[EC900]

A 4-5 year old battery might not hold charge, especially if it’s not an AGM type or some discount brands, so consider that.

The 12.5v you have might be too low to start, contributing factors; cold temps, N engaged clutch, heavy oil, prolonged storage w/o maintainer etc. and some hidden corrosion inside the cable.

Jump start and get a read on charge output for +13-14v.
Once started and running, check again next day to see if there’s a phantom draw or dissipation of the full charge of at least 12.8v.

Of course never a bad idea to regularly clean up connections.