1. Preheat oven to 200°.
2. Clip a candy thermometer onto the side of a 4-quart Dutch oven; add oil to pan. Heat oil to 385°.
3. While oil heats, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, chili powder, and baking soda in a medium bowl. Gradually add club soda, stirring with a whisk until smooth.
4. Dip onion rings, 1 at a time, in batter, coating completely. Add to hot oil. (Do not crowd pan.) Fry 1 minute on each side or until golden, maintaining temperature of oil at 375°. Drain onion rings on a paper towel–lined jelly-roll pan. Place pan in oven and keep warm at 200° until ready to serve. Combine salt, garlic powder, and black pepper. Sprinkle onion rings evenly with salt mixture just before serving. Serve with ketchup, if desired.
You'll have some of the batter left over, but don't decrease the amount you use. You need the full amount so the onion rings can be immersed in batter and get fully coated.