While working on the design for a future Voltaic product, I headed outside to get some preliminary
real-world solar generation data (A perk of working in the solar industry is the ability to spend some
time out in the sun). I had a few devices with me: an iPhone 4S, DriodX, and iPad 1, along with some
lab equipment that would allow me to monitor how the power was being used.

The devices charged faster when they were in the shade and relatively cool then when they were hot. Why?

Here’s what happened:
When testing the iPhone, the data looked great. It was accepting 1A of charge current from the
charger. Happy that the solar performance was as good as charging from wall power, I moved on
to test the DriodX. Plugging it in resulted in the phone displaying a ‘charging’ state, however my
equipment was telling me that it was taking a measly 50-100mA. What???? I checked the phone in
the lab and it would take up to 1.3A. I briefly checked the iPad, and found that it started by accepting
1A, but quickly dropped to 500mA, then 100mA, and finally settled at 30mA. Again, this was not
consistent with the 1.1A that it would use when charging from wall power.


As soon as I picked up the devices I knew why: HEAT! They had been sitting in the direct sun on a
hot day in a parking lot. The iPad’s black color, and the DriodX’s black plastic case made for a couple
of very hot devices. In any well-designed Li-Ion battery system, there is a temperature sensor on the
battery to prevent damage from intense charging/discharging operations at extreme temperatures. The
devices knew that it was too hot to take a high charge current, so they didn’t accept much power.

Sure enough, after placing them in the shade, I was able to continue testing and found that the
prototype charger was able to provide the same performance as wall power.

Something for everyone to watch out for: Overheated devices will look like they are charging normally,
but may be taking only a tiny sip of power. If you’ve found that your device has been ‘charging’ for a
long time, but not really increased in battery life – try slipping it into a shady spot!

2 Responses

  1. Chad

    I’ve encountered this with a number of different solar chargers. Some of the chargers work better than others in the heat, especially in a hot car. But most of them wisely turn off if they get too hot to prevent damage to the battery and other components.

    Reply
    • Independent

      Good post–thanks. There are snippets of information around on the web saying certain things when the mobiles become hot, say “do not charge your phone in the sun” and if the temperature gets above insert-some-specific-temperature-reading-in-degrees-C.. And that’s why. A thermistor in the battery telling the phone (I almost wrote computer) to stop or severely slow down the charging process.

      Reply

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