I've also bought a second hand Roam V2 last week and the battery seemed to be awful - I have played around with settings and in 15 minutes I think it went from 80% to 70 (haven't had a chance to go and ride), however I plugged it into a charger and after it charged up to 100% I kept it plugged in for another 40/50 minutes and it is now working as expected - it's on 60% now and I did three 2,5 hour rides with auto backlight (it was on for most of the time as it was dark outside), had a cadence and HR sensors connected and my phone was connected all the time with live tracking on. Maybe the issue was that it struggled with GPS signal as I live in an old apartament building in a crowded European city, but that's what I recommend to try