The wave form below is that of the vehicle battery, under start up conditions, and the main ground to engine, volt drop. The sample is from a 306 petrol engine car, that was suffering some poor running issues. This is one of the first checks I like to make, before getting too involved.
The Red wave represents the battery and the Blue represents the main ground.
This wave form is that of the same car, as above, with a new battery negative cable fitted. This action allowed a positive intialisation of the engine ECU and a solution to the running faults.
The wave forms below are taken from a DV4td, that had a broken earth cable. The result being the alternator had been damaged, which you can see buy the lack of charge on the blue wave and the ac coupled wave on the red.
Alternator signal as measured across a poor vehicle battery.
Alternator signal as measured across a new vehicle battery.