Sorry, this got a bit carried away, the techo in me escaped ....
Been building lithium batteries for 14 yrs now and I know what works and what doesn't, and the toy BMS and balancer in those prepacked batteries simply don't work for the capacity battery they are connected across. I've had roughly 30 brought in for me to tell them what is wrong with them and write a report as to why they failed so they could make a warranty claim.
The BMS and balancer used in these drop-in batteries was designed for push bike and toy batteries. They use a loss balance system that creates a controlled short across the cell through a resistor at a set voltage point. These resistors turn electrical energy into heat energy .... inside a sealed plastic case. They are designed to balance 1ah cells, not 40Ah cells, so when that cell voltage goes high, it will take those little resistors 20hrs to burn off 1ah .... the resulting heat build up burns out the zener diode and the resistor just remains on continuously .... eventually fully discharging the cell and because the cells can't be accessed, the battery becomes a throw away ....
The induction coil cell equalisers charge up each induction coil connected to that cell, then disconnect from the cell and link together so the voltage is equalised across the induction coils, then it links back to the cell.
If the voltage across the induction coil is higher than the cell voltage, the induction coil discharges into the cell until the induction coil and cell voltage as the same ......
If the cell voltage is higher than the induction coil voltage, it charges the coil until the voltages are the same between the cell and the coil
Then the cycle repeats
This way, any cell that is a higher voltage, discharges some of it's capacity into the cells with a lower voltage, but this takes place at cycles per second, so the cells are balanced very quickly and remain balanced when charging or discharging .....
The upper voltage for an LFP cell is 3.6v, past that and heat is being generated inside the cell, heating the electrolyte till it separates depositing rubbish across the graphite material coated plate, reducing the rate lithium ions can be collected and released, and causes the lighter elements in the electrolyte to boil off and gas, this is what causes the cells to swell .....
14.4v across a 4 cell LFP battery is 3.6v per cell if perfectly balanced, if they are not perfectly balance, at least one cell will be at a much higher voltage and another cell at a much lower voltage, a battery in the process of being murdered ....
T1 Terry
