lower wattage demand extends the battery life .. as does a blinking function.

D sized flashlight batteries generally are higher capacity, .. that's amp/hours data..

probably can put a big common battery in the cargo box,
but 2 AA, one per your mudguard perch could be adequate.

I got a bolt on mount that Busch and Muller offer to have a QR clip ,
for many handlebar mounted headlights * off the handle bar..

Alkaline disposables are 1.5v rechargeables are 1.2v so you need a 5th battery to reach 6v rather than 4.

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