Well this was a lead acid battery, so it probably wasn't much lighter than that brick of Ds! But on a 50 pound bike, it wasn't very noticeable. I left the light on one time to see how long the battery would last, and I wound up turning it off after two hours because the housing started to melt (my first housing was plastic. Mistake!). After two hours the light showed no signs of dimming, and I really never found out how long it would last.