From the datasheets of Durcaell batteries, we can construct the following table using alkaline batteries:
|Power consumption||2 AA||4 AA||2 AAA||4 AAA|
Now, what happens if you are commuting every day in the dark? How many batteries will you go through? Remember, making the light blink only helps in some situations and we may hit a LED efficiency wall.
Rechargeable batteries are a good idea... and with modern LEDs and batteries, you don't need to shove a battery in your water bottle cage.
However, remember that some of us also think the idea of riding long distances away from civilization sounds like fun. This creates problems, because a commuter won''t be far away from a power plug but a touring cyclist or randonneur will be.
If you make a light that can take AA or AAA batteries, your light needs to be able to accept a variety of battery types with different properties... which makes it hard to give the user a charge level indicator... but you can use a NiMH AA cell when you are close to home and carry some alkaline AA batteries if you want to tour. If you use a custom Li-Ion battery, you can pack more power in a smaller package but can't just buy disposable batteries if you are on a tour. If you have an optional cassette that takes AA batteries and a Li-Ion pack the user will lose the optional cassette. So we''ll have to see what sort of designs people generate...