That would certainly guarantee it was full since it corresponds to 3.65 V per cell. But in general you don't need to go that high. Reasons not to go that high are to increase the life of the cells and to avoid any cells going to the destructive 4.3 V if they are not well balanced. How are they balanced? If you're using EV-power CMUs then you will need to go to that high for them to do any balancing.PlanB wrote:I was going to declare the pack fully charged when it hits 58.4v
Coulomb counters will drift. You should reset it to 100% every time it goes to 58.4 V. But I assume that usually only happens once a day anyway.at the end of the first cycle then coulomb count from there to get SOC for any given 24 hr period.
Coulomb counting is the only way to give the user any idea of the SoC when it is between 40% and 95%. But using the coulomb counter to protect the cells from overdischarge is a bad idea, due to the aforementioned drift, and the cell capacity will reduce over time. Protection should be based on voltage (preferably estimated open circuit voltage). If you're happy to let the cells go down to an average of 5% SoC (which means if there is a +-5% variation in cell capacity, some will be at 0% and others at 10%) then you can just use measured voltage, as you can with your high 100% voltage, because you're so far down the steep part of the curve.
But if you want to prevent cells from regularly going any lower than 20% SoC then you will need to compensate the voltage for current times IR, and compensate the IR for temperature.
It's in Weng, Sun and Peng, the first reference I gave above.What would be really nice would be an equation for those cell voltage vs %capacity graphs.
Table 1 equation 6 combined with Table 3 right column.
Have you noticed how the basic curve shape remains constant even though the graph bounces up & down with temperature, discharge rate, etc? If we knew the line equation we could just use dV/d% to infer SOC from rate of change of pack voltage.
An interesting idea. You can see this plotted in Lu, the second reference I gave above.
http://www.cse.anl.gov/us-china-worksho ... %20BMS.pdf
Left hand graph on page 12.
One look at that graph should convince you that this is useless for detecting anything other than > 100% and < 10%. It is constant in the range 20% to 40% and has the same value in this range as it has in the ranges 75% to 78% and 99% to 100%.