chargersolar cell
I have an HTC Incredible phone which eats a lot of battery, and a cy-fi speaker which I use with the phone. I'd like to charge them both while riding my bicycle, and I think the best choice is a solar charger, so I can avoid carrying along tons of AA batteries.
How do I go about building a solar charger? I've soldered in the past, but have no specific electronics knowledge.
Don't use a bridge rectifier. It's inefficient, because you need two diode drops in series, and it doesn't allow you to use the frame as ground for a DC lamp (like, a modern LED bike light). Instead, consider a voltage doubler
simulate this circuit – Schematic created using CircuitLab
The 'regulator' can be a current regulator, or just a Zener followed by a limit resistor. Several LEDs in series can make use of the 'extra' voltage supplied. Accuracy, like efficiency, is less than important in this application.
I've used a 6V sealed lead-acid battery as the 'regulator'; overcharging seems not to be a problem, and a low-dropout regulator IC can ensure that the output to the lamps is a stable 6V.
Charging from a '5V' source is possible, but depending on such things is less than satisfying; the generator has all the power you need.
What is being requested is very well documented and widely available for DIY, and following Instructable links are examples of that.
In fact there are several questions on EE.stackexchange site, which are currently appearing on the right side panel, as I type this answer.
Check your favourite online/offline store and you'd find semi-flexible solar panels, in various dimensions. They claim to be flexible to 20-30 degrees, and could be quite fine for the messenger bag.
They key would be selection of a good solar panel -- since there can be some significant difference in energy efficiency between panels, i.e. the extent to which they can convert the sunlight into electrical energy.
Also note that, not all smartphones charge / charge-well with the standard USB charging, i.e. they make proprietary deviations from the standard, and can expect to be able to draw (much) more than 500mA @ 5V.
PS> I wanted to put this as a comment, but unfortunately, I still don't have enough reps to do anything like that. Surprisingly, I am allowed to post this as an answer.
Best Answer
I think it's unlikely you'll be able to do better than the cheap systems off eBay without spending a huge amount of time. Those systems are relatively rugged, weatherproof and have fairly smart charging systems. Building something equivalent means spending a lot of time finding cheap parts and making the bits you can't buy. If you have a 3D printer that will make it easier, but it will still take time.
I found a ~$40 one that folds and has a 10Wh battery that seemed like the best of the lot on eBay but don't have the link handy. Will edit the link in here tonight.
Given that $40 budget for a DIY system you'll need to start with some scrounged parts. If you can find a LiIon USB battery that will make your life a lot easier - all you need to do is add some scrounged solar panels. But I suggest trying to find all the small solar panels you can to start with. Test them to find which ones still work and what their output is. Work out how to stitch them together to get a single source (both mechanically and electrically).
Then spend most of your money on a smart LiIon charger and voltage regulator to feed 5.1V out the USB port you'll use to get power out of the device. I suggest a battery inside it to buffer the erratic solar input as some smart devices don't like that (my phone, for one). Even a smallish battery will do, a couple of Wh, as long as it can handle the max power you get out of your panels.
I would probably go with either a small 12V SLA or ideally scrounge a complete LiIon booster battery system that has a working charge/protection circuit. Then add a 5V regulated output you're good to go.