New battery loses power by standing a day or two! Dead/weak charger in the bike?

Something is not ok. Around midnight I tested the engine, started right away, idling good, that was when I measured the 14.50V, then switched off and it went down slowly to 12.80. But: today I tried to start with choke, sidestand up, killswitch disengaged, fuel opened, and the starter was spinning the engine good, but never started.

I also tried to bump start, without any success. Sometimes I heard a faint bo-bo-bo, like it was starting for one second, but still nothing.

I don't undrestand what happened, as the engine started right away last night.

And I tried so long, long minutes was the engine spinning, that I killed the battery. I'm recharging it now, but it was working heroically, turning the engine for long minutes.
I'm going to check the spark plug, but It's also new, never had any issues since 3 months with it, and it can't be dead since last night. Engine was running fine, etc.

Could it be a problem, that I filled it with 10w-50 instead of 10w-40? Haynes manual said it's also ok.

What can be the cause? Please..