Koyaanisqatsi
Electron addict
After paying close attention to the individual cells during the initial balancing of my new 12V/460Ah battery, I've come up with some various levels of safe voltages and techniques on how to find them for a given pack voltage and increase them safely.
Assumptions:
Cell max safe voltage: 3.65V
Balancing starts at: 3.4V and above while charging or floating.
Balancing threshold: 30mV between any two cells.
Your BMS may be different from above. But the math is the same.
Calculated safe pack voltage:
C = number of cells in the battery pack
"12V" = 4 cells
"24V" = 8 cells
"48V" = 16 cells
The theoretical max pack voltage is
Vpack = 3.65V x C
C=4: 14.6V
C=8: 29.2V
C=16: 58.4V
But that number is impossible to reach safely because at least one cell is certain to be slightly less than 3.65V, which would require at least one other cell to be above 3.65V, and thus not at a safe charge voltage.
Since the balancer will bring the cells to within 30mV of each other, the max pack voltage will vary between the following extreme cases:
1. One cell is 30mV lower than the others. All others are at max voltage.
Vpack = (3.65V x C) - 0.03V
C=4: 14.57V
C=8: 29.17V
C=16: 58.37V
2. One cell is at max voltage, and all other cells are 30mV lower.
Vpack = (3.65V x C) - ((C-1) x 0.03V)
C=4: 14.51V
C=8: 28.99V
C=16: 57.95V
Due to how a balancer brings together the voltages of each cell, the first case is common. But balance will drift over time and charge cycles. All well-balanced packs will have at least one cell at 3.65V, at least one at 3.62V, and the rest scattered between those points.
To be safe, even after balancing a battery pack, we should never charge higher than the second case, subtracting maybe a few mV more for extra padding. This assures there's no cell over-voltage as the balance of the cells changes over time and is affected by usage patterns and storage conditions. Charge above this voltage only under close observation and extreme caution.
This is still optimistic, and assumes you are regularly charging to high pack voltage and holding it there for a while to allow the balancing circuit to do its job. How often and how long are dependent on your setup. If cell balance drifts too far between full charge cycles, even charging to the second case could be dangerous.
When in doubt, start at a lower charge voltage and work up in increments, watching for cell OV or full charge alerts.
The lowest usable charge voltage is the point where the balancing circuit starts to operate. The charge voltage on a pack of unknown balance should be just above the balance circuit voltage (3.4V on my BMS). If you get cell OV alerts at this voltage or lower, your cells are extremely out of balance, and you should focus on balancing the pack.
The same symptoms can appear on new battery packs that have not been top-balanced before assembly. My new battery had cells that were over 150mV out of balance. The first couple charges to what should have been only 4.48V/cell caused cell OV alerts and the BMS shut off the charge FETs. The balancer brought that under control after a few charges to 3.45V/cell. But I still could only go to a relatively low pack voltage before the cell OV alert would hit again. It took 125 hours of holding the voltage at 3.45V/cell to balance the pack enough that I was able to increase the charge voltage. Then I incremented the charge voltage to 3.5V/cell and in small increments to 3.61V/cell, as the cell balance got better.
When "settling in" with a new battery, you need to be gentle with initial charge voltage, but hold at that voltage for many hours to allow cell balancing. Start low and avoid cell OV alerts. If you can monitor cells and balancing activity, you can add up real cell voltages to see what your max safe charge voltage could be.
As the pack becomes more and more balanced, increment the pack voltage in small steps until you have your highest comfortable charge voltage. This value will be used only when you want to balance cells. If your charger is capable, configure it to slowly ramp up to this voltage. I've settled on 14.45V (3.6125V/cell)
Your regular full charge voltage should be above the balancing circuit voltage by at least the balancing difference threshold. In my case: 3.4V balancer turn-on and 30mV threshold, so 3.45V/cell is 50mV above and would be fine. I use 13.97V (3.4925V/cell) because of quirks in my BMS.
Why do we want to balance cells at such a high voltage if the balancer works just fine at lower voltage?
As the pack voltage rises, the voltage difference between cells is amplified. What might have been 25mV difference at 3.45V/cell can become more than 100mV at 3.55V/cell. The balancer circuit operates if the cells are 30mV different or more. Pushing the pack voltage higher causes more balancing, bringing the cells closer to balanced than at lower pack voltage, yielding greater pack capacity.
Why don't we just charge to the higher voltage all the time?
Higher cell voltages cause greater stress and wear on the cells. Maximize pack life by charging to a lower pack voltage regularly, and running scheduled cell balancing every few weeks to few months, depending on your setup.
Assumptions:
Cell max safe voltage: 3.65V
Balancing starts at: 3.4V and above while charging or floating.
Balancing threshold: 30mV between any two cells.
Your BMS may be different from above. But the math is the same.
Calculated safe pack voltage:
C = number of cells in the battery pack
"12V" = 4 cells
"24V" = 8 cells
"48V" = 16 cells
The theoretical max pack voltage is
Vpack = 3.65V x C
C=4: 14.6V
C=8: 29.2V
C=16: 58.4V
But that number is impossible to reach safely because at least one cell is certain to be slightly less than 3.65V, which would require at least one other cell to be above 3.65V, and thus not at a safe charge voltage.
Since the balancer will bring the cells to within 30mV of each other, the max pack voltage will vary between the following extreme cases:
1. One cell is 30mV lower than the others. All others are at max voltage.
Vpack = (3.65V x C) - 0.03V
C=4: 14.57V
C=8: 29.17V
C=16: 58.37V
2. One cell is at max voltage, and all other cells are 30mV lower.
Vpack = (3.65V x C) - ((C-1) x 0.03V)
C=4: 14.51V
C=8: 28.99V
C=16: 57.95V
Due to how a balancer brings together the voltages of each cell, the first case is common. But balance will drift over time and charge cycles. All well-balanced packs will have at least one cell at 3.65V, at least one at 3.62V, and the rest scattered between those points.
To be safe, even after balancing a battery pack, we should never charge higher than the second case, subtracting maybe a few mV more for extra padding. This assures there's no cell over-voltage as the balance of the cells changes over time and is affected by usage patterns and storage conditions. Charge above this voltage only under close observation and extreme caution.
This is still optimistic, and assumes you are regularly charging to high pack voltage and holding it there for a while to allow the balancing circuit to do its job. How often and how long are dependent on your setup. If cell balance drifts too far between full charge cycles, even charging to the second case could be dangerous.
When in doubt, start at a lower charge voltage and work up in increments, watching for cell OV or full charge alerts.
The lowest usable charge voltage is the point where the balancing circuit starts to operate. The charge voltage on a pack of unknown balance should be just above the balance circuit voltage (3.4V on my BMS). If you get cell OV alerts at this voltage or lower, your cells are extremely out of balance, and you should focus on balancing the pack.
The same symptoms can appear on new battery packs that have not been top-balanced before assembly. My new battery had cells that were over 150mV out of balance. The first couple charges to what should have been only 4.48V/cell caused cell OV alerts and the BMS shut off the charge FETs. The balancer brought that under control after a few charges to 3.45V/cell. But I still could only go to a relatively low pack voltage before the cell OV alert would hit again. It took 125 hours of holding the voltage at 3.45V/cell to balance the pack enough that I was able to increase the charge voltage. Then I incremented the charge voltage to 3.5V/cell and in small increments to 3.61V/cell, as the cell balance got better.
When "settling in" with a new battery, you need to be gentle with initial charge voltage, but hold at that voltage for many hours to allow cell balancing. Start low and avoid cell OV alerts. If you can monitor cells and balancing activity, you can add up real cell voltages to see what your max safe charge voltage could be.
As the pack becomes more and more balanced, increment the pack voltage in small steps until you have your highest comfortable charge voltage. This value will be used only when you want to balance cells. If your charger is capable, configure it to slowly ramp up to this voltage. I've settled on 14.45V (3.6125V/cell)
Your regular full charge voltage should be above the balancing circuit voltage by at least the balancing difference threshold. In my case: 3.4V balancer turn-on and 30mV threshold, so 3.45V/cell is 50mV above and would be fine. I use 13.97V (3.4925V/cell) because of quirks in my BMS.
Why do we want to balance cells at such a high voltage if the balancer works just fine at lower voltage?
As the pack voltage rises, the voltage difference between cells is amplified. What might have been 25mV difference at 3.45V/cell can become more than 100mV at 3.55V/cell. The balancer circuit operates if the cells are 30mV different or more. Pushing the pack voltage higher causes more balancing, bringing the cells closer to balanced than at lower pack voltage, yielding greater pack capacity.
Why don't we just charge to the higher voltage all the time?
Higher cell voltages cause greater stress and wear on the cells. Maximize pack life by charging to a lower pack voltage regularly, and running scheduled cell balancing every few weeks to few months, depending on your setup.
