solar charging 9vdc

Discussion in 'The Projects Forum' started by donnierod, Mar 18, 2012.

  1. donnierod

    Thread Starter New Member

    Mar 18, 2012
    I have a 12 volt solar charger that I use to charge a 12v 7.5 Ah battery with.

    I want to charge the Energizer 9v, 175mAh battery with the 12 volt solar charger.

    Is there a simple way to do this, like adding a resistor to the positive lead of the solar panel and hooking up the battery?

    or will I have to look at putting together a circuit to make this work?

  2. wayneh


    Sep 9, 2010
    You need to protect your battery against two things, over-current and over-charge. There may not be a current concern, but you need to know. To be safe, you could use the short-circuit current of the panel and compare it to the battery's max charge rate. There's no way you'd actually get that much current, and it would be nice to know.

    I have some solar lights that start dumping current once the battery is charged to a specified voltage. They use a pretty simple circuit and it would probably work well for you. I've posted it here somewhere.
  3. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Energizer has an applications manual for their Ni-MH rechargeable batteries on their website. They say detect a full charge then reduce the trickle charge current to 1/40th (less than 0.5mA) or disconnect the charger.

    My Energizer Ni-MH charger has only a timer to disconnect it after 6 hours. It severely overcharges (and cooks) cells that are not completely dead.
  4. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    I noticed recently that Energizer has changed the label on their "9V" (actually 8.4V, it has 7 cells) Ni-MH 175mAh battery. Maybe it is not made in Germany anymore.

    A store near me still sell Energizer's "9V" (actually 7.2V, it had 6 cells) ni-MH 150mAh battery that is probably been there for at least 5 years.

    My 175mAh battery is 10.15V when fully charged.
    My "12V" solar charger is 17V with no load so it will detroy the little 9V battery.
    You need a battery charger IC made for charging a Ni-MH battery.