Pylontech HV balancing
- Thread starter topaz
- Start date
Korishan
Administrator
- Joined
- Jan 7, 2017
- Messages
- 7,746
Not quite balanced, but pretty close. If you charge to 100% they should finish out balanced.
I mean, <5mV deviation is really good, tbh.
For comparison, I have some LiFePO4 cells that's been being cycled every day <10% of capacity and they have drifted about <15mV after 6 months of use, without a bms connected, and they are still well within their specs. (I have the bms now & will be installing over the weekend)
Generally 20mV is a good deviation value and you want anything higher than that to be balanced.
Curious, you have 8 cells shown, but their respective numbers are way different. Are these part of a much larger bank, or just cells that were closely matched and retain their labeling from the sorting/evaluating process?
I mean, <5mV deviation is really good, tbh.
For comparison, I have some LiFePO4 cells that's been being cycled every day <10% of capacity and they have drifted about <15mV after 6 months of use, without a bms connected, and they are still well within their specs. (I have the bms now & will be installing over the weekend)
Generally 20mV is a good deviation value and you want anything higher than that to be balanced.
Curious, you have 8 cells shown, but their respective numbers are way different. Are these part of a much larger bank, or just cells that were closely matched and retain their labeling from the sorting/evaluating process?
Ok, so I'm gonna wait till 100% then.
Impressive for you battery pack to derive so little over 6 months.
The serial number is the one from the pack to better match visually when I had to swap the banks to charge evenly because I bought the batteries in many waves (3 then 1 more then 4 more).
Impressive for you battery pack to derive so little over 6 months.
The serial number is the one from the pack to better match visually when I had to swap the banks to charge evenly because I bought the batteries in many waves (3 then 1 more then 4 more).
OffGridInTheCity
Moderator
- Joined
- Dec 15, 2018
- Messages
- 2,893
@topaz - Agree with @Korishan that <5mv is good balance.
@Korishan's level of minor drift over several months is perfectly reasonable in my experience. I'd be happy with <5mv and monitor things over the next 6mos, 12mos and if it stays that way... up to the 20mv then you're doing very well.
This pic is from this morning on a DC House 16s 48v @ 50ah battery I have hooked to an APC UPS. It's been sitting for several months (almost fully charged because of APC UPS) and this is as good as I've seen it balance.... #9 at 3.459v is 15mv less than the 3.474 cells but it's been stable like this for several months so I'm not worried about a unhealthy cell and this does not affect operations.
@Korishan's level of minor drift over several months is perfectly reasonable in my experience. I'd be happy with <5mv and monitor things over the next 6mos, 12mos and if it stays that way... up to the 20mv then you're doing very well.
This pic is from this morning on a DC House 16s 48v @ 50ah battery I have hooked to an APC UPS. It's been sitting for several months (almost fully charged because of APC UPS) and this is as good as I've seen it balance.... #9 at 3.459v is 15mv less than the 3.474 cells but it's been stable like this for several months so I'm not worried about a unhealthy cell and this does not affect operations.
Last edited:
not2bme
Member
- Joined
- Oct 16, 2017
- Messages
- 523
Everyone's right on the money on the drift of the batteries over time. With lithium ion batteries you shouldn't have major drifts between cells over a period of time. If you do then you have issues with your batteries.
Now the deviation is another story. Depending on how balanced your pack are, (ie. if they're from a same source or if they're from all different ages and sizes), these deviations can vary greatly. Now going to 100% may run into large deviations if the capacities of the cells are different, as the curve of going from 90% to 100% is very fast so you will see wild changes. You can 'top balance' the pack so that at 100% your packs can be perfectly balanced at 100% but when you get down to 10% it may deviate a lot. So what you want to know is deviation between 20-80% where it is more stable.
Below are examples of my packs.
Left Bars - My Nissan Leaf packs. They came from the same battery so they're perfectly in line, no matter at 20% or 80% they remain under 15mV. I've never charged them to 100% and since it's winter, my solar production is low so it's rare I even get to 80-90%
Right Bars - My Original pack deployed 7-8 years ago. B1-B7 have a slightly lower capacity than B8-B14, so even the deviation is great, the groupings between them is very close. So you can see 40mV difference but I'm not worried. I just know if it grows to 100mV then I have to worry. This is also another thing, I set my balance high at around 80mV. That way I can notice if one pack is drifting higher than others. That would indicate if there's an issue with the health.
Edit: Oh I forgot to mention my cells are 18650s or NMC batteries where the curve is fairly linear. I have no clue about LFP or LiFePO4 batteries, as their curves are fairly flat throughout the capacity and I have no experiences on how they would behave...
Now the deviation is another story. Depending on how balanced your pack are, (ie. if they're from a same source or if they're from all different ages and sizes), these deviations can vary greatly. Now going to 100% may run into large deviations if the capacities of the cells are different, as the curve of going from 90% to 100% is very fast so you will see wild changes. You can 'top balance' the pack so that at 100% your packs can be perfectly balanced at 100% but when you get down to 10% it may deviate a lot. So what you want to know is deviation between 20-80% where it is more stable.
Below are examples of my packs.
Left Bars - My Nissan Leaf packs. They came from the same battery so they're perfectly in line, no matter at 20% or 80% they remain under 15mV. I've never charged them to 100% and since it's winter, my solar production is low so it's rare I even get to 80-90%
Right Bars - My Original pack deployed 7-8 years ago. B1-B7 have a slightly lower capacity than B8-B14, so even the deviation is great, the groupings between them is very close. So you can see 40mV difference but I'm not worried. I just know if it grows to 100mV then I have to worry. This is also another thing, I set my balance high at around 80mV. That way I can notice if one pack is drifting higher than others. That would indicate if there's an issue with the health.
Edit: Oh I forgot to mention my cells are 18650s or NMC batteries where the curve is fairly linear. I have no clue about LFP or LiFePO4 batteries, as their curves are fairly flat throughout the capacity and I have no experiences on how they would behave...
Last edited: