We were recently asked by a customer what the charge setting would be if he wanted to lower the voltage settings on his BMS to increase its life span.
Although this is possible it is not something I would recommend, but I would love to have input from other users with their thoughts.
An LFP cell has a very non-linear voltage charge profile. The challenge is that from about 20-90% the voltage curve is very flat and it is difficult to use cell voltage alone to estimate its State of Charge (SoC). Once it drops below that threshhold, the voltage drops very fast for the last few percentage of change and when charging it acts similarily. This is why we have found that active balancing on LFP cells is not the big advantage we would hope for as we really can not enable it with confidence much before 3.4V as we could mistakenly be balancing a cell who's actual capaicity is higher or lower than its cell voltage would indicate, meaning that a cell at 3.30 v suggests that it may be lower than a cell at 3.31, but when charged we find that once it gets into the measurable part of the curve above 3.4 volts that cell might be 3.45v whereas the other cell is only 3.42.
Taking that into account we have may have a bit of flexilibility for adjustment but not much in my opinion.
Note that as there are slight variations in chemistry and anode materials these numbers are only for example only and would need to be tested by the user to ensure it meets his needs. The charge parameters to modify are:
BMS Charging Parameters
- BVOL Balance end voltage = 3.55
- BMIN Balancing start voltage = 3.38
- CHAR Cell End of charging voltage = 3.45
- CMAX Cell over-voltage switch-off =3.60
BMS discharge parameters
- CMIN Cell-under voltage protection switch-off = 2.95
You can see here where the LFP cell chart is very flat in the mid range whereas the LI chart is a gradual and more measurable increase vs SoC.
