12.6 is fully charged battery.
% of charge at a given voltage drops rapidly.
12.0 is very discharged (Google for exact - it is posted all over the web)

A click with key to start is bendix moving out but not cranking engine. It is usually caused by low battery voltage. It is up to mechanic (you) to confirm and determine reason for low voltage.

I'm waiting to catch a deal on cheap volt meters so I can keep 1 in each car. Always begin by measuring battery voltage and watch how far it drops with key to start. My SD cranks with only 12 volts & not much drop when all (obviously except charging or battery - otherwise voltage would be 12.6) is working.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.