Simple Solar Charger Question
Brian Carpenter
Posts: 728
i have a 12 volt battery that will be supplying both a 3.3v source to a prop and a 12 volt source to a motor. The prop will be on all the time but the motor will only run for about 3 minutes every other week.
I have a 12 volt 5 watt solar panel that i want to use to keep the battery up. is this as simple as connecting it to the battery via diodes?
The battery is an SLA
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I have a 12 volt 5 watt solar panel that i want to use to keep the battery up. is this as simple as connecting it to the battery via diodes?
The battery is an SLA
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It's Only A Stupid Question If You Have Not Googled It First!!
Comments
If you just want to keep the battery topped off, then that's all you need. To do any charging you need at least 2.16V to 2.4V per cell
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
What is the Amp-hour capacity of your SLA battery? Some 5 watt panels can deliver about 400 mA at 14 volts on a sunny day, and that would be too much for a small SLA battery once it reaches full charge. Overcharging shortens the battery life. The current into a fully charged battery should not be more than the capacity of the battery divided by 100. So into a 7 Ah battery, 70 mA max. Depending on the load and the battery, it may not be a problem.
Some solar panels are well matched to battery charging, in that the current drops off as the battery nears full charge. Test it! Take the panel and fully charged battery in a loop with an ammeter out on a sunny day, and measure the current. That kind of panel usually has an open circuit voltage in the neighborhood of 17 or 18 volts instead of 21 or 22.
Lacking a fancy charge regulator, you can get overcharge limiting by putting more than one diode in series with the panel, but that will also limit increase the charging time. There is an integrated circuit float charge regulator, the PB137, you can buy from Mouser. It is not ideal for solar panel charging, but it works if you have a panel with enough excess voltage.
That is the difficulty with solar panels, to get enough on gloomy days but not too much on brilliant days.
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Tracy Allen
www.emesystems.com
Thanks for the reply. In sunny arizona, we seldom have gloomy days. the motor will literally only run about 3 minutes every other week. I will give it a try
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