Add the beans, onion, garlic, cumin, water, ¼ tsp salt, and ¼ tsp pepper to a 6-quart slow cooker. Stir to combine then place the lid on the slow cooker and cook on LOW for 8 hours.
Turn off the slow cooker. Drain the beans and reserve the liquid. Transfer the drained beans back into the slow cooker and mash with a potato masher while stirring in some of the reserved liquid until you reach a desired consistency. Place the lid back on the slow cooker to keep the beans warm.
Add the ground beef, chili powder, and ½ tsp salt to a skillet over medium-high heat. Cook the beef, breaking it up with a spatula as it cooks, until well browned and cooked through, about 5 minutes. Add the beef to the slow cooker with the beans and stir to combine. Season with salt & pepper to taste.
