I posted this over at candlepowerforums the other day, and realized I should post it here as well. I've been reading this forum occasionally for several months, and profiting from the helpful, knowledgeable folks here. If you're interested in this, and want to reply, I suggest reading the
thread at cpf to get updated, since I'm keeping up with that one regularly, but I'll keep up here as well, so you can post here if you don't want to register at cpf.
A frequent frustration for me when on long bicycle tours is the difficulty of keeping my batteries charged for my bike lights, cellphone, camera, laptop, and GPS. Now that I've found that hub dynamos can
produce far more than the 6V/3W they're rated for, and now that I have money to invest in an electrics system for my touring bike, I've decided to build a system that does what a car electrics system does: generate electricity from motion, store that electricity in batteries, and use it for lights and other things. I drew up a quick-n-dirty block diagram in MS Paint to help me understand how it will all work, and thought I should include it here. It's at the end, but you may want to skip down and reference it as you read my long and complex explanation of the system.
The power in the system will be generated by a Schmidt SON hub dynamo. All of its power will go to charge a Li-Ion battery pack. The pack will have twelve
18650 cells (wired in 6S2P, output voltage range of 18-25.2V), since that's as many as I can fit in the water bottle they'll be housed in. I plan to charge them by sending the varying AC output of the dynamo to an automatic switching voltage doubler/fullwave rectifier
circuit to give me at least 15V at low and high speed, and feeding that into a switch-mode buck regulator which will output 12V. That regulated 12V will power two battery
chargers designed to charge a 3S Li-Ion pack from 12V, with their inputs (from the regulator) wired in parallel, and their outputs (to the battery pack) wired in series, so that their total output voltage to the 6S pack will be the 25.2V it needs.
My front light will be 4 Cree P4s controlled by a
bFlex, hooked straight to the battery pack. I'll also use a switch-mode buck regulator to output 12V from the batteries, and use that to run 12V car battery chargers for my cellphone, camera, laptop, and GPS batteries.
I've already bought the parts for my front light, but they're not all delivered yet, so I'll assemble that when they come. I also have 4 adjustable switch-mode buck
regulators rated for 60V input, which I mentioned above, and my Schmidt SON is on order. I haven't yet bought the Li-Ion batteries or chargers, and my design for that system is not yet finalized, so I'm looking for input.
I have some design constraints. Using 4 Crees for the front light requires a minimum 17V input to the bFlex, so I have to wire my batteries in at least 6S so that I can deliver that voltage when they are almost drained, at 3V/cell. That high battery voltage also means my buck regulator can always deliver 12V to my various other battery chargers. But I can't find Li-Ion battery chargers that can take 12V and charge a 6S pack, so that's why I'm planning to use two chargers designed to charge 3S packs from 12V, and wire their outputs in series.
Almost as an aside, my laptop takes 19.5V, so I will probably set one of my buck regulators to output 19.5V from my Li-Ion battery pack and plug that straight into the laptop, instead of buying a step-up converter designed to power my laptop in a car, since that would be unnecessarily stepping down to 12V and then back up to 19.5V. Also, the laptop power pack is rated to deliver 3.34A at 19.5V when plugged into AC wall power, so I could use one of my switch-mode buck regulators to step that down to the 12V that my Li-Ion battery chargers need, thus enabling me to charge my Li-Ion pack from AC wall power.
One potential problem I see is wiring the output of the Li-Ion chargers in series to charge a 6S pack with two 3S chargers. Does anyone have experience with this? Will the charge regulation be affected?
As I've said, I'm asking for your input, both problems you see and ideas you have. I don't have any experience working with dynamos or building lights, and have only limited experience designing and building electric circuits, but I've been reading and understanding a lot (mostly from the excellent posts here), and I think I have a working design. The details of the light design and building deserve their own thread, and I'll probably need to start a new thread for adapting Martin's voltage doubler/fullwave rectifier circuit to power battery chargers through a regulator, too. Right now I'm just trying to get this electrical system design out of my head and into the real world, and I need your help.
Alex
