NIMH solar charging questions

Discussion in 'General Electronics Chat' started by mbohuntr, Jul 22, 2012.

  1. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    Morning, I bought a cheap solar panel (6V, 250mA) from radio shack to expieriment with as well as a lm317 and 25 ohm pot for limiting current. I quickly discovered (duh) that the max output for this panel never exceeds c/10 (250mA for 2500mA batteries) so the LM317 wasn't needed. My question is this, each cell is 1.2V, would putting 4 in series drop the current too much for the panel to be useful for one day charging because of the internal resistance?, and, if I put 2 in series, is the supply voltage harming the cells? should I pop in a couple of diodes to drop the voltage?

    Thanks, Mike
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    You can put the batts in series and they will all charge at the same current, so that would give you 6v-4.8=1.2volts, if you charge at 200ma that would be a 6ohm resistor in series with the nicads.Or you can use the lm317 in series set it to 200ma constant current mode.
    Last edited: Jul 22, 2012
  3. MrChips


    Oct 2, 2009
    No need to drop the voltage. The batteries will charge as long as they can receive a surplus voltage. The only concern would be overcharging. You need a way to stop charging when the terminal voltage has been reached. This is where a voltage regulator comes in handy.
  4. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    Thanks guys, I tried dropping the voltage using the r1(220ohm) R2 (5k pot) configuration and got around 1.2V-4.7V. Problem was that the output current dropped to 40mA regardless of output voltage?? I was looking to charge a couple of D cells to power the wifes portable fan in the camper. The 12v model we use now burns up the camper batteries in about 4 nights, and these are 2 -27's in parallel! :eek: I was looking to charge a set during the day, and burn another set at night. 2 sets would allow me a day without sun and not hurt too bad.