Maybe remaining 5V is enough to survive. I dunno what's the limit for intelligent chargers to actually recognise there's a battery attached to the end of the wires and start charging?
They all vary, normally anything below 10V, for a sustained period is deadly, because of the growth of lead sulphate crystals, which it must definitely have..
It started well enough this morning, a bit slow, but, probably if it doesn't start quickly and I need to keep cranking it will fall away rapidly
Intelligent chargers I think operate in the 10V -11V range, although, I've had success previously with a 16V supply onto a 12V battery, but with the current limited to just 100mA and left alone like that for a week. It got a few more months out of it anyway.
I'm going to need a battery, but I was just happy it started first time considering it's old fuel and it's been outside all winter with just a basic cover over it 
Ohlins, PC3, fuel cut defeat, +4deg timing, 17" front wheel.