If you can, check battery voltage AT THE BATTERY TERMINALS when the car is cranking. It'll be a bit down on 122v but hopefully not much.
Check all the high current connection: Battery to solenoid, solenoid to starter, starter (or engine block) to chassis, chassis to battery.
If there is a poor connection somewhere, you MIGHT be able to feel it if a terminal or connection is warm IMMEDIATELY after cranking. - That did work for me once but it's a bit of a log shot.
You could also check for volt drops by measuring the voltage across each connection whilst the engine is cranking.
Battery terminal itself to the metal of the connector; Should read zero whilst cranking. Metal of the battery terminal connector to the starter terminal; Should read zero whilst cranking. Starter motor to battery negative terminal: Battery terminal itself to the metal of the connector; Should read zero whilst cranking.
One other thought is it could be starter motor brush problems.
Battery in the car with the engine running should read maybe 14.5 volts. Lower would suggest not charging. -possible alternator or regulator issue.
safea
November 2017