Easy Slow Cooker Barbecue Chicken

to 4 thawed boneless skinless chicken breasts (2+ pounds preferably)

bottle of your favorite barbecue sauce (I prefer Sweet Baby Ray's Original)

bottle beer (optional)

brown sugar

garlic (minced (to taste - we use 2))

of a medium sized sweet onion (we love onions so sometimes I use a whole one)

vinegar

Salt and pepper