Nah I don't have a multimeter, think I'm gonna have to invest in one pretty soon.

The battery isn't completely flat, just flat enough that the car won't start. I checked the warning light bulb, and swapped it for a known good one. Still no change.

I just went to the car, roll started it and it was happy to sit there and idle/run with the battery fully disconnected, but as soon as I turn the headlights on it'll stall. If I reconnect the battery and roll start it again, it'll run, again. I can turn the lights on with the battery connected, but not disconnected.