Why is 2005 SRX sputtering when I use heat control?

Why when I turn heat on for 2005 SRX car seems to sputter when at stop lights, stop sign etc. also headlamps seem to dim then brighten back up almost seems as though vehicle will shut off, when I don't use heat car drives fine…recently replaced tensioner and pulley .Car has been like this for about two months…Would battery be issue ,not sure if I have the right size or amperage for this car…THX