Use a vegetable peeler to peel the potatoes. After peeling and rinsing, use a sharp knife or a mandoline slicer to cut the potatoes into thin, even slices, about 1/8 inch thick. This will ensure the potatoes cook evenly in the slow cooker.
In a large mixing bowl, combine the sliced potatoes with minced jalapeños, 2 cups heavy cream, 1 cup shredded Parmesan cheese, 4 cloves minced garlic, 1 teaspoon chili powder, 1 teaspoon salt, and 1/2 teaspoon black pepper. Toss until the potatoes are evenly coated.
Transfer the potato mixture to a 4 to 6-quart slow cooker. Cover with the lid and cook on high for 3–4 hours or on low for 7–8 hours, until the potatoes are tender when pierced with a fork.
In the last hour of cooking, sprinkle 2 cups shredded cheddar cheese over the potatoes. Cover and continue cooking for the remaining hour until the cheese is melted and bubbly.