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Old 07-02-2020, 01:22 PM   #1
Anthmatic2
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Default What's an acceptible cell difference?

Most of my leaf cells stay within the same voltage range while charging and discharging but the last battery has dipped lower while discharging a few times.

I've had cell differences of up to .1v which doesn't seem like much but I assume that the voltage difference could get pretty big. The BMS (Chargery) will balance the cells but obviously the harder the BMS has to work, the less life I can expect out of it.

I'm going to try to measure the internal resistance of that last cell, but I was curious if anyone else sees numbers like this? Does this seem reasonable or out of range?
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Old 07-02-2020, 01:59 PM   #2
DaveTM
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Default Re: What's an acceptible cell difference?

This is not unusual after the end of a charge.


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Old 07-07-2020, 05:48 AM   #3
Anthmatic2
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Default Re: What's an acceptible cell difference?

Thanks DaveTM. I haven't seen anything higher than this so hopefully in the clear
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Old 08-16-2020, 09:58 AM   #4
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Default Re: What's an acceptible cell difference?

How about cell difference during discharge?

My carts been running fine for weeks but all of the sudden today I had one cell drop down 400mV while driving/discharging.

I’m wondering if that battery’s “knee” is higher than the others and I just haven’t hit it before?
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Old 08-16-2020, 02:22 PM   #5
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Default Re: What's an acceptible cell difference?

Recently I noticed my cells, although balanced after a charge were fine, but when approaching the lower end of a discharge were out of balance more than I cared for them to be. (I don't recall the specifics).

My ZEVA has a "Dynamic Balancing" function that will constantly balance cells during a charge or a discharge.

I set it to Dynamic Balancing for the next cycle, and once done I changed it back. Now the cells are discharging at about the same rate with no large fluctuations.

If your Chargery doesn't offer that option, you may wish to manually bottom balance the cells and see if that helps.
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Old 08-17-2020, 09:51 AM   #6
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Default Re: What's an acceptible cell difference?

Chargery recommends that you set cell difference during discharge to as high as 800mv. During discharge, cells can vary a good bit. At least according to them, that is fine. Setting for balance during charge is 60mv, still more than what DaveTM is seeing. His are really close.

I've set my chargery at 600mv and 60mv for balancing during charge. I've not read anything that you need your cells balanced much closer than that. Hopefully, one of the experts will come along and correct me if wrong.
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Old 08-17-2020, 12:58 PM   #7
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Default Re: What's an acceptible cell difference?

IMHO (and my opinion and $1.25 will get you a Coffey) when my cell voltages differ by 100 millivolts, I get "concerned".
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Old 08-20-2020, 07:50 AM   #8
Anthmatic2
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Default Re: What's an acceptible cell difference?

I'm surprised that 800mV would be considered acceptable. I agree that anything over 100 sparks some concern for me. I'd be curious to see if any experts chime in on that.

After charging the pack back up, the cell difference came back into line (<60) very quickly. I'm still thinking that there might be a capacity issue with one battery module. I'm going to try to learn some more about these batteries and I'll report back.
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Old 08-20-2020, 08:16 AM   #9
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Default Re: What's an acceptible cell difference?

The actual capacity of the batteries is what controls the differences in voltage between top and bottom.

Here are some previous posts I made on this subject:

Quote:
Originally Posted by Sergio
Good questions, I will add more detail in case other members have similar questions.

The concept of "balancing" is to bring cells to the same known and returnable SOC level.

The analogy is that the cells are like water cylinders of the same height but different diameters and therefore hold different amounts of water.

If You stack them side by side and remove 1/2 of the water from each, the water will be at the same level in all of them, but the amount of water you removed from each cylinder was different.

Lithium cells connected in Series have the same amount of energy removed from each of them.

In the water cylinder example above if some of the cylinders have 60 gallons capacity and some 40 gallons capacity, removing 20 gallons from each of them will result on the 40 gallon cylinders being at 50% level/SOC and the 60 gallon cylinders being at the 66% level/SOC.

If you then add 20 gallons back to each cylinder (charging), they all go back/return to the 100% level/SOC.

That is why it only makes sense to use a BMS to top balance since we know if the cells have different capacities their SOC and voltage will differ as You discharge them in Series.

Some folks prefer to bottom balance the pack or bring the water level to near empty 0%level/SOC and not use a BMS at all.

As you add 40 gallows to each cylinder, You will fill up the 40 gallons but only reach the 66% level/SOC of the 60 gallons cylinder.

This is again expected since all you are concerned is that when You again remove 40 gallons they will all return to 0% level at the same time.

Differences in manufacturing or changes in internal resistance as cells age take a long time to develop and it is only a concern for folks that bottom balance their pack since a BMS is always top balancing and easily corrects those differences.

Sometimes the differences the BMS is correcting are introduced by the BMS setup itself due to differences in the ADC converters or wiring and connections resistance.

If You have positive experience doing some kind of "Mid Balancing", please share with us.


As far as increasing the longevity of Lithium packs by not charging them to 100% SOC, that is a very well documented fact due to stress caused to the cells.

Just look at this one article for example, Table 4 indicates how much longer You can expect your pack to last if You don't charge to 100%.
https://batteryuniversity.com/learn/...ased_batteries
You can also look at pretty much all electric cars on the Market and their "Daily charge" vs "Long Range" charging recommendations.

Quote:
Originally Posted by Sergio View Post
Just like you mentioned, unless your cells are perfectly matched in capacity, they will deviate in voltage from each other at different SOCs.

Exaggeration for purpose of illustration, a 100AH cell and a 50AH cell will both be at their 100% SOC at the same voltage level.

Now if you take 40Ah out of each (which is what happens when they are drained in Series), the first one will be at 60% SOC while the other one will be at 20% SOC and have totally different voltages.

If You try to balance at that time, the BMS will continue to drain the 60% SOC cell trying to get it to the same voltage level as of the 20% SOC cell.

Now when you charge then back again, the opposite happens, the BMS needs to keep discharging the 20% SOC cell since it is coming up in voltage much more than before as the bigger capacity cell now has more "room" after being discharged by the BMS shunt.

Basically the BMS cannot get out of its own way to stop the unnecessary current shunting.

My suggestion on that thread is to use the dynamic balancing (for extended shunt time) until the cells are balanced when at the top (end of charge), then switch back to the normal method to maintain the pack.

This also explains why just "top balancing" and then running without a BMS to provide LV protection is a bad idea since there is no guarantee that the cells will continue to have a perfect slice of that "average voltage" and one cell could easily bottom out without the pack getting below minimum value.
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