This project will maintain a sealed lead acid battery which will power my back yard weather station. I will be using a small solar panel 5w and a 7.2ah sealed lead acid battery.
I have a solar charger purchased off ebay but the problem is during the day it floats the lead acid at 14.4volt which is too high for a small battery. The other issue is during the night when there is no solar power it keeps a bunch of status leds on which consume an excess of 20ma's.
My project will have 2 charging stages.
1 - If the battery voltage is below a certain threshold and the solar panel voltage is high the solar panel will be connected directly to the battery until float voltage is hit. I only expect this to happen after a few days of cloudy weather.
2 - If the battery voltage is above a threshold a regulator is connected to the battery this regulator has the voltage set to 13.5v which is the optimum float voltage for a lead acid.
I have finished soldering together a prototype board.
Please excuse my messy soldering skills. The small blue wire connected to the top of the Arduino right near the atmega328 is for aref. The arduino pro mini (ebay clone) doesn't break this pin out but there is a 100nf cap there to help keep a stable aref? Using some small gauge wire it wasn't hard to solder a wire onto the back of the capacitor.
Left to add is a simple 7805 to power the Arduino off the incoming solar power. Hopefully it will still run when the solar panel is charging the battery directly, will need to test this. During the night it will loose power and switch off. Since there isn't any decisions to make about maintaining the battery during the night this is fine.
I have just successfully tested this board (usb powered). Using a 5w solar panel and a simple sketch to switch between regulated charge or direct charge.
- add the 7805 for powering the arduino
- Write some code that will decide how to charge the battery
- Look into possibly a weekly charge from the solar panel into the 14v's. Will research this a bit more to find out if this is required for long term battery maintenence.