Cell Balancing Strategy for MC337761C under load condition

[luthersam]

Hello,

I am trying to derive a strategy for cell balancing using MC337761C battery cell controller.

On going through the data sheet I could understand that the minimum cell balancing time required in 0.5 minutes (30 seconds). So considering the cell voltage is a dynamic quantity and is going to considerably vary on a dynamic scenario while the cells are under load, the 30 seconds is too long a time to wait for the cell balancing time to complete. I mean even if we finalise a cell balance target voltage to which all cells must be balance that target is not going to same even after 50 ms while the cells are under load.

I can understand with this cell balancing time,  it can be comfortably done during a sleep mode / no load condition.

The data sheet describes it can be done during a normal scenario. Normal scenario I understand as a scenario while cell are under load.

1. Can any body guide me how this can be done while the cell is under load.

2. Do you have any application note to guide me on this ?

Regards

Sam

[luthersam]

Thanks Jozef. That was very helpful. 

I have some couple of queries more: 

1. I found there is a Manual Pause bit in SYS_CFG register. So If i pause the cells which was undergoing a balancing should I still wait for 3 ms before I read the registers for a measurement. [ I was seeking alternate ways not to stop the cells from balancing before I take the measurement because of 3 ms delay time involved ]. 

[JozefKozon]

Hello Sam,

yes, after any balancing if you want to perform accurate cell measurement sequence you should wait certain amount of time, depending on the filter type. For the filter type in the Table 88. in the MC33771C datasheet the recommended waiting time is 3ms. Even if you only pause the balancing with CB_MANUAL_PAUSE bit. 

With Best Regards,

Jozef

[luthersam]

Thank you Jozef for your support. I was off so could not reply to your message. 

[JozefKozon]

Hello Sam,

you are very welcome. No problem at all. Wish you merry Christmas and Happy new year.

With Best Regards,

Jozef

[JozefKozon]

Hello Sam,

please see below an answer from an application engineer regarding the Cell balancing. Unfortunately we do not have any application note regarding this. For the information please refer to the section 9.9 "Cell balance control" in the datasheet and pages 12 and 21 in the MC33771C Safety Manual. The safety manual can be downloaded from the MC33771C product page under the secure files. 

You can consider to use our MC33775, which is however in preproduction.

DESCRIPTION

Yes, if you turn on the cell balancing longer than it should, then this will lead to a situation of imbalance, where you will have to balance all the other cells.

Now, the situation that you describe indicates that the cell you are using has a very small capacity (or is almost dead), and in this case the balancing current is probably too high. You should increase the balancing resistors to adapt to the cell that you are using, and then it will be easier to manage the balancing time.

You can also manage the balancing time directly with the SW to stop the balancing without using the internal timer of the IC. As you are suggesting, you could also monitor the cell voltage to stop the cell balancing at the target voltage (but note that when cell balancing is turned on,  there may be some voltage measurement error induced by the balancing current in some parasitic resistance, on the cell terminal cabling for example - you could pause the CB to perform the cell measurement, but then again, pay attention that the filter on the CT pins will force you to wait some time before the measurement is valid).

On MC33775A, it is possible to program a voltage limit where the cell balancing will stop at.

With Best Regards,

Jozef