That redundant ground has come up before. James1549 brought this up years ago and I had been thinking about it as well. James created a thread on the topic here:
https://www.crossfireforum.org/forum...le-engine.html You are not the only one to see the weakness of the original design. Grounding to the block thru the chassis seems silly to a lot of us !

As to your device, that is a voltmeter and it's "Capacity" reading is not accurate at all. YOu cannot determine capacity of a battery by monitoring voltage alone. Capacity is determined by monitoring cell voltage versus discharge current*. Note that it claims higher capacity just because the alternator is charging - starting the engine DOES NOT increase battery capacity, only voltage.

*(There is some evidence that, with lead acid batteries, you can also monitor cell voltage and charge current and calculate charging effectiveness, but in the lab we always go by cell voltage versus discharge current. Our battery conditioners uses this to condition batteries, in fact).

Mark Christopher
Systems Solution Engineer,
EF Johnson Technologies
Irving, TX