# How do I build a solar charger for my phone / other devices?

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.

• Other possibilities than solar would be a hub generator (expensive), "bottle generator" (dynamo that presses against the tire), or using magnets and coils similar to systems for lights only (commercial version) to turn your bicycle wheel into a dynamo (generator). – freiheit Apr 28 '11 at 16:48

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.

Another solution is to wire in a power adapter connected to your motocycle's battery that can charge them via a 12v>5v USB adapter. It would be by far the simplest approach and probably a cheaper one than buying batteries and such. Make sure to add a fuse and a cutoff switch.

Unless you're in an area that gets lots of sun, a solar panel probably won't be able to pay for itself with short term use as they are relatively expensive. You can also look at projects like the Mintybuck:

http://www.nycresistor.com/2010/05/04/mintybuck/

The Mintybuck is a DC-DC Switching converter (Higher Efficiency than a Linear Regulator) that runs off Li-Ion batteries based off of Ladyada's Mintyboost project. You can pick up Li-Ion batteries from plenty of vendors and even laptop batteries. 18650's are between \$3-8 each and pack plenty of power if connected together.

If your mind is set and you want a solar charger I suggest just buying one pre-made from a company such as Brunton. You will need to have a smart charger for the battery as well as a proper DC-DC converter as the voltage can drop in the solar panels which means you'll need a SEPIC configuration converter.

A great generic DC-DC converter IC you can use to get your hands dirty would probably be the 34063, or the LT1302 if you only want 5v output. Lots of information on these particular chips.

Something to keep in mind is that USB powered devices can draw more current than the standard 500ma upwards of 1A. These devices usually sense the device they are attached to by shorting the data leads or applying a voltage/resistor to them. You can find more out about that here: http://www.ladyada.net/make/mintyboost/.

• I think this is a bicycle, not a motorcycle. But a generator hub would work in that case. – neilfein Apr 28 '11 at 7:03
• Well it was definitely migrated from bicycles SE so if it isn't then the OP was in the wrong place initially too. – Amos Apr 28 '11 at 22:19
• Yep, I'm one of the ones who migrated it. – neilfein May 2 '11 at 4:26