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BQ76920EVM: Dynamic load with energy recuperation

MateuszM
Prodigy 105 points
Part Number: BQ76920EVM
Other Parts Discussed in Thread: BQ76920, BQ78350-R1

Hi!

I'm looking at the EVM schematic and I'm struggling to define what happens in the circuit when:
- the Battery is sourcing the energy (discharge state) at first, both DSG and CHG FETs are turned on.
- then the load acts as energy source - let's say It's a motor with regenerative braking.
- plus everything can happen in short/long uncontrollable peaks.
Does this smart battery act like a normal battery or some sort of protection kicks in? Can someone explain what happens with the extra energy?

From the notes i see that if DSG current is small enough the DSG fet is turned off, so the current could only flow by the body diode. But then probably CHG FET would be off so battery could not charge,

  • 0
    TI__Guru 54715 points

    Hi Mateusz,

    The BQ76920 does not have an overcurrent charge detection, but one is available in the BQ78350-R1.  Since the gauge protection requires measurement of current which occurs on 250 ms intervals from the coulomb counter, the response times are long, the system will not normally respond to a pulse or several unless they integrate to enough current to trip the gauge threshold.

    The BQ78350-R1 will normally keep both FETs on and current can flow in or out of the battery as needed.  If the regenerative braking produces enough charge to drive the battery into OV, the charge FET will turn off and the battery will not provide a load for the braking.  Similarly if the charge current from the braking is long enough and large enough to trigger the OCC protection, the charge FET will turn off. In the event the battery were discharged to UV, then there were regenerative braking, it would charge the battery.  If current is low it will flow in the body diode of the discharge FET.  The BQ78350-R1 has body diode protection so if the current exceeds its threshold the discharge FET will be turned on to avoid heating the FET.  If the braking charges the battery to meet the recovery condition, it will recover with both FETs on.  See the BQ78350-R1 technical reference manual for description of the faults, recovery and available parameters.

    If using the BQ76920 with your own MCU instead of the BQ78350-R1 gauge, the MCU must control turn on of the FETs and can turn them off as well.  Consider how you want your system to function.