“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

## Saturday, July 06, 2013

### Solar sells - anything

Let's take a look. The specifications are scant at best and hard to find. I've been able to determine that the battery is a 410 mAH (milliamp hour) Li ion battery. I'm going to assume that this is at 5 volts (the required charging voltage for an i-Device). I'd really like to have specifications for its capacity in watt hours. But, assuming as stated, it's 2.05 watt hours. Now, an iPhone battery has a capacity of 5.4 watt hours, so a full charge, neglecting efficiency, on the solar charger would charge a dead iPhone to about 38% of capacity. An iPad has a 42.5 watt hour battery and thus the charger will provide just under 5% of a full charge. Mine shuts down at that percentage. The "instantly" claim isn't supported by current battery technology in the devices, so the devices must simply operate off of the charger (to the extent that it works at all).

But what about the sun? The Amazon page for the charger lists its dimensions as 5" L X 1.5" W. Using the free and terrific Tracker Video Analysis software (which works just fine on jpegs) I determined that the "high efficiency solar cell" is about 1.46" X 0.85". This is $8.00*10^{-4}{m}^{2}$. I'll also assume the cell to be 20% efficient. I'll use $350\,{\frac {w}{{m}^{2}}}$ as a good average for available insolation in my area. That yields a charging rate of about $8*10^{-4}*350*0.2 = 0.056$ watts and a time to charge to capacity of $2.05/0.056=36.6$ hours. Others at various review sites cite (sites cite? I like it) two days, which doesn't seem unreasonable. On the web site video, they show the unit being charged under a table lamp. Umm... good luck with that.

I actually have several solar chargers for my various devices, all of them are quite bulky as you'd expect. Solar power density is just not that high. It's amazing that a profit can be made by slapping a trivially small solar panel on what is otherwise an extremely poor quality auxiliary charger for your portable device. And, don't forget, for an extra \$10 you can get it with a three bulb LED flashlight. Finally, they'll send a second charger free (just pay separate processing and handling). My rating for this product: