# Charge batteries with a bike dynamo?

I am experimenting with ways to charge batteries while cycling. Standard bike dynamos produce 3 watt 6 volts.  That's not a lot of power but 12 hours of cycling should charge a 36 watt hour battery.  I am thinking about about making my own battery pack.  It needs to be 4s1p 14.4 volts.

What I need help on is what sort of battery chemistry to get: lithium ion, LiMiNi, lithium poly, etc.  The batteries I need should charge efficiently at low current (about .25 amps).

The other thing I need help with is how to turn the dynamo power into charging power.  I know I need to rectify it and smooth it.  I have heard about Zener diodes.  They are used in solar applications because they are more energy efficient.  Should I use those? Lastly, dynamo power varies depending on the speed of the bike (Between 0 volts and 12 volts.)  Lithium ion cells need a 4.35 volt charge.  Would it be better to use a regular voltage regulator you get a radio shack or a switching voltage regulator.  I have an anyvolt 3.  Maybe there are PCBs you know of which automatically regulate a vacillating input voltage.

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6 years ago
No idea about the chemistry part, but some points to take into account:
- your dynamo rated 3W will produce this at full speed, on lower speeds the voltage and current will drop
- to charge a battery holding x Ah (Ampere hours) you will have to put more than x Ah in. Calculate at least with a factor of 1.3, the actual factor depends on the charging current and the chemistry. The remaining energy is lost as heat.
- any kind of regulating circuit will lose energy. I'd expect you have to produce twice as much energy as will be stored in the battery in the end.

Zener diodes? Good for stabilizing in _some_ applications - when energy preservation is premium, not so much. Energy efficient? In what respect?
For rectification go for germanium diodes or Schottky diodes - low Uf, low losses.

As batteries don't like to be charged with fluctuating voltages/currents, you might want to design a kind of buck/boost switching regulator and use an intermediate storage (like a gold cap).
aruselowski2 years ago

what about using solar cells to suplament your power requirements? I have no idea how to set something like this up but I was thinking about getting a dynamo for my KMX cart and hooking it up to a 12 volt battery. I have been thinking about suplamenting it with a couple of panels from flexsolarcells.com

http://www.flexsolarcells.com/index_files/OEM_Components/Flex_Cells/pages/17-PowerFilm-Solar-Cell-Module-P72-75.php

dasimpson19815 years ago
you could charge your 3.7v pack with a daynamo as the charge controller will stop it if it over charges
seandogue6 years ago
If you go with Lithium, consider using LiFe type. They're much more tolerant than traditional lithium batteries. You'll still need to monitor temp, but they can be charged using a far less complex circuit than others.

You'll of course want to regulate the source voltage, and no, they're not going to like a fluctuating voltage.
Noblenutria (author)  seandogue6 years ago
If you were going to make a battery pack would you use a premade PCB or make your own?
6 years ago
Well, I'm not your average bear. I've been designing circuitry for 30 years, and I did so professionally for 25 of those.

For a homemade project? I'd probably build my own. Whether or not I'd actually layout a PCB and etch it is a question of the complexity and the necessity of a compact design. For a bicycle? I'm guessing that I'd build it using copper plated perfboard. I don't have an perchlorate, can't stand using Ferric Chloride, and due to my living situation, I don't really have a suitable work area for doing chemistry.

In terms of how much current, Li batteries are not really suitable for trickle charging . In fact, afaik, the only chemistry suitable for trickle charging is Lead Acid.

FTR, fluctuating current is not really a problem it's fluctuating voltage that will bite you in the ...
Noblenutria (author)  seandogue6 years ago
Is there any problem in charging these batteries at very low currents?i know it will take a long time but is it more or less efficient to charge at higher currents?
onrust6 years ago
You have to check this cat out! fossilfool here on instructables. THEN, he is on flickr as Rock The Bike. I have also done mail order business with Rock The Bike and stand tall on the "fool" ........ his human powered "band stage" rocks San Francisco parks while blending drinks!
orksecurity6 years ago
The fancier the battery chemistry, the more careful you have to be about controlling the rate at which they charge and not overcharging them. Overcharged or overheated lithium or NiMH cells can ignite (!) or explode (!!).