Estimating Battery Charge Time from Solar
Jeff Crystal – Voltaic DIY Solar
You have a 2 Watt, 6 Volt panel and a 1,000 mAh, 3.7V battery, how long does it take to completely charge? The quick and very wrong answer would be to figure out the Watt hours of the battery (3.7 * 1Ah = 3.7 Watt hours) and divide. The reality is about 2.5 times longer.
There are three main reasons for the difference, even in ideal conditions. First, the Wattage rating on the panel is the open circuit Voltage multiplied by the peak current. When you connect a panel to a battery, the Voltage drops down to that of the load, in this case 3.7V. Finally, all the power that enters the battery does not get converted into storage energy. Some percentage is lost as heat as the process to convert the incoming power into stored power takes energy.
In field tests, we’re seeing that the combined loss factor is about 2.5. So Divide the Watt hours of your battery by the Wattage of your panel and multiply by 2.5. In our example above you would get 3.7 Watt hours / 2.0 Watts * 2.5 = 4.6 hours to fully charge. If you have cloudy conditions, your panel is not pointed at the sun or your panel’s Voltage is not well matched to your battery, this could increase.
With large-scale systems, maximum power point tracking is used to increase production efficiency. We haven’t seen any cost-effective examples in small-scale systems yet. If you know of any, please drop us a line.
http://voltaicsystems.com/DIY/estimating-battery-charge-time-from-solar/
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