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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?
Charge it completely full, until it floats at 14.8 V or whatever is recommended.
Then hook it to a modest load, perhaps a 1 A tail light, and plot the voltage as it discharges. Stop when you get to 8 V and immediately recharge it so you don't do too much damage.
This should give you an idea of the remaining capacity in the battery.
Read the whole of batteryuniversity.com and also dig on the internet for datasheets of similar batteries, for their discharge curves. And read on StackExchange, in fact, the right hand column is full of similar questions!
I used to run several vans for work and batteries were always a problem. I found that an hydrometer to check on them was very useful. It will show you exactly which cells are 'bad' and whether they could be recovered.
This sort of thing: Mighty Max Battery Hydrometer
