# Solar Panels

##### Ash
Feb 9, 2013
424
Hi Guys,

I have a few recycled solar panels about, so I decided to string them together.

I'm getting a steady 4v on the output, with 8 small panels. I'm trying to connect it to a 0.9v-5v to 5v DC-DC Booster Module. The problem is the 4v drops when connected to the module, but rises when its disconnected and left open.

Any ideas?

Ash

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The maximum current you'll be able to get is the current that the smallest panel can supply (kinda). Also, the voltage falls as you draw more current, If you're getting 4V open circuit, determine the load required to drop it to 3V -- that's going to be pretty close to the maximum power.

Because solar panels operate more like a constant current source when heavily loaded, an increase in the load actually reduces the power available. Attaching them to a SMPS is exactly the wrong thing to do because if the demanded power exceeds the maximum available, the voltage will collapse to zero (or close to it)

##### Ash
Feb 9, 2013
424
I see, so if I arrange more panels in parallel I can increase the current?

It would be nice to achieve an output of 5v and ≥ 250mA, perhaps if I have two 1.2v NiMH AA batteries charging from the Solar Panels then the output to the boost converter to achieve my 5v. Thats if my panels have enough power.

Would the batteries still charge from the solar output while still under load? Or will I need to switch the panels off while under load.

#### davenn

Moderator
Sep 5, 2009
14,011
I see, so if I arrange more panels in parallel I can increase the current?

Yes

It would be nice to achieve an output of 5v and ≥ 250mA, perhaps if I have two 1.2v NiMH AA batteries charging from the Solar Panels then the output to the boost converter to achieve my 5v. Thats if my panels have enough power.

But that's not going to improve your current capacity .... with the boost converter being ~ 80-90% efficient
its going to chew up some of your current as well ..... better off just to use more panels if you have them

Would the batteries still charge from the solar output while still under load? Or will I need to switch the panels off while under load.

not if the drain from them by the load is the same or higher than the current supply from the panels

sounds like your panels are 0.5V ones ... at this stage current capability unknown

Dave

#### BobK

Jan 5, 2010
7,682
I expect that just connecting the cells as you have them through a diode to the two NiMH batteries in series would make a trickle charger. Measure the current in that configuration with fully discharged cells. I expect it will be 100mA or less which is fine for a trickle charge. Of course it will take about 24 hours (of sunlight) to charge them if they are typical 2000mAH NiMH AA cells.

Bob

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##### Ash
Feb 9, 2013
424
Can I measured the current openly? Or do I need a dummy load to measure the current? Lets say 20-40 Ohms?

I'll probably order some more panels to play with in a few days

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Solar panels are kind of the exception to the rule of not connecting an ammeter across a power supply.

Sure, you can just wire it up across the cells.

However, it would be better to measure the current when connected up to charge batteries. That would give you a realistic estimate of the actual charge current.

Naturally this will vary with illumination.

##### Ash
Feb 9, 2013
424
Thanks guys,

I will ask more questions at a later date, if I need too. Learnt a lot from this thread. =)

Ash

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