Tips for my Nexus 5

Hey guys so my nexus 5 is having battery problems with wrong time for percentage and 1% drain every minute. I thought it was snapchats fault but maybe that isnt it. I did tend to spin my phone on the table so would that be it?
Anyways my options are to either fix the battery, sell phone as is(how much do u guys think), or downgrade to lollipop(i tried this but the fastboot flash wouldnt work and im having this error: is not recognized as an internal or external command).