Roswell Bob
Solar Enthusiast
I am curious if the bargain cells (that many seem to be happy with) have any problems staying balanced? Are there some balancers which do a better job than others?
Is balancing an issue with lesser cells?
- Thread starter Roswell Bob
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joanna_lannie
Maybe sleepwalking now
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To be honest, I don't quite understand what you're talking about. But battery balancing should be the basis, BMS has a certain balancing function, but it would be better to have a balancer. If neither, maybe you need to consider manual balancing.
John Frum
Tell me your problems
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Yes.
The lower grades of cells tend to have higher variance in capacity and internal resistance.
Both of these factors contribute to their tendency to drift.
Yes.
Andy from @Off-Grid-Garage has recent videos on building a pack with balance leads connected to a terminal block.
Makes it easy to connect a bms and an optional second device(think active balancer).
I did the same thing when I made my diy battery.
So far I have not needed to add an active balancer though.
To minimize the drift...
Top balance during the commissioning process.
Charge into the high knee on a regular basis.
Enable balancing on your BMS but only in the high knee and above float voltage.
Don't discharge the pack so far that weak cell nosedives into the low knee.
This just makes the weak cell weaker.
Roswell Bob
Solar Enthusiast
I spent top dollar for 'matched' grade A cells. I've been running for a couple of months and see no evidence of any balancing. They follow each other very nicely. I am curious to know if I would have had any problem had I purchased the cells I was offered at a reduced price. Hope this clears it up.
RCinFLA
Solar Wizard
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The more unmatched the cells, the faster they will deviate in state of charge balance.
Other thing that accelerates state of charge imbalance is running high discharge current rates. High current amplifies effects of mis-matching of cells.
As cells age, their internal impedance rises . All cells do not necessary degrade at the same rate.
Matching means for the same current, each cell has the same overpotential terminal voltage slump. Overpotential voltage slump is greater the higher the percent C(A) current rate. Same AH capacity does not always mean the same load current overpotential voltage drop per cell but they are closely related.
Overpotential voltage slump is also a function of cell temperature so if due to cell physical location in battery array it runs warmer or cooler it affectively reduces cell matching. This will cause some cells to age faster than others. Greater overpotential voltage slump for given cell current means greater cell internal losses, and internal self heating at currents above 0.5 C(A).
You will really start to notice the increase in cell voltage slump below +10 degs C. with moderate load current.
Older cells have higher impedance and greater internal losses so they may get warmer at moderate discharge current rates. When it is said an old cell has 80% capacity, it is not the loss of capacity that hurts the most, it is the greater terminal voltage slump with moderate load current that is usually more of an issue to actual use case performance.
Overpotential voltage slump takes one to three minutes at fixed current rate to reach equilibrium and has a exponential decay. Same for no-load recovery time to rested unloaded cell state. At low state of charge (<15%) the time to reach equilibrium increases.
Right after full charge there will be surface capacitance charge that artificially raises cell voltage above full charge voltage of 3.43v to 3.45v. The surface capacitance is caused primarily from negative graphite layer and only amount to about 0.01% of cell capacity. It can be dissipated with a 1 to 3 ohm power resistor in less than a minute after which the cell unloaded rested voltage will be 3.43v to 3.45v. If you do not dissipate the surface charge manually, a cell at 3.65v from the charger may take a day to several days to drop to 3.45v on its own leakage current rate.
Other thing that accelerates state of charge imbalance is running high discharge current rates. High current amplifies effects of mis-matching of cells.
As cells age, their internal impedance rises . All cells do not necessary degrade at the same rate.
Matching means for the same current, each cell has the same overpotential terminal voltage slump. Overpotential voltage slump is greater the higher the percent C(A) current rate. Same AH capacity does not always mean the same load current overpotential voltage drop per cell but they are closely related.
Overpotential voltage slump is also a function of cell temperature so if due to cell physical location in battery array it runs warmer or cooler it affectively reduces cell matching. This will cause some cells to age faster than others. Greater overpotential voltage slump for given cell current means greater cell internal losses, and internal self heating at currents above 0.5 C(A).
You will really start to notice the increase in cell voltage slump below +10 degs C. with moderate load current.
Older cells have higher impedance and greater internal losses so they may get warmer at moderate discharge current rates. When it is said an old cell has 80% capacity, it is not the loss of capacity that hurts the most, it is the greater terminal voltage slump with moderate load current that is usually more of an issue to actual use case performance.
Overpotential voltage slump takes one to three minutes at fixed current rate to reach equilibrium and has a exponential decay. Same for no-load recovery time to rested unloaded cell state. At low state of charge (<15%) the time to reach equilibrium increases.
Right after full charge there will be surface capacitance charge that artificially raises cell voltage above full charge voltage of 3.43v to 3.45v. The surface capacitance is caused primarily from negative graphite layer and only amount to about 0.01% of cell capacity. It can be dissipated with a 1 to 3 ohm power resistor in less than a minute after which the cell unloaded rested voltage will be 3.43v to 3.45v. If you do not dissipate the surface charge manually, a cell at 3.65v from the charger may take a day to several days to drop to 3.45v on its own leakage current rate.
