When I riding fast, the hub generator on my commute bike puts out more juice than required to power the headlight, particularly on downhills.  So I put together a little gizmo to charge AA batteries with this extra energy, while still having power going to the light. 

This gizmo (a.k.a., the budget battery charge regulator, or BBRC) is based on a schematic from Steve Kurt of the touring and bikecurrent e-mail lists.  My apologies to Steve if some of the words below are too blatantly stolen from some of our e-mail correspondence back and forth (engineers do that sort of thing all the time!  ;->). 

The circuit is intended primarily as a standlight, and as a battery charger second. The standlight allows the headlight to stay light when stopped at a traffic light, or during a slow speed turn.  The circuit doesn't try to completely charge the battery, but instead tries to get it up to 70-80% charge (this varies with temperature).  I've built the BBRC exactly to Steve's schematic, except that I added a switch between the light and the circuit in order to turn off the light while charging the batteries during the day. 

The led isn't required for the circuit to operate. It's just there as a secondary means of telling you that the battery is getting somewhere close to being fully charged (this is *not* a precise indicator!). The primary means is the fact that the transistor and/or R2 are getting very warm! Both Q1 and R2 will get quite warm, and need to be connected to a big heatsink.

The only time the circuit should heat up is when the light is off and the battery is well charged.  This is a result of using a shunt regulator. If the light or battery isn't eating up the 3W of power that the dynamo is providing, the shunt regulator will have to. Not very efficient, but it is cheap and simple.

The voltage will vary with speed and load, but should be near 6.8V or so.  As the batteries charge up and approach the 6.8V level, their charge current will decrease. Whether the light is on or off, you can charge the batteries all day.  The best charging will probably result from riding with the light off. Turning the light on may result in poorer battery charging. A higher voltage setting will more fully charge the battery, but also risks damage to the battery on a warm day.  This circuit is definitely not fancy!  It's a much faster charger than the type that trickle charge the battery, but trades off a full charge for a fast, safe charge.

(Hmm, Steve's got me thinking now - maybe during colder weather I can charge 4 AA batteries instead of 5 AAs.....  )

The hardest part about the BBRC is figuring out where to mount the thing!  I used a Vistalite seatpost clamp to mount it to the head tube.  I have a piece of aluminum between the BBRC and the clamp to act as a heat sink, but time will tell whether or not it is necessary.  If I'm charging batteries while touring, the batteries will go in a holster in the handlebar bag.  If I'm using the circuit as a standlight for commuting, I'll have a homemade pack of 5 AA batteries shrink wrapped together and duct taped to the head tube.

I still need to do an extended road test with this, but it seems to work fine based on a few spins around the neighborhood with a battery pack and voltmeter in the handlebar bag!

Resources

These are all Radio Shack part numbers to make life easier.  This is everything except the Molex connectors, wire, and mounting bracket.

Pictures

Parts used....

Assembly of prototype in progress....

Finished prototype mounted to the handlebars.

Here's the assembly of the nicer looking version.  This is much nicer with Molex connectors instead of the 18 gauge wire!

Here's how it looks on the touring bike.  Note the use of the Vistalite light seat post mounting bracket to install the BBCR on the head tube.  The bracket is designed for a maximum seat post diameter of 31.8 mm (1.25"), which is the outside diameter of the head tube!

Copyright
Last Revised: 21 September 2002