Here's the power supply setup for my latest project. In the middle we have a Lithium Ion Polymer Battery - 3.7v 2600mAh (
http://www.adafruit.com/products/328) sourced from A
dafruit.com, to the left is a 6V 2W (medium size) solar panel (
http://www.adafruit.com/products/200) also from Adafruit and and finally on the right is nifty charging circuit from Adafruit (
http://www.adafruit.com/products/390). The charging circuit is pretty clever. It lets me charge from the solar panel, a USB port or a 5V power supply. It also has a load output with a bypass circuit that I'll use to power my project. I'll have to be a little careful though, when I'm receiving power from the battery I'll be getting somewhere between 3-4 volts but when I have full sun steaming up my solar panel I might be getting 6 volts. Fortunately including a regulator in my project is easy so I'll be covered up to 16 volts.
Time for a little math.
First lets start with the capacity I have from my battery and charge setup.
Battery: 2600 mAh
Charge rate: 330 mA (2 W / 6 V = 333 mA)
Hours required to provide a full charge: 2600 mAh / 330 mA = 8 hours.
Average full sun hours for Portland, Maine-
Average: 4.51 hours per day
High: 5.23 hours per day
Low: 3.56 hours per day
So in the best conditions I can't expect to be able to recharge a fully discharged battery each day and the winter will be worse. Also, I'd like my project to be able to make it through a couple cloudy days without running out of juice completely.
Lets say I want to make it through 3 days with effectively no sun, from a full charge. That's roughly 84 hours. With a 2600 mAh capacity battery that means 2600 mAh / 84 h = 30.95 mA. We'll call it 30 mA. So I'd like the average load to be less than 30 mA.
So far we've talked about the power capacity, now lets talk about power rate.
The load I want to power has a draw rate of 40 mA for the low power option or 295 mA for the higher power option. Those currents exceed the budget of 30 mA that I set for myself. Fortunately we're talking about averages and we can use that to our advantage. Consider the concept of PWM, the idea that you can trick an LED into operating like it had 50% voltage by powering it with a very rapidly pulsating signal with a 50% duty cycle (on half the time and off the other half). Using this same concept I can extend the range of my battery by switching my load off (putting it into sleep mode) and only turning it on intermittently.
So what kind of duty-cycle would I need? For the low power option 30 mA / 40 mA = 75%. For the high power option 30 mA / 295 mA = 10%. So to cover either option I can sleep for 9 seconds and wake up for 1 second. Or to be safer sleep for 19 and wake for 1. If I really want to extend the battery range I could even wake up for 1 second every 10 minutes, for a duty cycle of 1/600 which would be an average load of .5 mA for high power and .07 mA for low power. That comes out to half a year on battery for the high power option and over 4 years for the low power option! Of course that's all completely theoretical, I think the self discharge from the battery itself would limit me to a month or so at most and the sleep mode for my loads are not truly power free. But the idea that I can power what I need to consistently with the supply I have seems to be feasible. It might even be worth considering a smaller panel and battery if I wanted to make the project more compact.
Another rather sophisticated modification would be to tailor the load to current and future conditions. In the winter it could automatically decrease the duty cycle or if the weather calls for a few days of rain make sure the battery is fully charged and decrease the duty cycle. If it's sunny and the battery is fully charged, set the duty cycle to the max and take advantage...
