Simple solar charging experiment fail (!?)

Hi there!

Im trying to investigate in some solar charging. I tried a very simple setup as you can see attached.

Solarcell:
4V, 35mA

Batteries:
4x1.2V (4.8V), 700mA

When measuring the cell alone without any circuit attached I get approx. 3.2V in the shade as well as approx. 15mA over the diode.

When I measure voltage directly at the battery its allways 0V. I left it an afternoon in the sun but its also 0V at the battery (with and without the circuit attached).

Is there something Im doing wrong here? Or does it just take ages to charge at that mA rate?

Thanks in advance!

You need a higher voltage at the solar cell then the battery pack. All you are doing is using the solar cell as a load due to the potential difference

My Solar system for a 12V SLA peaks at 21V. At 13V it stops charging

-- Mark

I use Sparkfun's expensive, but really efficient "Sunny Buddy" board to charge a LiPo battery and run a middling load of a LED based digital clock.

If you measure 0V across the batteries they are drained completely and is destroyed - the small charging current from the solar cell will not be able to bring them back to life.

Also to charge NiCD/NiMH chemistry you need approx. 1.4V per cell to charge fully, so you would need 5.6V to charge your pack.

// Per.

This panel has an open circuit voltage of about 8 V in full sunlight, and will charge 4xAA (NiMH) cells.

Don't forget to use a diode between the panel and the cells to prevent discharge during dark periods.

I suspect these are old NiCd cells with internal whiskers shorting out the electrodes. These can normally be zapped with an electrolytic capacitor and the short removed, but any rechargable cells that have been allowed to drain to 0V are essentially ruined beyond recovery and then some.

Over-discharge ruins secondary cells, no matter what type they are, AFAIK.

Are you sure the cathode of the diode is connected to the positive battery terminal? I cannot make out how the terminals on the battery pack are connected internally, but I would think a series connected pack would have one terminal at the end, which in your case is the positive terminal (yellow gator). Perhaps not, but worth checking.

Thanks guys for your replies!

I checked the batteries with an advanced charger and the charger identified 2 of the the 4 batteries as completly dead. These batteries are pretty old, maybe that was one problem.

I have two of these solar panels so I will put them in series to get 8V when in direct sunlight. That will probably charge the 5.6V that is needed.

Thanks for your input! Allways fun to tinker with electronics and get so much support and expertise from the community! :)

Hi,
Did you check the way your diode was connected?
As pointed out in post#6.

You diagram is not like the picture, look at the positive and negative of the battery pack.
And positive and negative of your panel.
130cb92c0663e16309ecce7be9fc839b8f50c191.jpg

Tom… :o