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BQ77915: Battery protection for x2 high current 6S LiPo batteries in parallel

Matthew Griessler
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: INA302, LM5050-2, BQ76925, BQ76930

Hi,

I'm from the University of Florida. I have a robot that has 8 thrusters, a computer, and several sensors, cameras, and other small actuators. We have been running with a 48V rail comprised of two 6S LiPos in series. We're changing thrusters so we're switching to a 24V rail and taking the two batteries we're running in series and running them in parallel. We're taking this opportunity to finally add in battery protection. This is an area I do not have much experience with so I am looking for advice.

The overall topology I'm imagining something like this:

The biggest thing I want to protect against is over-discharging, or under-volting, the batteries. For at least this initial version, we will continue to remove the batteries and put them on a dedicated charger. This circuit only needs to protect against discharging events. OT and OC protection would be nice. We'd also like to be able to monitor the battery voltage and current.

This is what I'm thinking based on what I've found poking around TI's website:

Battery protection:

The BQ77915 looks like a chip that would meet and exceed our needs, but we'd have to use two because our batteries are 6S and put multiple FETs in parallel because our maximum power draw at nominal 24V is 1200W per battery.

Alternatively, I was thinking we might have to implement a solution where we monitor the battery voltage with a uC and then cut power if the batteries get too low. We have a relay on each thruster that we used for our "kill" functionality and we could add a relay to the computer/sensor power. The benefit here would be that we don't have to do FET paralleling but the disadvantage would be the time to design and test the circuit.

Battery monitoring:

We have a separate circuit that does this that we've used on other vehicles in our lab. It is based on the INA302

Battery paralleling:

We've used the LM5050-2 to do this before, we plan on continuing to use the chip on this vehicle as well.

  • 0
    TI__Guru 54715 points
    Hi Matthew,
    A protector like bq77915 is a simple to use solution to provide protection at the levels set. It will need to sense current which will mean a sense resistor and some resulting loss in each battery monitor circuit. Some systems will have the battery protect itself from abuse at its terminals if they are exposed when the battery is removed. Others will signal the system not to use the battery if conditions are out of limit. This works similarly in the system but may not be helpful if the battery is subjected to some abuse outside the system. Which method you use depends on your requirements.

    A monitor device such as bq76925 provides a way to measure voltage of the cells so that your MCU can make decisions on protection or control. This part has analog output, so a MCU must convert the signals and assemble the voltages. It also has a temperature measurement method and current measurement and threshold system. You may be satisfied with your INA system for current measurement.
    A more sophisticated monitor such as bq76930 contains ADCs and provides a digital output for voltage and current as well as thresholds for OV, UV, OCD, SCD. Temperature can be measured. A MCU is still required for decisions such as to turn on the outputs, evaluate temperature, and control balancing.

    When connecting batteries in series with protection, be sure to consider the applied voltages if one pack protects. In parallel consider the effect on the other battery if one protects and the load shifts. Interface level shifting or isolation is needed for different potentials if you talk to the packs. Also the possibility of one being above the other, your LM5050 solution likely addresses that.

    You might look at the capabilities of the parts noted and look at how they will meet the needs of your architecture.