Q: startung the car issue

my car always start just find, i park it and left the hazards lights on for 45minutes , , because of the busy street i was on, came out turn the key, no sound at all, but light,horn, radio work, find no light on the dish board came on for the battery,aaa tow service, guy connect what he call a boaster, start, within a 1/2 second,told to get battery check, did and 5-6 errands, in the midst of the errands, turn car off/on , car start right up with no problem,had the battery check and was told the battery was low which i do not know what that means ,(not by my regular mechanic) consider the light did come on,i was told i need a new battery, the warranty of the battery was 4 years, which it still had a1year and 4 months left ,of course i purpose, one, just to make sure i did not have the same problem, just had alternator install within a routine check -up and service by my regular mechanic ,no starting problems at the time . the next day same problem, was it the battery .

My car has 74890 miles.
My car has an automatic transmission.