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NiMH 12-cell charger

Khoi Nguyen83
Expert 1180 points
Other Parts Discussed in Thread: BQ25630, BQ25600, BQ24115, BQ2002

Do you have any recommendation for 12-cell NiMH charger?

We need 12VDC and 5VDC regulated output so that's why we look into 12-cell charger.

I only found up to 4-cell charger on TI product. Perhaps we need to do 4-cell charger and a booster.

This is for Medical device which needs backup battery and the capacity is roughly 14V, 1500mAh.

-Khoi

  • 0
    TI__Intellectual 1980 points
    Hello

    Max 7-cell, like bq25600, bq25630. You could connect your battery in parallel.

    Regards
  • 0
    Prodigy 5 points
    Hello Khoi,
    Charging NiMH is not as easy as Lithium based chemistries.
    1. Fast charge must be >=C/2 in order to force the chemical reaction to show -dV/dt and +dT/dt.
    When a NiMH is full its voltage actually starts to decrease, -dV/dt, and its temperature rises exponentially, +dT/dt.
    2. Trickle charge can be <=C/10
    3. NiMH can not be charged in between C/10 and C/2 or cell damage will occur.

    Putting 12 NiMH in series will require cell balancing, further complicating the design.
    I don't recommend this many in series, although it has been done before.

    For charging NiMH we have some older devices which still work well.
    These can be found in the Power Management - Battery Management - Battery Charger IC section, filter for NiMH chemistry.
    www.ti.com/.../battery-charger-ic-products.page

    Up to 4 or 5 in series is manageable. Of course you can add parallel cells to increase the total mAhr rating.

    There is also this App Note on "Using the bqSWITCHER, bq24115, as a Nickel Charger"
    This uses a BQ24115 integrated fet 2Amp LiIon switching charger but controlled by a BQ2002 NiMH charge controller.
    The BQ2002 does the charge termination criteria for Nickel Chemistries.
    The design works very well.

    For your design I would use 4 or 5 in series, add parallel cells to get the mAhr you need. xSxP
    Maximum cell voltage would be about 1.5V when full, so a 4S pack would be ~6V full, 5S would be 7.5V
    Minimum usable cell voltage is ~1V, 4S=4V, 5S=5V

    So with a 5S pack, you could buck to 5V, the buck would need to be able to get to high duty cycles as the battery discharges.
    Or you could use a LDO if the currents are fairly low.
    Boost would be needed for the 12V rail.

    Here is a good paper on battery chemistries;
    Characteristics of Rechargeable Batteries
    www.ti.com/lit/an/snva533/snva533.pdf