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BQ24400 recommendation?

Sebbe
Prodigy 170 points
Other Parts Discussed in Thread: BQ24400, BQ24115

Hello,

We are designing a product that is normally driven by a 7.5V 1.5A AC/DC adapter, but has 4x AAA NiMH batteries connected in series as backup power.

We are looking for a backup battery charger that maintains the battery charge level. Fast charging is not a key feature since the batteries act as an emergency backup during brownouts

Preferably the charger has a powerpath solution or application examples that seamlessly switches between AC adapter to battery drive.

This product will have an option to be run on batteries only, and due to minimum uptime we are contemplating to use Alkaline batteries in this configuration.

Q1: Will the charger detect that its 4x AAA alkaline in series and disable charging if the user inserts the AC/DC adapter?


Q2: Would you recommend BQ24400 as a suitable charger for the job to maintain backup battery charge for 4x AAA NiMH batteries?

Q3: When using 4x AAA batteries (single cells, not packaged), where do you put the NTC thermistor?

Q4: Considering to set a very low trickle charge/maintenance charge,  NiMH batteries are 850mAh, 1/80C approx 10mA as constant trickle, or do I want to set a maximum time for this?

Q5: If I want to shut down or restart the charger with our MCU, whats the most effective way? Do I have to power cycle the charger or can I float or sink a certain pin?

Q6: Do you have a suggestion for a power path solution?

Best regards,

Sebastian

  • 0
    TI__Genius 13700 points

    Hello Sebastian,

    Q1.

    The charge does not have the capability to diagnose between a rechargeable and a non-rechargeable without the potential of damaging them.

    Q2.

    The BQ24400 will fit this application as long as the external components are sized accordingly for the power and the feedback is designed to accommodate the number of cells in series and the chemistry.

    Q3.

    The placement of the NTC thermistor should go at the point that you expect to be the hottest. However if you are not anticipating high ambient temperatures or hig charging rates that placing it close to or in contact with one of the cells should be sufficient.

    Q4.

    The charge rate is dependent on the batteries that you are using. Power Stream povides some good feedback on the impact that charging rates can have a NiMH batteries here:

    http://www.powerstream.com/NiMH.htm

    Also Panasonic provides some good chemistry related advice to not charge a battery indefinitly.

    http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_ChargeMethods.pdf

    Q5.

    The best way to control whether the device is charging or not would be to retain the ability to to actively pull the TS pin to GND. This will suspend charge until the it is released and the voltage on the TS pin returns to an acceptable range.

    Q6.

    Unfortunately, the size of the NiMH is not as large as what is available for Li-Ion. As a result there are no chargers for NiMH that also have power path.

    Here is another possible option. If you look in the Tech Documents for the BQ24115 it discusses charging nickle based chemistries.

    http://www.ti.com/product/BQ24115/technicaldocuments

    http://www.ti.com/lit/an/slua411/slua411.pdf

    Let me know if you have any follow up questions.

  • 0 in reply to Ryan May
    Prodigy 10 points

    Hello Sebastian,

    We are doing a system design with 3 NiMh/NiCD cells with considerations very similar to yours. Hence I would be very keen to know the choices made by you and how they worked out.

    Regards,

    Mohan.