BQ77915: Please could someone check my circuit for 12S LiFePO4 cells?

Ian Kelly2

Part Number: BQ77915
Other Parts Discussed in Thread: CSD18535KCS, BQ76952, BQ76972,

Part number BQ7791514
12S LiFePO4 3.2V cells (36V battery pack)
Charging voltage 43.8V
Charging current 4A
Discharging current 10A
Balancing: External 3.9R @5W resistors (12 resistors connected via J1a, J1b & J1c)
5mOhm is 1.5W rated

Also if the MOFETs were changed to be TI CSD18535KCS (which have gate charge 63nC) and the charge and discharge currents are up to 180A, still using BQ779150214. What value should R33a the current sense resistor, and R23a, and R23a the DSG & CHG resistors be please?

Is there anyway to lower the max. cell charging voltage of the BQ7791514 to 3.55V?

Many thanks,

Ian

over 1 year ago

0 Luis H. S. over 1 year ago

TI__Mastermind 42060 points

Hello Ian,

You should not need to change the value of the resistors due the new MOSFET. The only resistor you may need to change is R23a, if you find the turn-off speed too slow during testing. However for now I would not change it.

You could also have FETs in parallel to handle larger charge/discharge currents, if needed.

Unfortunately there is no way to change the OVP of this device to 3.55-V. If you wanted more control, you could look at monitor/protector devices like the BQ76972 or BQ76952, both which can do up-to 16s.

From what I can see, the schematic looks good!

Best Regards,

Luis Hernandez Salomon

0 Ian Kelly2 over 1 year ago in reply to Luis H. S.

Prodigy 60 points

Hi Luis,

Thank you for checking my circuit and the answers.

Could the current sensing resistor also be safely shared by any of TI's coulomb counter ICs? Thus saving a component and extra another voltage drop. Preferably with an SPI interface please.

Best regards,

Ian

0 Luis H. S. over 1 year ago in reply to Ian Kelly2

TI__Mastermind 42060 points

Hello Ian,

Yes, that is completely okay! But I would suggest that any device that you use should have its own separate filtering components, these should not be shared.

If you want a device with coulomb counting that can do 12s LFP cells, I would actually suggest to not use the BQ77915 at all, and instead use the BQ76952 or BQ76972. This device has an integrated coloumb counter, all protections can be adjusted, has high-side FET drivers for protections and can work anywhere from 3s to 16s.

In addition, SPI can also be used with this device .