To preserve the condition of the Lithium battery, when the Boomer isn't used for an extended period, the batteries should rest at 50% charge. To achieve this, simply charge the Boomer to 100%, hold down the button for 5 seconds. This will turn the light on and automatically shut the Boomer off when the battery reaches 50% charge.
This light (and its twin rear light) is fantastic - blindingly bright if you look directly at it and yet it fits in the palm of your hand. The diminutive size coupled with the simple wrap-around method of attaching the light to the bike mean it takes seconds to put the light on or take it off and it can be popped into a coat pocket. USB charging is a great idea.
This light from Knog is quite bright enough for my urban cycling, it indicates when it is charged and cna charge with the USB either with a power point or through my computer, no more suddenly finding I need a battery to get home in the evening.