72v li-ion battery problems
- Thread starter Halifaxe
- Start date
goatman
10 MW
I find it interesting that the odd numbered pgroups voltages are different than the even numbered pgroups or column 1 is higher than column 2. maybe its the way the bms operates and not the battery.
bww129
100 mW
What battery is it? Are the BMS cell voltage taps wired in exactly the same position for each cell? It doesn't take much extra added resistance to mess up a measurement at high currents: 20A through 1mOhm is 20mV.
bww129
100 mW
Do you know the voltage accuracy of the BMS? Some of the higher quality ICs used in BMSs are accurate down to 1-2mV, but that depends on implementation as well.
You probably shouldn't worry too much about small difference in cell voltage difference under load, especially if the pack is new and uses quality cells. Since you have the ability to monitor cell voltages you can track them over time which will be a better indicator of cell health.
You probably shouldn't worry too much about small difference in cell voltage difference under load, especially if the pack is new and uses quality cells. Since you have the ability to monitor cell voltages you can track them over time which will be a better indicator of cell health.
bww129
100 mW
goatman said:
Some of the ICs used in BMSs have dual ADCs and can sample two voltages at the same time. It is possible that the cells in column 1 were sampled by ADC1 and cells in column 2 were sampled by ADC2, hence what appears to be two different groupings, but it isn't possible to know for sure without any knowledge of the BMS, what ICs it uses, and exactly how it is wired.
did not read all
A balanced pack is only "balanced" **at a particular voltage**
Cells vary, both in capacity and in resistance / performance, the relation between SoC and voltage level.
That is why balancing is needed.
Doing so does not make the above variations go away, just "lines them up" at the top (or bottom or middle)
A balanced pack is only "balanced" **at a particular voltage**
Cells vary, both in capacity and in resistance / performance, the relation between SoC and voltage level.
That is why balancing is needed.
Doing so does not make the above variations go away, just "lines them up" at the top (or bottom or middle)
thundercamel
10 kW
What John said. From what I've seen with my batteries, it's normal for them to all be balanced uniformly at the top of charge. They get out of line a little as the state of charge decreases to empty, but then they all line up again next time they are charged.
