If you are doing what I think, you are doing. The meter is saying the cables are having a bad connections conniption, or are just bad inside them.

When using the meter in line with the one side of the battery post to their other connection point the voltmeter reading should be far less than 0.2 volts.

Anything higher means there is a large resistance. The 0.6 v is the bad cable. The 0.2 is not so good either and may be a reading related to his not so well connected or a well grounded cousin most of the time.

When checking charging voltage the voltage should be at least one volt above the rested voltage on the battery. With the car running, the meter should hold 13.2v to 14.2v across the posts. The average varies with the accessories loading and the total conditional charge of the battery.

The bad connections affect the feedback to the alternator/regulator system. This controls output of current within those voltage ranges.

Phil