Problem starting my Movano van?

I have a Movano van conversion, 2500cc, W reg, diesel. It's been off the road for about 4 months with no battery in it. I charged the battery and it started, I ran it for about 15 minutes, the next day there was nothing at all. The battery is about 18 months old. Does this sound like I need a new battery or there is a problem with the van?