Key
 

Hi my first post,
Key to my v50 was playing up unlocking ,changed battery know it won't work at all ,checked voltage

how about the other key does that work fine? These fobs can play up mainly due to wear and tear ... That is why it is vital to have 2 working ones ...

If it worked before battery replacement, try the original one again. Presumably it can only go in one way round, and check its clean and making good contact.