Every engine start can be considered a load test, comparable against itself.
Kaqiqi 5-Digits Voltmeter DC 0-33.000V (0-33V) Digital Voltmeter 5-Digits bit High Precision Voltage Meter: Amazon.com: Tools & Home Improvement
www.amazon.com
This^ 5 digit , 3 wire voltmeter claims to sample voltage 5 times a second.
While it might not catch the absolute minimum voltage the battery is able to maintain when turning the starter, if one were to watch it each engine start, but especially ovrenight cold engine starts, one will see it fall lower as the battery ages and the colder it is, or when it is undercharged.
A good healthy battery will likely hold 11v+ starting an overnight cold engine in mild ambients.
A weak battery will likely fall to the 9v range.
A battery at death's door will fall to 8.
Some vehicles hate voltage falling this low during cranking. It's certainly harder on the starter.
The third yellow wire is for voltage sense.
Ideally, both ground(black) and sense+ wire( yellow) would go right to battery itself, the red wire to a source that turns it on with the ignition.
Battery life Nazis will likely benefit by insulating the battery from radiant, and convective heat from engine/radiator. Depends on the vehicle as to how effectively this can be accomplished, or if it is worth it the effort, but the battery which stays 10C cooler on average, will likely last 2x as long, all other factors being equal.
Trunk mounted AGMs usually report some of the best lifespans, and AGMs are not really happy when they are not regularly topped up to true full.
But keep em cool, and fully charged, and they will still fail in their duty, at some point.