Nevilleh wrote: I guess you put your own charger control circuitry in it. Can you show what that is please?
Here is mine:
This is for a TC Charger charger, and is straight out of its manual. I used one of the leftover optocouplers from the BMS modules. Made it up on a piece of veroboard.
The 12V on the left hand side is the normal accessories 12V that feeds the BMS master. The "Charger Control" is from the BMS master.
On the right hand side, the 12V is from the charger. This 12V is not very well isolated from the traction battery, so I treat it as HAZV, using orange wires and conduit. The ENABLE and 0V are also from the charger. This charger expects a voltage on ENABLE from 2-5V. 2 = off, 5 = on full bore, and continuously variable in between. To get this, you pulse the optocoupler on and off.
Neville was prescient in using an output for the charger that could be configured for PWM. Once I worked out how to set it up, using PWM is simplicity itself. Write a higher number to a memory address and the mark/space ratio goes up. Write a lower number and it goes down. This gives very good control to the charger.
I don't have a current clamp, but I do have a Watt meter for 240VAC. So I can tell that my charger uses about 2kW at full bore. This roughly translates to 12A DC delivered to the battery. My BMS modules can bypass about 200mA. I have set up the master to dial back the charger once a cell gets to 3.60V, and the cells also start balancing at 3.60V. Dialed back, the charger uses about 200W, which should be about 1A (the numbers get a bit fuzzy down here, because I don't know how efficient the charger is).
It all seems to be working so far. Without dialing back the charger the voltage on the highest cell quickly went from 3.6 to 3.7 to 3.8 (my "turn off the charger" value). Now that I have it dialing back it takes a lot longer. 200mA is a good fraction of 1A, so all the bypassing cells are getting less charge than the others. Each time I charge I have more cells going into bypass mode. My lowest cell is still 3.1V though when the charger finally turns off.
I do have one concern, I don't have modules on three of the cells. I managed to blow them up, and the replacement microcontrollers are taking a long time to arrive. I probably should go out and measure them with a voltmeter to make sure they aren't too high...