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


    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


    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.