car won't start - 1990 200T

[lee@wheelman.com (Lee M. Levitt)]

Aargh!

1990 200T, 90K miles, 4 year old battery, two year old
alternator...everything good up until yesterday, when the car didn't have
enough juice to start. Turned over slowly, accompanied by fast clicking
sound...

After putting it on the charger briefly, the car started right up...

I let it idle for about 10 minutes, then parked it again.

This morning same trick. 

Anyone have thoughts on whether this is a battery problem, an alternator
problem, or something more nasty?

What *should* the voltmeter read when the ignition's in the on position and
the engine is not running?

Lee