I have a 12 volt starter motor on a diesel engine that is rated at 2000 W. Typically a fully charged lead acid battery is 12.7 volts. Therefore the current draw would be 157.48 amps but for only a few seconds. What is the correct way to size a battery to this starter? Do I look at amphours (battery capacity) or CA/CCA?
It depends on the battery climate temp. If sub 0’C use CCA , CA rating is sometimes 30% higher.
Since diesel has 22:1 compression ratio the start torque is much higher than a car at 8:1.
The proper way to choose a starting battery depends on the Battery/Motor ESR ratio under any condition as any drop in battery voltage is due to battery ESR and load current when starting to crank the pistons is due to the sum of both Battery ESR and starter ESR.
Battery ESR rises with Depth of Discharge rapidly above 80% and motor ESR rises due to the Reverse EMF of the motor rising with RPM. The motor rated current will stall at same voltage at about 8~10x rated current or ~ 1000A in your case.
Considering many users replace a battery if CCA test <50% (or less) of rating you need to consider coldest temperature of battery cost and turning speed and motor oil viscosity and starting reliability /risks you want for the life expectancy in your application.
In Winterpeg, I had lots of CCA battery experience with cars and slow turning engines at -40'C and sometimes relied on battery and/or oil heaters or inline water heaters. Usually everyone had jump cables in the trunk for helping others that underestimated their battery needs and would willingly stop and help anyone. Occasionally to help themselves... For these climate conditions you always choose more CCA margin than you need in the summer.