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bq2002 charging 4 NiMH cells with 5V source?

Fabio Verna
Prodigy 40 points
Other Parts Discussed in Thread: BQ2002E, BQ2002, LM317

Hello,

I have 4x 1.2V NiMh cells that need to be charged from a 4.7V-5V source (0.5A). Here is what I've learned so far by reading the doc and application notes on this website:

  • I need a fast charge controller
  • Charge termination can be detected with PVD (dT/dt is better but requires an external sensor)
  • Cells are charged at a constant current

The bq2002e is an interesting low-cost product. But I'm really not sure whether I can charge a 4.8V battery with a 4.7V source. Can this be achieved or do I need a higher-end product (with maybe a built-in voltage booster) ?

Thank you very much for your attention

Fabio

  • 0
    TI__Guru*** 145970 points

    No, you cannot charge a 4.8V battery with a 4.7V source with the bq2002.

    In addition, your charger will need to output a higher voltage 4 x 1.2V.  At fast charge termination, the voltage on each nickel cell could be as high as 1.6 or 1.7V.  Thus, we recommend to give your charger the ability to maintain the constant charging current up to a 2V per cell pack voltage or 8V in your case.  The fast charge current must remain the same throughout fast charge to get an accurate full capacity indication on the pack voltage.

    The bq2002 requires a 5V power source to operate and controls an external current source.  So, you need to build a current source with the LM317 or discrete transistors.  Then, you will need an input voltage to the current source of 8V plus whatever the dropout of it is.  Thus, you will need a boost converter to get this voltage from your input.  You can search for one at power.ti.com.

    Your 4.7V to 5V input source can directly power the bq2002.

  • 0 in reply to Chris Glaser
    Prodigy 40 points

    Dear Chris,

    Thank you very much for your answer. That helps a lot. I'm going to work on a new design with the bq2002 and the lm317.

  • 0 in reply to Fabio Verna
    Prodigy 30 points

    Hey,

    We are also working on same.Can you share me the reference schematic.

    As per discussion,i understood we have to provide 8V VCC to charge 4 series cells (NI-MH).Is it correct?