Suspect the battery is low or just bad. (New battery can go bad at any time. Had a new battery go bad in just 5 months once.)
Be sure the battery is good and then you need to ensure the alternator output is good.
Does the car have a battery voltage gauge? If so the battery voltage should be in the 13V+ range.
This can be "tricky". The output can be ok from a cold start but over time -- minutes in my experience -- the alternator output can drop and after a while the battery is being called upon to supply the car's electrical needs. I've seen a Panamera with a dying alternator and the car/engine behaves in a very erratic manner which can have one suspecting all sorts of problem when it is just marginal electrical power.
Otherwise you might have to start the engine and let it idle while you monitor battery voltate over say 15 minutes.