Hi Nikroc

It was left outside but not subzero temp but 3-6 deg Celsius on the coldest nights.

The battery is about 6 years old. Never gave me any problems. On its own nothing switched on. No dash, lights or anything. As if no battery at all.

When I connected bike to car battery everything switched on. But when I pressed start button to turn the engine, everything shut dead as if no battery at all. No battery does anything to bike now. Completely dead and stays dead with both batteries.