Friday, September 21, 2007

Solar Powered Mobile Phone

I recently purchased a new phone from Virgin Mobile, a UTStarcom model they call the 'Slice'. It's about 3/8ths of an inch thick and very rectilinear. The standby time is 11 days on a 900mAh battery. Presuming that the phone uses the entire 900mAh over the 11 days, that is an average standby current of 3.5mA at 3.7v for about 13mW standby power. That is really low.

The Electronic Goldmine carries a small solar panel that it says produces, in full sun, a short-circuit current of 20mA with an open circuit voltage of 2.4v. Unfortunately, they don't give a figure for the power generated by the panel. I have no idea how to calculate that from the given figures, because the short-circuit voltage was not given. So, I'm going to try a ball-park estimate.

The panel is about 800 square mm. The photovoltaic material covers maybe 40% of that area, for a collector area of 320 square mm. Full sun is usually estimated to deliver around 1000W per square meter, or 1 milliwatt per square mm. The solar panel then receives about 320mW of power. If the solar cells are 6% efficient then the power output in full sun would be approximately 20mW.

If they produce 20mA into a short circuit in full sun, and if my estimate of 20mW is sane, then I should get sane values if I calculate the voltage and resistance of the circuit. Working out the voltage ( P=VI ) gives me 1v, which is sane. Working out the resistance ( P=I2R ) gives me 0.05 ohms, which is also sane.

So, one of the panels should provide, in full sun, about 20mW at 1v. Because I'm not often in the full sun, I have to derate this to about 1/3. To compensate I'll use three panels which, as it happens, fits nicely on the back of the phone. Connected in series I should get around 6mA at 3v in good lighting.

The phone requires a stable 5v supply for it's charger, so I'll need a power conditioner to boost the panel's output power to 5v. I can do this with a simple blocking oscillator, a small and simple circuit.

The phone charges its 3.7v 900mAh battery from a 5v supply in about 2.5 hours. Presuming constant current over the 2.5 hours that is 360mA at 3.7v, or 1.3W. Round it up for charge inefficiency for 1.5W. At 5v that is about 300mA from the charger, which is a switching model with a nameplate rating of 1A.

Assuming the phone draws around 300mA from the supply for charging purposes, and that if it can't get enough current it will behave poorly, I'll need to store up enough power so that the phone can charge for a resonable period before I run out of juice. Since the phone turns the screen on for 10 seconds when the charger is connected and disconnected and since that causes the phone to use much more power than the standby current (I'll have to measure that directly, no way to calculate it), I need to keep charge cycles to a minimum.

I'll need to use either a small 5v bank of rechargeable batteries (4 1/3AAA NiMH cells) or about 5F worth of super capacitors. The batteries store more power and have a better discharge curve, so they are probably the way to go. Four 1/3AAA NiMH cells at 1.25v each would put the output voltage right at 5v for most of the discharge curve, and they are good for around 200mAh, so they'd give about a 40 minute charge cycle, which is really good. A super cap would not provide as stable an output voltage, but depending on far out of spec the phone will allow the voltage to get before giving up, this may be ok. The super caps will handle the full 5v on a single 2.5F cap, so two of them would work.

If the panels provide 5mA it would take around 40 hours of full sun to charge the batteries. Of course I don't have to fully charge or discharge the batteries.

It seems like this is a fairly complicated scheme involving multiple power conversions and a large external power source. While I'm sure it would work, it's not something I'd really want to carry around on my phone or as a seperate device. The alternative is to bypass the phone's charge circuits and access the lithium battery directly. Since this is a trickle charge I wouldn't have to worry much about overcharging the battery. I would have to be careful about sending too high a voltage to the phone's internal circuitry.

I can use the panels to run a simple blocking oscillator with the output connected to the battery terminals. When the oscillator fires it will direct a short charge pulse into the battery. The voltage of this pulse would probably not be too much higher than the battery voltage, but in order to protect the phone I'd need to include a simple filter, perhaps just a small coil. I could also disconnect the phone from the battery and let it run from a capacitor while I take a few milliseconds to charge the battery, but that would take more components, and I'd have to make sure that the charge doesn't happen during a call when the phone is using high current (otherwise it would take a significantly large cap).

I would have to intercept the connections to the battery so that I could put the filter inline. A simple way to do this is to make a double sided PCB that can slide down between the battery and phone contacts. There is about 0.5mm of clearance between the battery and the contacts, so probably just enough room to get a thin board in. I could also open the phone and modify the battery contactor, but that runs a higher risk of breaking something.

This solution is much better than using batteries because I can do all the work with surface mount components and then mount the resulting circuit on the back of the phone with the solar panels. This gives me a very sleek installation that only makes the phone a bit thicker.

The solar panels I've got are probably not the best for tight spaces. As I noted above they only have about 40% coverage with acutal solar material, and the PCB backing they are on is pretty thick. I could probably get a bare cell and fabricate a more powerful, thinner panel. This would do a better job of keeping the phone charged under less than ideal lighting conditions.
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