my group is building a device that consists of multiple sensors, a micrcontroller and a xbee wireless module. The device turns on once every hour and takes a measurement then goes into sleep mode. The total powered required to operate the device is about .2W. We're using 4 rechargeable D batteries to power the entire thing. The D batteries will be connected in series to a recharging IC I got off of digikey (http://pdfserv.maxim-ic.com/en/ds/MAX712-MAX713.pdf). A solar panel will be used to charge the batteries. The recharging IC requires a voltage of 1.5 more than the total battery voltage (4.8+1.5=6.3V). I know the voltage needed from the panel to charge the batteries but I'm having a hard time with the current. Each D battery has a capacity of 10,000mAh(http://www.batterymart.com/p-powerizer-d-nimh-rechargeable-battery.html). I can determine my charge time by taking the capacity and dividing it by my input current. I was hoping to find a panel with an output current of at least .9A, so that my charge time would be 10Ah/.9A=11.1hours. However, I think D batteries have a standard discharge rate of 10 amps. Would the .9A supplied by the solar panel be enough to charge the batteries. Any recomendations on solar panels would be greatly appreciated.

 

The output from the solar panel is 0.9A at noon in the middle of summer. It might average 0.3A per hour during one sunny day. In winter it is much lower. Do you think it will work on cloudy days?

The battery does not charge perfectly. The manufacturer recommends a charging time 1.4 times what is calculated.
It might take a few sunny days in summer to fully charge your huge battery.