In a large, deep skillet, heat 3 inches of oil over medium-high heat until it registers 380 degrees on a deep-fat thermometer.
Meanwhile, in a large bowl, combine the pancake mix, chili powder and 1 1/2 teaspoons salt. Whisk in the seltzer until smooth. Working in batches, coat the onion rings with the batter, letting the excess drip off; add to the oil and fry, turning once, until golden and crisp, about 2 minutes. Drain on paper towels and sprinkle with salt.