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BQ2970: Recharging two parallel BL-5C Li-Poly battery pack with a single charger

binu pandian
Prodigy 20 points
Part Number: BQ2970
Other Parts Discussed in Thread: BQ25896,

Hi,

We have a design requirement with max. power 11W and average power consumption 4-5W.  We have BL-5C batteries available and have decided to use two BL-5C packs to meet our requirements.

Battery spec: 3.7V, 1000mAh.(We have tested the batteries for maximum discharge current 2A before cutoff)

We are planning to use the 2 BL-5C in parallel and use a single BQ25896 charger.

Regarding the parallel connection, we have the following queries,

1) Should we consider using BQ2970 for short circuit protection to prevent one depleted battery from discharging the full battery or is this not required in case of li-poly pack?

2) Based on our understanding, two identical batteries in parallel should not cause any problem.  However, for practical considerations, with batteries of different age and internal resistance, we believe issues like 1 battery getting charged/discharged faster than the other might rise.  What will be the effect on lifetime of batteries in such cases?

2) Is it better to use serial or parallel configuration when connecting two batteries?

Kindly provide your suggestions.  Also, provide link to any references that might help us in a better understanding on the subject.

Thanks and Regards,

Binupandian S.

  • 0
    TI__Guru* 76230 points
    Hi Binu,
    When two batteries are connected in parallel, you increase the overall capacity, i.e it is doubled. When in series, the capacity is the capacity of just one cell, but the series voltage increases (doubles). So it depends on what you are trying to do. Does your application need higher voltage or higher capacity.

    so for question 1, if the cells are connected in parallel, the bq2970 will see the pack as just one battery. you need protection for li-ion cells
    2. when connecting cells in parallel, you need to ensure they are of the same age and the same type to avoid one draining the other. The cells in parallel will always be the same voltage though regardless of if one is more aged than the other , so this will not cause an issue other than you wouldn't be getting the full capacity from the pack

    thanks
    Onyx