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Help on creating a lockout protection for a 4 cell battery pack on the system side

MohammedAl-Amin
Intellectual 540 points
Other Parts Discussed in Thread: BQ24610

Hello,

I have been having issues on getting my head around how to protect the Li-Ion battery pack which is a 4 cell pack which has its own protection circuit, but it is advised that this should not be used as the primary protection.

The battery is a 4 Cell in series Li-Ion Pack which provides only two wires positive and negitive.

What I was hoping to just create is a Under voltage-lockout and  Over voltage lockout circuit,  something which is simple to implement. But allow the charger to charge the battery, when its depleted.

Please could any one help me on this it would be much appreciated.

Mo

  • 0
    TI__Guru* 80963 points
    Hi Mo,

    For a multi-cell battery pack, over-voltage and under-voltage detection must be done at the cell level because each cell will have a different voltage. Does the protection circuit in the pack do cell balancing to keep the cell voltages close to each other? If the cells are unbalanced, simply calculating the cell voltage as Pack_Voltage/NumberOfCells will not be accurate.

    For a system side circuit, you could consider a window comparator capable of driving a FET. However, this is beyond the scope of this forum.

    Are you using a battery gauge or charger IC on the system side that might already include these features? Is there a safety standard you are targeting to meet?

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 540 points

    Hello Matt,

    Thank you for replying back to me, the system is to be used for medical device and would need to adhere to EM and electrical regulations (EC).

    I have Implemented a battery charger which is the BQ24610 and a system side Fuel Gauge which is the MAX17361. but the issue is that overprotection is handled by the BQ24610 but the Undervoltage is only sensed by the fuel gauge (MAX17361). This, therefore, relies on MCU to be used with the fuel gauge. This is fine with the high-end version of the product but the lower end version would not include an MCU and so an analog approach would need to be taken which can automatically protect it when the voltage reaches 10 volts to lock or shut down the load to the system.

    As for the battery, comes with its own internal protection, such as OVLO, UVLO, over current, undercurrent protection and PTC, I will have to look into about the cell balancing, as most 4S battery packs I've seen have not mentioned about cell balancing.

    I was thinking about the use of a window comparator to drive FET but was not sure about where to start. Also, I was thinking as the rest of the system is connected to DC-DC converters and a load switch, if there could be something to sense the voltage and enable the EN pins of the load switch and DC-DC converters. 

    I have attached a picture as you can see the boxes in Red are the supply to the various parts tot he system, as they are load switches and DC-DC converters which have enable pins would there be a type of circuit or IC which could enable them on when the voltage is above 10V and disable when below 10V?

  • 0 in reply to MohammedAl-Amin
    TI__Guru* 80963 points
    Hi Mo,

    Many fuel gauges are equipped with a strong array of protection features with FET control output, but access to the individual cells is required. At least your battery pack is providing protections for OV and UV at the cell level, but the best approach is to have primary and secondary protection. In this case since you only have access to PACK+ and PACK-, does your gauge have any configurable Alert pins that could be used to control external circuitry? Some battery gauges do provide this feature.

    For help with window comparators, you can start here: www.ti.com/.../products.html
    There is another E2E forum where you should be able to get help for any of these products. Unfortunately I cannot recommend a specific non-gauge solution since I don't have experience with this.

    Best regards,
    Matt