Going batteryless...
They're a little expensive, but a generator hub (among others,
shimano and
Sturmey-Archer make one) put out enough power to feed a maglite, and, they don't drag like a bottle generator does. If you're not pulling power, they don't drag at all... Another advantage of the hub-style generators is that they are completely enclosed; away from the elements and road gunk.
I like the Sturmey-Archer, as it is a combination hub - with a brake and a generator.
If you're using a bottle-type generator, the old fashioned schwinn types add a lot of drag to the bike. (They're horribly inefficient.) The German-made Busch & Muller Dymotec 'bottle' generators are much more efficient (at about 40%) and cause very little drag. They also use larger, replaceable rubber faced rollers, which won't wear your tire sidewall like the metal wheels of the old-style bottle generators.
BTW. You can buy
headlights &
taillights designed for use with these type of generators; some have standlight capability. (They store up a charge while you're moving, then, when you stop, they release that stored charge to keeps the lights lit for a few minutes.
Unfortunately, like all else, the good stuff's expensive.
For side visibility, add a spoke magnet, and a pickup coil for each wheel. The output of the pickup coil (similar to a tach pickup) feeds an LED lamp. The LED flashes each time the wheel rotates. No (well,
unmeasurable, anyway) drag, and no batteries.
Reelight manufactures one of these pulsing lights. So does
freelight
Ref
Bicycle Lighting Wiki