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Get Powered Up! Certified Energy Manager Jeff Yago answers your alternative energy questions

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Archive for the ‘Amp-hours’ Category


Amp-hours for Solar LED lamp

Friday, April 17th, 2009

Hi Jeff,

Thanks a lot for your help.

I forgot to mention that I am planning to re-charge my batteries with a solar cell. I was wondering how can I choose a solar cell that charges a 3.6V battery.

I am trying to figure the Amp Hours (Ah) that I will need in order to have a constant light output of 14 lumens for at least 4 hours.

How long will it take for the solar cell to re-charge the batteries and do I need a regulator for the current when I charging the battery

Thanks a lot



Yes, you should have a solar charge controller between any solar module and a battery. A small unit for the size system you are considering would be very low cost and protect the battery while maximizing the solar charging process. Although it’s important for the voltage of any solar module to be properly selected for the battery voltage, voltage has little to do with sizing the battery to match the load.

You first need to determine how much “power” your lighting system will draw. This will need to be in “amps”. Next, how many hours per day will this light operate?, Now multiply those numbers together and get amp-hours. For example, let’s say you want to power a light for 10 hours per day that draws 2 amps. This is a 20 amp-hour load per every 24 hours of battery and solar system operation. Now let’s say your area averages 5 hours of direct sunlight per day (typical for most middle and southern states). This means your solar array will need to provide 4 amps of charge just to keep even. (20 A-H / 5 H = 4 A).

This means each day the example light takes out in 10 hours what the solar puts back in 5. However, this does not include any charging efficiency losses, the chance for bad weather, or occasional need for longer operating hours, so the solar array in our example should be up-sized at least 50%, or 6 amps (1.5 x 4 A). So for our example, we would select a solar module that has a 6 amp normal operating charging amp rating, with a voltage that is correct for the solar charge controller and battery we are using. Now we can select the battery.

Since we do not want to discharge a battery more than 50% on a daily basis, we will need a battery that has a 40 amp-hour rating just to handle the daily charge-discharge cycling. (20 A-H/ 50% = 40 A-H). Now lets say we want the battery to power the light for 3 days of bad weather and no sun. This would require a 80 A-H battery (40 + 20 + 20) .

Now follow this example using the ratings for your LED light, solar module, and battery.

Good luck,

Jeff Yago



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