I had a weak battery once and the symtoms are exactly like yours. There was a weak/shorted cell and although it puts out the 12+ volts with a voltmeter the weak cell dies very quickly and intermittently, ie. overnight and with the stereo on while you fiddle away or no reason at all. It drops one cell down (2V) to 10+V and you will not be able to start the car. Doing a load test on the battery will show up this problem. When you are bench charging, this cell will show up as the one that doesn't bubble the same with the rest.

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1992 124 E200 (Euro)