Take the batterys Watt Hours Like this battery = https://fotodiox-inc-usa.myshopify.com/products/vbatt-f130s

130Wh

Divide that by the Power Draw of the light = https://fotodiox-inc-usa.myshopify.com/collections/flapjack-led-edgelights/products/led-flpjk-c518asv 

 40W

130 Divided by 40 = 3.25 WPH

so the battery is 3 hours 25 minute

on the Fotodiox C-518ASV