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Apologies to those waiting for the next installment of this project, I have been busy with various client sites…

As I mentioned in part 1, I decided to make some real world measurements of the performance of the solar panel and also the power requirements of the iPhone 3G.

I hooked up a DC power monitor to a 12 volt battery and for a period of exactly 1 week I charged the iPhone exclusively from the 12V battery via a Kensington car charger. In 1 week of my normal use, (ie with WiFi and bluetooth switched on 24/7, email checks every 15 mins, some light browsing and probably 3 or 4 hours of calls) the phone used 43.5 WH (watt hours).

(To publicly answer those who have asked, the iPhone charges at 3.5 Watts continuously but slows down as the battery gets close to full.)

So how did my little 2.4 Watt test solar panel fare? I measured the output over a period of 7 days although I will point out that due to the time of year (sun low in the sky and thus a shaded panel) and a few other factors the panel only had direct access to sunlight for a few hours per day, so it could do better…

Day 1 (10am - 4pm = 6 hours): 0.1 Watt/Hours (heavily overcast)
Day 2 (10am - 4pm = 6 hours): 0.1 Watt/Hours (heavily overcast and raining)
Day 3 (10am - 2pm = 4 hours): 1.8 Watt/Hours (blue sky, v few clouds)
Day 4 (12pm - 3pm = 3 hours): 0.9 Watt/Hours (blue sky, some cloud)
Day 5 (11am - 3pm = 4 hours): 0.9 Watt/Hours (lightly overcast becoming clear blue later)
Day 6 (10am - 2pm = 4 hours): 2.1 Watt/Hours (blue sky, v few clouds)
Day 7 (12pm - 2pm = 2 hours): 0.6 Watt/Hours (blue sky, some cloud)

That’s a total of 6.5 Watt Hours in 7 days. Due to the limited number of hours the panel was setup I suspect I could get another 30% on top of that figure so about 8.5 Watt Hours in 1 week from a 2.4 Watt rated panel.

The phone used 43.5 Watt Hours in 1 week so I need a panel about 5 times more powerful than my little test panel, which would be a 13 Watt rated panel…

But of course this is the UK, in winter. I could save money and buy a smaller solar panel along with a decent sized battery and fully charge it over summer. In the winter the iPhone would slowly deplete the battery as it would be taking more current out than a smaller panel could put in, but as long as the battery had enough reserve to make up the difference until spring I’d be ok. Then over spring/summer the panel would generate more power than the iPhone needed during that time and thus replenish the battery. At least that’s the theory…

So part 3 will be some calculations (yes more math, sorry!) to see what size battery I might need in order to buy a cheaper less powerful solar panel than 13 Watts. Maybe the sums won’t add up or the battery would have to be so big it would be cheaper to buy the more powerful solar panel…

Filed under: Gadgets