car will not start unless jumped

bmw 328i 2007 150k miles. in the mornings the car would start fine but after a few drives it doesnt want to start unless having an active jump from another car. ive put 3 different new batteries in the car and the alternator has been replaced. tried different grounds on the car to the engine and it still wouldnt start. pretty annoying when i have errands and places to be. i also took a multimeter to the car while running, the alternator read 14-15 volts. the battery was also reading 13-14 volts. can someone please explain what would be wrong? i dont have any extra money to be spending so budget friendly as well. thanks