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#### CubeRider

Mar 15, 2013
32
Hey guys

Right I have a question about solar technology.

I have just purchased this:

Its a solar panel and the technical data says:

Nominal Voltage: 1V
Nominal Current: 200mA

So my question is...

How long would it take for that solar panel to charge a 1.2v recharageable battery ?

Or an even better question would be what equasion would I need when calculating the charaging capactity of a solar panel for a particular voltage battery.

So does this panel suggest the most output it can give in direct sunlight is 1V ? and that at its peak it would be putting out 200mA's ?

As you can see I dont have a clue lol but its exciting and I like the idea of a renewable energy source and using solar power to top up a recharageable batter for free power.

Last edited:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,898
How long would it take for that solar panel to charge a 1.2v recharageable battery ?
That depends...

Let's work it backwards:
What capacity has your battery? Let's assume it is a typical NiMh cell with nominal 1.2V/1000mAh capacity. And assuming the battery is fully drained (you see, I have to make a lot of assumptions due to lack of data).
To fully charge this battery you will need at least 1000mAh of charge. Unfortunately the charging efficiency of such a battery is only about 66%. Which means that the charger has to deliver a total of 1000mAh/0.66=1.5Ah.

Next: a typical NiMh cell has a nominal voltage of 1.2V, but the voltage rises to ~1.35 V during charge. You will need a higher voltage to charge the battery. Otherwise you would discharge the battery. Let's assume you have a charger that operates from 1.5V. For simplicity's sake let's assume the losses within the charger are negligible (they are there, we just don't consider them).

You will need a booster (step-up regulator) to inncrease the voltage from the solar cell (1V) to the input voltage of the charger (1.5V - or more). A good step-up converter may operate at an efficiency of 90%.

We now have all the numbers:
Power from solar cell: 1V*200mA=0.2W = power at the input of the step-up converter
Power at the output of the step-up converter: 0.2W*90%=0.18W=180mW
Voltage at the output of the step-up converter: 1.5V(minimum).
Current available at the output of the step-up converter: Iout=180mW/1.5V=120mA.
Assuming an ideal charger, all that current goes into the battery: Icharge=Iout=120mA
Total charge required: 1.5Ah=1500mAh.
Charge time: Tcharge=1500mAh/120mA=12.5h

These figures will vary with the battery (capacity, age, state of charge, type etc.) and the quality of the assumptions I made.

#### CubeRider

Mar 15, 2013
32
Oh WOW !

Thank you SO much Harald for that write up, it sure would be nice to have that information off top of my head.

I have read that about 4 times now and each time I read it I understand it a little bit more, am sure I will come back to this page over and over again in the future

One part I am not familiar with is the step-up converter, so I just did a quick google search and saw that I may be able to make this up from components too : http://www.miniinthebox.com/dc-dc-c...p-down-solar-power-supply-module_p406452.html

Is that the kind of thing you mentioned ?

Which then makes me wonder could I make my own Solar Powered Charging Station for free rechargable battery top up ? That would be an awesome project for me to make.

Thanks also for the battery charging basics link, I shall bang kettle on and have a read of that now.

Cheers again

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,898
That kind of converter is a step in the right direction.
However, I would not recommend to build this without some thorough knowledge of electronics. The one you found needs at least 3.5V input voltage - if that were available to you, you wouldn't need the converter because you could charge the battery from 3.5V.
You may want to consider buying two additional panels, That gives you 3V/200mA if you connect the panels in series. From that level you can easily charge a 1.2V battery. Here is a selection table for suitable charger ICs.

A very simple solar battery charger is described here.
And here is some background information on charging batteries from solar energy.

Btw: could you educate me on the expression "to bang kettle on"?

#### CubeRider

Mar 15, 2013
32
Thanks for the links there Harald, I think additional panels would be the way to go.

Bang the kettle on, erm in the UK I guess you could say we drink a lot of tea, when we have a problem or want to talk or read up on something its not uncommon to first make a nice hot cup of tea and settle down to read something.

Some really great links there, thanks again.

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