Originally Posted by LittleBigMan
Great idea. I agree that bicycle lighting is too often weak and sickly for the speeds I reach in the dark. The last thing I need is to suddenly notice a branch in my way going downhill.
If I've missed anything with this next comment, just disregard it: you figured your battery size (total amp/hrs) by doing the math. My experience with a different battery (sealed lead acid) showed me that the higher the watts, the less accurate the math was. You might find you need more amp/hrs than you first thought, just to prevent this effect. The smaller the battery, the more prone it is to "hit bottom" before it's projected run-time. It's better to have too many AH than not enough.
As someone else pointed out, batteries are rated at low current draws. For higher current draws typically you'll get less than the rated value. I don't know how the curves work out for lead/acid batteries, but I suspect from what you're saying there must be a more pronounced difference at a higher current draw. I stayed away from lead/acid because they are significantly heavier than newer technologies and not really all that much cheaper.
From what I've calculated on the battery I've ordered, I SHOULD be somewhere around 65-70% depletion on my battery even allowing for less capacity at the higher current draw. This is pretty much optimal as far as I'm concerned. I wouldn't want to get much more than 80%, so this leaves me with just a bit of fudge factor. I'll test the battery and lights as soon as I get them and report on the actual performance. If I find the battery performs at much less than my expectations, I will buy another just like it (for around $35 I think) and run it in parallel with the first one. This will double the capacity. I think I can fit two of these batteries in a 33 oz water bottle.