Hi there Mega Hunter, I have got and am using the Nite-Site NS50, and as you i found the battery is heavy so had to change because couldn't see myself carrying that in and out farm building and hunting in woodland. So i bought my replacement from E-Bay and the item number is 250919245956 at a cost of under £20 i have a replacement that is no larger or heavier than my mobile. Hope this is of a help to you.
ATVB
Roger