Had this happen...............I found out that the alternator had died and the the car lived off the battery until it did exactly the same as yours, at 1st I thought the batt had given up as it was old, so I just changed that, 2 days later same thing........had the Alt checked and it was fine on test until a mechanic came over and said rev it to 2000rpm and sure enough the output from the Alt cut out.
Changed the Alt, has been fine for 2 years and I never got any warning light on the dash that it wasnt charging.
So it maybe be your Alt.
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