post
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#8
I have 3 sets of 14s120p (48v) = 42packs (a mix of 6,000 cells mostly from alarmhookup / used but good). Each pack was built different cell counts to keep overall packs at <1% difference between them. I have run (for longer life) at max charge of 56.0v(4.0v/pack) and low (inverter off) at 47.5v(3.4v/pack) and have cycled over 150 times so far.

I recently got my Batrium BMS online, did some balancing around 3.9v/pack but then stopped (set Batrium balance to kick in at 4.10v) because for the most part, 37 out of the 42 stay (up and down) withing 0.06v of each other. I have 5 'bad packs' that I am in process of fixing that are 0.11 outliers - but setting those aside.

My question is.. based on comments above...
Isn't is OK for my packs to be withing 0.06v as they go up/down daily - especially since my operating range is 4.0v to 3.4v so worst case is 4.06 to 3.34v. And that using Batrium balancing to try to keep them perfect (withing 0.2v?) is overkill / wastes power?.
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#9
0.2V or 0.02V?

0.2V is too much and any pack that is that far out from the others needs attention. Even 0.1V difference could lead to a major issue.
0.02V is fine, but I'd be worried if the variance starts to increase. 0.4 is when you'd really want to start tracking down the issue.

Also, with the increase in cells in parallel, that variance increases needing attention as the Wh of wasted energy goes up quickly. And this also makes the bms work harder to keep everything in balance.
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#10
I'd suggest 0.06V (ie 60mV) is not too bad for variance.
The Batrium cellmons have 10mV resolution.
Top balancing (like the Batrium gear does) should be targeting the same voltage anyway.
In the Batrium s/w, under Menu > Hardware > Cellmon > Bypass Extra Mode have you enabled "Auto Level"?

You mentioned you have different numbers of cells per pack to get equal capacity. This does mean cells in packs with less cells would get worked slightly harder, but this shouldn't be enough to make a difference.

Part of the question is: "what's the trend?"
The packs you mentioned that are 0.11V (110mV) out, do they
- both get to minimum (go flat first) & maximum (get to full earliest)? Eg the cells have lost capacity.
- never get to full voltage (always lag lower), eg they have 1 (or maybe more) self dischargers or higher capacity than the rest.
Some packs internal resistance may have climbed = uneven charge/discharge vs other packs.
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