Greetings. A few months ago I purchased 4 AA 1.2V NiMH rechargeable. I think they were rated
to 1,500 mAh. I used them for a while with a discman and then decided to employ them for a school project involving a home made solar charger.

I employ a 4.5 V solar cell that produces about 0.1 Amp. Since I didn't had a 3 AA battery holder, I employed one 2 AA battery holder and a single AA battery holder. I connected the positive terminal of the single battery holder to the negative terminal of the 2 AA battery holder, in order to have 3 batteries connected in series. Then I connected the positive terminal of the 2 AA battery holder to the positive source of the solar cell, and the negative of the single battery holder to the negative of the cell.

After 18 hours of full sun exposure, I removed the batteries and tested them. Only the battery in the single battery holder was fully charged, while the other 2 batteries had almost no charge at all. I replaced the battery in the single battery holder with another battery that was fully drained and run the test again, receiving the same results.

Does this means that the 2 batteries in the 2 AA battery holder may be damaged, or shall I connect the batteries in parallel for charging them? I also included a diode in order to avoid
discharge of these when connected to the cell and tested it to see if it was properly connected and it was. I will appreciate any feedback you may provide me in order to solve this problem, if it is not too much to ask.

Thanks again for the help provided and keep up the good work.

best regards,
Laurentino De Vega Joly

 

NiMH batteries are more difficult to charge than other types and are easily damaged by overcharging. You need a proper charging circuit for them (meant specifically for NiMH). I don't have any recommended circuits, but you should be able to find something on the internet.

You may wish to check if you have damaged any of your batteries. You could try charging them one at a time in a proper NiMH charger.

 

Charge the batteries in series, not in parallel. Perhaps your 2-cell battery holder is mis-wired.

Robert Scott
Real-Time Specialties

 

Hi Laurentino:
When you are charging batteries with solar cell, sould be to connect in parallel, anyway the AA battery AA may be damaged.The current in the serie circuit is the same in each charge, so the batteries connected together in a one holder, must to have a few charge.

In a profesional solar system you must to use an peaks supressor connected from the solar cell to the battery, and a charger connected from the battery to the charge, to avoid complete discharge of the battery and to avoid also overcharge.

 

Maybe someone on the energy forum of http://www.motherearthnews.com can help you.

 

Whether you charge cells or batteries in series or parallel depends on the charging voltage. The charging voltage must exceed the nominal voltage of the cells/batteries connected. Using your 4.5V charger for three 1.2V cells seems about right.

Three 1.2 V cells in series is 3.6 volts. If I recall correctly, a forward biased silicon diode drops about 0.7 volt. So you solar array has to generate in excess of 4.2 volts in order to charge the cells.

Your charger is nominally 4.5Vdc. Have you checked the charge voltage in full sunlight? Have you put an ammeter in series to check the charging current?

I'm curious as to how you determined one cell was fully charged and two were not? Did you use a battery checker that puts a load across the cell while measuring the terminal voltage?