Yes, a 'solar charger helmet tail light'. That's a mouthful. In my world, every item I have to carry has to have one great function or even better, multiple functions. Basically the second option gives you more bang for your buck.
So, here's the idea: I have a 2-AAA tail light that I can mount to the back of my bike helmet. I also have a solar panel that will charge 2-AAA batteries. But why have two devices that hold the 2-AAA batteries? Could it be possible to combine the devices, getting rid of the second battery holder? Why not! When the sun is out and the solar panel is able to charge the batteries, the tail light would not be used or even needed in daylight. And at night, when the tail light gives me some safety on the road, the solar panel wouldn't be able to provide any power anyway because it is dark.
In the simplest scenario, I would just wire the solar panel to the tail light. But without any charging regulation, the batteries could overcharge, damaging them. I could add a switch or a charging regulation circuit but where. A simple idea would be just to put a disconnect plug inline between the light and the panel. If I don't use the light, there is no need to plug the solar panel back in. And, with a plug, I can now swap other devices to connect to the solar panel.
For example I could make a separate battery holder for charging AA batteries. Or, for my bike tour across Canada, I ended up making a single-AAA battery holder so I can charge the battery I use in my new 'TechnoPipe'. Or I could at an extension cable so I could maybe charge other devices located elsewhere, like on a backpack strap, or inside a backpack, or maybe in my pocket.
For this project, I'm using a PowerFilm Solar panel; extremely small and lightweight (but a bit pricey). I've glued the panel to a backing to give it a little more durability. I added blade-type wire connectors on the back of the panel so I can use this for future projects without a chance of damage trying to un-solder other connections. I can use the battery holder for another project but left the diode connected to prevent battery discharge.
I've made a cover out of nylon and vinyl, using ribbon to seal the edges. This is not waterproof (I have a helmet rain cover so I don't have to remove the solar panel in case it starts to get wet). I have added velcro, which also can be used to store excess power cable. By convention, I connected the red (+) lead from the solar panel to the the center pin of the power cable.
Since the tail light is so small and compact, there is absolutely no way to install panel mounted DC plug. The other option is to add an inline plug. I took the tail light apart so I could safely drill a hole for the power cable. I soldered the power cable to the battery connections on the circuit, with the center pin power cable wire going to the positive (+) terminal. This should work when reassembled because when the batteries are charging, the tail light will be off. When the tail light is on, no power will be provided by the solar panel because it is dark.
All this sounds so easy. The most difficult portion is finding the power plugs, especially the female inline jack. There are at least a dozen 'standard' male DC power plugs, but not all those sizes have the corresponding female jack available. After repeated trips to several electronics suppliers, I settled on 5.5mm OD x 2.1mm ID (but even with this there are two lengths available).
Now, it's all done and it's time to mount the whole system. With so many helmet designs out there, it would be impossible to come up with a system that would work with them all, but using shock cord might work for most. For my helmet, I threaded the shock cord through the mounting slot for the chin straps, and then tied them off in the back of the helmet near the tail light. Using the velcro flap, I attached the solar panel to the front of the helmet and then fed the shock cord through the solar panel loops to hold down the back end.
Like I said earlier, with the DC plug connections, I can swap out other devices. This will help me more if I take the solar panel backpacking.
Ultimately I want my solar panel to charge my MintyBoost, which then can recharge multiple devices like my iPod. With that in mind, I have a few future projects to work on.
MintyReBoost: MintyBoost using 2-AAA's with charging plug input
MintyReBoost II: MintyReBoost with a battery capacity indicator
MintyCameraBoost: MintyReBoost able to charge camera battery