I've got a rarely used backup battery light too, but mostly I rely on the dynamo. I generally only need the backup light on very wet rides if the dynamo begins to slip on my wheel. BTW-I'd trouble shoot the connections at the lamp and bulb if you've got a problem with losing light when hitting bumps, etc. Among problems I've like this are poor or loose ground connection at the lamp housing, and insufficient tension on the spring contacting the back of the bulb.
I'm also interested in continuing the previous discussion regarding charging batteries with a dynamo while riding. First of all, many better dynamo lamps and taillights now have a 'standlight' circuit that stores enough charge in a capacitor to keep the lamp on for 5 minutes +/- after stopping, so you still have lights while you're waiting at an intersection, for example.
There was also a previous post with some comments on charging conventional rechargeable batteries with a dynamo, that discussed use of rectifiers to rectify AC output and diodes to prevent backcharging.
My comments on this are that rectification should seldom if ever be necessary with a bicycle dynamo, as I believe almost all bicycle dynamos put out DC and not AC current. If they put out alternating current, they would probably be called 'alternators', and not generators. The output rating and type of current should be indicated on the dynamo shell.
Since diodes only allow for one way current flow, it makes sense to me that the use of a diode in the charging circuit would prevent backcharging or draining of the batteries while you are charging them.
IMO, that still leaves two unresolved issues:
1. How do you control the rate of charging, and is this necessary or not?
2. How do you control overcharging (shutting off the charging system) when the batteries are fully charged, and is this necessary or not?
It seems like once all these questions are answered, a pretty simple schematic could be drawn up, and such a charging device could be built inexpensively with readily available electronic components.
Also, there are some very low drag bottle dynamos on the market today. They cost a little more, but if efficiency and drag are significant concerns for you, you might try one of these low drag models. You will pay more for one of these units (figure $100.00 +/- for the dynamo alone, excluding lamps), but it may be worth it to you.
B&M Dymotec S6, available from Peter White Cycles:
http://www.peterwhitecycles.com/dymotec.asp
Dynosys Lightspin, not sure about US distributor:
http://www.dynosys-ag.ch/indexe.html