1981 300D 101k

I searched the forums, but need some direction. Went to start the car today 22F. She usually takes a 2nd try below 30. This time she tried a few times and on the 4th try I just get a click, one single click. Tried a jump and still just the click. About to head to wally world to get a battery tester.

My question is, that single click that I am hearing. Is that a dead battery or something else.




Recent Oil change (full syn), fuel filters change, diesel purge, new air filter. Waiting for Diesel Giant to send wrenches for a valve adjustment.