Bicycleman.....You could always put the light outside in the cold and let it run to test how long it goes until it needs recharged. Mfg estimates are usually a bit optimistic but close. I calculated the runtime on mine and it worked out pretty close. I get 22 hours on low, 8.25 hours on medium, 5.25 hours on high, and maybe 2.5 hours on turbo mode but never used turbo mode. I use rechargable 18650 3400 Mah Panasonic batteries. If it is a dark night and there are no other riders, the low setting is plenty up to about 16-17 mph and that is pretty good at night. I need more light riding with others because of the shadows cast from following riders. If it rains during the day, you are supposed to burn your lights and that is when I use my low power setting. I carry a spare set on 600K and longer. My rule of thumb is to double my needed battery capacity if all goes per plan but you know what they say about plans.
You get 13.5 hours to do a 200K and assuming 10.5 hours daylight in good weather, you will not need much in the way of batteries. Just make sure you have as little extra.
