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Solar powered NiMh charger for 4 cells

tolu collins
Prodigy 110 points
Other Parts Discussed in Thread: BQ2000

Hello, I want to design a solar/usb powered battery charger for a pack of 4 NiMh AA batteries in series.

I've read from various posts that the inconsistent current from solar panels will make end of charge detection difficult, so i was thinking of a design where the solar can be used to trickle charge the batteries (meaning the peak current of the solar cell will be withing a safe trickle charge current of the battery) and a usb to fast charge the batteries.

My questions are:

1. What is the best solution for this application?

2. If i'm using the bq2000 series chips to charge the 4 batteries in series what minimum voltage across the batteries will i need to ensure proper charging? I'm not too clear about this part from the datasheets.

Thanks

  • 0
    TI__Mastermind 31120 points

    The bq2000 is a fast charger and needs a constant current and >c/3 charge to get proper termination.

    The majority of the Nickel cells in the world are slow charged at C/10 when ever in their cradle (home cordless phone).

    The slow charge full voltage is less than 1.5V/cell so for 4 cells one would need about 6V with a current around C/10.

    Most slow chargers are a resistive transformer with fine wires rectified and run into the cells.

  • 0 in reply to Charles Mauney
    Prodigy 110 points
    Thanks Charles. So if I get you right, For a 2200mAh battery I could use a solar cell with a Vmpp of about 6v with an lmpp of about 220mA, and this should be able to provide trickle charge for my battery pack, while the bq2000 series can be used from mains supply when present to fast charge the cells. If this is right then what voltage will I need to fast charge the battery using the bq2000, will 2v/cell (8v) be fine? Thanks.
  • 0 in reply to tolu collins
    TI__Mastermind 31120 points

    Yes, a slow charge from the solar cells and a fast charge from an adapter would work.

    Adapter voltage (1.75V to 2V per cell plus 1 to 2V over head) of 8V should be ok.

    Remember to have  at least C/3 charge so you get a good termination with the adapter fast charge.