Battery dead?

Hello everyone,

I have an intelligent battery charger from Monsana and I’m trying to charge the Yuasa battery of my Yamaha R6 RJ15.

However, when I connect the charger, it only shows about 0.8 V and then displays the error message “Err 1.”
The polarity is connected correctly (plus to plus, minus to minus).

I had it charged up all the way around 2 month ago.
The Battery stood warm inside by House all winter long

Does anyone know what could be causing this?