Step 8: The Hardware: an ultralight solar charge controller
The solar charger has four main subsystems. I'm going to spend a few minutes going over them. Feel free to skip this page if you care more about the "hot to make this" than the "why this works," as it runs a little long.
The solar cells. The cells we are using are flexible, thin-film plastic cells. They convert sunlight into electricity. In our case, our solar array puts out 3 volts and 150 milliamps in full sun, and less than that in cloudy, hazy conditions. The voltage and current provided by the cells is variable, so we'll need a way to store the energy in a more useful form. For this, we need
The storage battery. We'd really like to just hook the solar cells to the cell phone and call it a day, but this doesn't work because much of the time they can't put out enough instantaneous power. So we'll store the solar energy in an intermediate battery which can provide high power.
The solar controller. This circuit detects when the batteries have stored up a useful amount of energy, and turns on the cell phone charging regulator. Then it monitors the amount of energy in the storage battery, and turns off when the storage batteries are depleted.
The DC-DC converter. The solar system runs at a variable voltage between 2v and 3v. To charge the battery we need a regulated 5V DC. This is accomplished by using a DC-DC converter, in this case a lightweight universal converter called an AnyVolt Micro.
When I initially envisioned the charger section, I had planned to do it the easy way: a solar panel feeding a regulator which would trickle charge the battery. Unfortunately, Motorola decided that this wouldn't be possible - my Motorola V325 phone requires at least 100 milliamps of charge current, or it will continually cycle the charging circuitry on and off. This actually depletes the battery rather quickly instead of charging it, which is the opposite of what we want. In full sun, the solar cells I'm using provide 3V and 150 milliamps when wired in parallel. This isn't enough power to sustain a 100 milliamp charge current under real-world conditions, where we have cloud cover, tree cover and time of day to contend with.
This is the exact same problem that full-scale solar power systems have to contend with: peak power demands often exceed the abilities of the solar cells. We're going to solve the problem in the same way: by using a storage battery. In this case, I've used two AAA NiMH batteries in series. These are charged at all times by the solar cells through a diode. The power output of the solar cells is moderate enough that we don't need to add any other circuitry to prevent overcharging the batteries.
However, we will need a circuit to start charging the cell phone when the storage batteries are charged, and stop charging the cell phone when the storage batteries are depleted. This will prevent the cycling problem - the batteries will charge up for, say, 20 minutes at 100 milliamps, until they've reached a high enough state of charge to deliver an appreciable amount of energy to the cell phone. Then the cell phone will charge from the storage battery at, say, 250 milliamps (the actual charge current depends on how dead the cell phone's battery is) for ten minutes, and then with the storage battery depleted, shut charging off for the next twenty minutes until the storage batteries are charged again. In this way, we charge the cell phone at an acceptable charge current even though the solar cells can't put out enough power continuously.
An advantage of having a storage battery in the system is that you can leave the solar charger outside or in a window to collect energy during the day, and then charge your phone using that stored energy in the evening after the sun has set. This makes it possible to use the solar charger as your everyday cell phone charger, instead of being for emergency or off-the-grid use only.
Another advantage of having a storage battery is that it means the solar charger can function as a solar battery charger for AAA cells. This is useful if you're carrying, say, a digital camera that runs from AAA cells. If you needed AA batteries instead, use an AA battery holder in place of the AAA holder in the instructions.
The last nice thing about this system is that in an absolute emergency, say, after an alien invasion has blotted out the sun, you could replace the rechargeable NiMH batteries with disposable alkalines and charge your cell phone from them. In most circumstances you'd never need this capability, but if you have to contact the president regarding the aliens' one weakness, it might just save the planet.