Citroen Relay Battery Issues

[dilby]

Hi all -

Recently bought a 2017 Relay in order to do a conversion over the winter, but 3 weeks in and the battery has died. I've only done a couple of short runs in that time, but wondering if any alarm bells should be going here, and

1) Is there a way to find out if anything is draining it?
2) Should I avoid a jump cable on this as I've heard you should stick to trickle chargers to avoid any issues caused to other components (not sure how true this is).
3) Is there anything i should be checking while I'm still in warranty period?

Thanks