If Li-ion, it does matter if you run them down. I just talked to L&M the other day. I have the Li-Ion Ultra.
*L&M lights will not completely discharge the battery but will turn off before the battery is drained and will not turn back on until enough juice put back into it.
*not charging the battery, even if unused, will over time discharge the battery, bad for battery, shame shame. I'm going overseas for an extended period and they said charge it fully once a month and it will be fine.
*you can leave them plugged into the charger without harm, but the guy said if you use it for 30 minutes, don't bother recharging it, use it 30 minutes the next day, etc. I've heard conflicting advice on this particular point. One said X recharge cycles, other said plug in every night, regardless of how long it was used won't harm anything
*when not using it as a bicycle light, be aware they have to stay cool. They told me that if you're sitting still (i.e. no air passing over it) on a warm night for a long period, the light won't cool properly. Sitting still on a ride for a few minutes won't hurt anything.
