BQ77915: Charging issues with cell imbalances

Evan Yurko

Part Number: BQ77915

Hello,

We are having difficulty getting this part to work when we have a significant imbalance in cell voltages. Currently we have one part and 4 series batteries. Current issue is when the cells are imbalanced, the discharge fet will stay on but the charge fets gate voltage is around 1-2 volts. I have changed R29 to 10k and R32 to 3.3Mohms. Only way I could get the batteries to keep charging was to connect CTRD and CTRC to GND (not B_COM). If anyone has an ideas as to what is happening it would be greatly appreciated. Currently: Cell 1: 3.2V, Cell 2: 3.4V, Cell 3: 3.7V, and Cell 4: 4.2V.

over 1 year ago

0 Max Verboncoeur over 1 year ago

TI__Mastermind 25980 points

Hi Evan,

The 08 version of the device has the OVP threshold set to 4.2V. If Cell 4 is indeed at 4.2V, then you are most likely triggering an OV protection and turning the CHG FET off.

On a side note, R31 is recommended to be 1Mohm.

Regards,

Max Verboncoeur

0 Evan Yurko over 1 year ago in reply to Max Verboncoeur

Prodigy 30 points

So in the imbalance introduced, the batteries will not balance during charging?

and I forgot to mention that I did change that to a 1Mohm resistor.

0 Max Verboncoeur over 1 year ago in reply to Evan Yurko

TI__Mastermind 25980 points

Hi Evan,

The cell should balance, you may be able to see it better with an oscilloscope. This video explains the algorithm in more depth (link).

The FET will still turn off and remain off until the cell voltage drops below the hysteresis threshold, which on this device is ~4.1V.

Regards,

Max Verboncoeur

0 Evan Yurko over 1 year ago in reply to Max Verboncoeur

Prodigy 30 points

I looked at it under a scope, it appears as VC0 and VC1 nodes are attempting to balance. Reading from the charger part we are using, the charge current is 44mA. Does this sound like what is supposed to happen?

0 Max Verboncoeur over 1 year ago in reply to Evan Yurko

TI__Mastermind 25980 points

Hi Evan,

Do you have waveform captures to share? It may help my understanding of what the device is doing if you have a waveform that shows the voltage across each of the filter capacitors.

I also recommend watching the video I linked in my previous reply to get a better understanding of what cells will balance when.

Regards,

Max Verboncoeur

0 Evan Yurko over 1 year ago in reply to Max Verboncoeur

Prodigy 30 points

Watched the balacing video. Gave a better understanding of whats going on. I captured the voltages across the filter caps. I believe this is working but the only thing is maybe beefing up the cap filter on the adjacent battery.

+1 Max Verboncoeur over 1 year ago in reply to Evan Yurko

TI__Mastermind 25980 points

Hi Evan,

This looks good.

The slight spike on the adjacent cells is somewhat unavoidable. This is caused by a slight mismatch in the timing of the internal balancing FETs turning on/off. Keep in mind that the recommended value for the filter caps is 1uF when using internal cell balancing, so increasing them may not be advisable.

Regards,

Max Verboncoeur

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BQ77915: Charging issues with cell imbalances