Li-ion batteries need to be charged at a several times year.
Originally Posted by
noglider
My Cygolite Hotshot (tail light) is two or three years old, and the battery no longer holds a charge. I contacted the company from their web page, and a person named Paul from the company said I could return it for battery replacement. I'm willing to pay, but he hasn't yet told me if I have to or how much it will be. So far, so good. This is the normal lifespan of a LiIon battery, so I'm not complaining.
Li-ion batteries slowly leak away their charge, if they get to zero charge they are very susceptible to damage and complete failure. Charging them up every three months should do the trick.