Specialised Bike Light Optics

[Steve K]

5.8v float charge seems high for 4 nimh AA's. I used to charge my nicads at 1.35v/cell, and nicads don't mind float or trickle charging. If I were you, I wouldn't change anything right now, but if the batteries don't last long, consider dropping the voltage.

Using a charger like this can be tricky if you are using a series regulator. At a given speed, when the current decreases, the dynamo voltage increases. For my Schmidt, if I used a 98v zener to clamp the voltage, the dynamo still produced more than a few milliamps of current (at high speeds). If you want to clamp the dynamo voltage, it's best to do it at low voltages. If you clamp at 25v and the dynamo still produces 200mA, then the zener has to dissipate 5 watts instead of the 3 watts that would be dissipated at 6 or 7 volts.

Steve K.

 

[tspoon765]

Before I got my switching sussed out 'properly' I only had one switch and was running the LED through the regulator too - hence the highish voltage. Although on my DAP I can see the voltage across the batteries as they are charging and it is routinely up aroung 5.80 volts, within 2-3 hours of a 12 odd hour charge cycle starting, ending up around 5.87 ish most of the time. As my commuting ride is 90 minutes each way at the most generally I'm not too worried about overcharging. Usually one way of my commute is in darkness so for that 75-90 minutes the blinkies draw their 100mA happily.
Might be a different story when I go on tour, although then I will be recharging DAP battery packs at a fairly steady rate.
Good point about the zener voltage too. It hasn't really sustained any damage all the time that I've run it so far, so I'll probably leave that anyway. Its interesting to see what other people have come up with when designing their LED bike lights, particularly the Guy who runs his dynamo up over 10v most of the time anyhow and uses a voltage doubler at low speeds.