One of our cars recently had problems starting on a ~70°F day, after having the headlights on for 10-15mins with the engine stopped, post a 20 mile drive.
The decision was to replace the battery, which was done and we have had no issues with the car starting/running since. However, I am interested in the state of the "old" battery.
What method(s) would the community suggest for someone with a moderate understanding of electronics and electrics, to test this?