Could possibly be a loose battery connection. I had a very similar problem as you're describing on my former Kawi C14. Tightened it up and off I went.
Nope at that voltage your battery is done, and when you then put aload on it trying to start it's draging it even lowerI checked the terminals. All buttoned up and tight. I'd think the engine would still try to turn over at 11.5V but all I get is a solenoid-style click...
This is what I'll be buying when mine next needs a replacement:OK, I haven't had the need to add acid to a battery for ages, always buy sealed batteries.
OK, I see now that it ships dry, which will be the first sealed battery that I've bought that ships dry.