# Basic Solar Charging AA questions

#### Matt D.

Joined Nov 10, 2013
65
I know another solar panel charging batteries question!

Ok so I have a solar cell that puts out about 9v and 90ma with no load.
I connect it to 3 AA NiMH cells and it drops to about 4.6v and 60 somethingish ma.

I was surprised the voltage dropped that much- is that normal?

If I had a 9v and 330ma solar panel would that make less of a voltage drop since it technically has more power?

Also the 3 batteries have a combined voltage today of 3.96 volts. Connected to the solar cell going through a blocking diode it reads about 4.6v, but removing the diode and connecting directly to the solar cell the voltage just read the 3.96 volts. Is that because the batteries have more power than the solar cell?

BTW I'm only trying to trickle charge the batteries at .03C or less - they will have a rest at night and when the sun isn't bright enough. I know the setup I have can be made to work with the 3 cells, but I will probably be charging 4 cells.

I've got some more questions about a 6v regulator for 4 batteries but don't want to throw out to many questions at once!

#### enggricha

Joined May 17, 2014
87
Ok so I have a solar cell that puts out about 9v and 90ma with no load. - how do u measure current without any load??

I connect it to 3 AA NiMH cells and it drops to about 4.6v and 60 somethingish ma. - this is possible, but when the voltage drops, usually the current would increase and not decrease

If I had a 9v and 330ma solar panel would that make less of a voltage drop since it technically has more power? - yes that would reduce the voltage drop

Also the 3 batteries have a combined voltage today of 3.96 volts. Connected to the solar cell going through a blocking diode it reads about 4.6v, but removing the diode and connecting directly to the solar cell the voltage just read the 3.96 volts. Is that because the batteries have more power than the solar cell? - this is because of the voltage drop across the diode, approx 0.6V

#### Matt D.

Joined Nov 10, 2013
65
Thank you enggricha.

Just measured the short circuit - is it called ISC?

#### Matt D.

Joined Nov 10, 2013
65
But the voltage is higher when connected with the diode - not lower.

I assume when the voltage is reading 4.6v when connected to the solar cell then it has already dropped the 0.6v

#### Matt D.

Joined Nov 10, 2013
65
I want to use a 6v regulator to supply the voltage and current to the batteries. Please see the attached data sheet.

I'm not sold on this one by any means but it is one that I think might work - the one for 6v out.

Does the output voltage = input voltage + 1v mean the input voltage has to be at least 7V(I assume)?

Also does this voltage regulator need to be turned "on" with pin 1 connected to .6V. If it means that then could I just connect the positive lead of the solar cell to pin 1 - in addition to input pin 5?

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#### #12

Joined Nov 30, 2010
18,223
Yes, the datasheet says the control pin should be, "high" to let the chip run. That's between .6 volts and 16 volts.
Page 8, Fig. 7 shows the necessary extra voltage to supply the regulator chip, and it's less than 0.3 volts at 90 ma.

I want to kill a misconception you have. The voltage at the battery during charging will be just a few tenths of a volt more than the battery voltage would be if it wasn't being charged. You can not expect the battery to suddenly support 9 volts just because the solar panel can go there when it has no load.

NiMh batteries can be very picky about their charging. Please study at www.batteryuniversity.com to find the correct method. You might find it is different from ni-cad or lead-acid.

#### Matt D.

Joined Nov 10, 2013
65
Thanks to both. I just didn't realize the volts would drop that much.

I'm only planning on using about 30 or so ma per day so the charging current can be quite low. I don't know what the self discharge rates are for the different nimh cells - the old ones and the new low self discharge ones.

Do you really think the cells would be damaged or have memory problems at such a low charging current? Remember there would be a rest at night and when the solar cell cannot supply the 7v to the circuit. About 30 ma is used during the day and then there is the
queiscent current. My point is that the cells would never get overloaded with a lot of current.

#### #12

Joined Nov 30, 2010
18,223
You're right. Shutdown voltage must be 1.8 or more to let the chip run.

However, the graph of dropout voltage seems to show a rather low requirement of about .3 volts at 90 ma.

Yes, ISC means, "Current, short circuit".

http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

or from post #7,

"NiMH batteries are fickle fiends to charge, exhibiting temperature-dependent changes in charge and discharge curves. They also don't have a float voltage, so constant voltage charging doesn't work."

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#### Matt D.

Joined Nov 10, 2013
65

"Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging."

This is still on the current Energizer datasheet

#### ronv

Joined Nov 12, 2008
3,770
So your not trying to recharge them, just maintain them?

#### Matt D.

Joined Nov 10, 2013
65
Yes. But only use about 25ma a day. So that is a very small amount of use that could easily be replenished at even a c/100 rate.

I'm not sure if it would be better to use the new low self discharge ones or the old type where more of its capacity is lost everyday.

#### Bernard

Joined Aug 7, 2008
5,788
Best power from SP will be when it is loaded down to around 6.3V, but as you only need 25 mA, SP will be loaded down to about 8 V. Try a diode, & 120Ω, 1/2W R, check I & adj. R to your liking. Regulator not needed.

#### Matt D.

Joined Nov 10, 2013
65
Thanks Bernard that is great! I was getting tired of looking at datasheets. I'll do some experimentation.

So the higher voltage at 8v for four batteries would be ok? Should I look for a lower voltage SP to get closer to 6v loaded?

#### Bernard

Joined Aug 7, 2008
5,788
Nope, just pick proper R.