battery reads 0 v when running ?!?

[sandstorm]

today my 190e 2.3 died while driving after hard braking for the traffic light. I couldn't restart the car (the battery was dead) so I took it 2 blocks furder and they recharged it (it was empty). after that the car runs fine (propably until the battery depletes again in next 5-6 days.

when I got home I connected the multimeter to the battery (engine off ) and it was reading 11.56
then I started the engine and it was reading 0 ????
what is the problem. (I have replaced the regulator few days ago and the and the alternator belt was tightened.
do I have the short somewhere?
do I need to replace the alternator?

On my other car (mazda) the batery reading is 12.55 (engine off) and 14.35 with engine running sosomething is weird with my benz...

Any Ideas?