If you didn't get the service records from the previous owner, check for a round date sticker on the 12V battery. It will indicate its vintage. Knowing that date won't reliably tell you if the battery is good or not. 12V batteries need to be tested with a load tester. Check with your local parts store if they will do this for free.
The 12V battery supports all sorts of auxiliary functions, like the circuits you use in cold weather (wipers, window defroster, fan, etc).
It gets topped up during recharging, and possibly during driving if it gets low.
2016 Spark EV 2LT with DCFC