cyccommute 

It goes even further than that. Lumens and lux are weighted based on the sensitivity of the human eye to certain wavelengths. Some of the wavelengths that lights put out...infrared, for example...we can't see so it's not included in measurement of the 'luminous flux' used for measuring lumens.

cyccommute 

I recharge after every ride. It's just habit. A battery pack may make one trip to work or four but the problem is when it makes that half a trip to work. It's a little appreciated fact that it gets dark when the sun goes down. You get a bit more of an appreciation for that fact when your light blinks out and you have a few miles to travel. Due to some planning errors on a Gilligan's Island ride...aka a 3 Hour Tour in mountain biking...I learned the hard way what happens when the sun goes down. I now carry a backup light on my person at all times and a backup to the backup to backup the redundant systems on my bike. Stumbling through a forest by feel and by the light of a squeezy keychain light puts the fear of dark in ya.

As for the recharging, use a smart charger. If you use a lithium based battery, you have to use one. If you use nickel batteries, you'd be better off using one.

smasha

li-ion has a higher energy per volume than alkaline batteries, so a li-ion battery the same size as a AA or AAA would still have more power. and weigh less.

if you want to take it to extremes, you can power car with AAAs.

without getting into DIY, one is generally better off just starting with a more robust power source than AA/AAA.

Burton

Just so you know - I have both rechargable AAs, AAAs and 3S1 Lipo cells. If you're looking for something 'more robust', 18650s are no different than AAs compared to larger Lipos. The thing that hasn't been mentioned is that rechargable AAs are less than half the price of a lipo and will last a lot longer. Some of mine are currently 12 years old. So use what you want - lets just not pretend there's only one answer.