For 10 beer brats, you'll need:
10 fresh bratwurst
10 brat buns (hot dog buns are an abomination, don't use them)
Charcoal briquettes (about 30) or lump charcoal
Holding Sauce ("batter"), see recipe below
Condiments (ketchup, brown mustard (yellow mustard is an abomination - don't use it), chopped onion, sauerkraut)
Mound lump charcoal or charcoal briquettes in a pyramid. Start the charcoal. (Use an electric starter or crumpled paper, avoiding the use of hydrocarbons. When the coals are partially white, spread in a single layer. Allow the charcoal to burn until coals are covered with white-ash glowing embers (about 20-30 minutes) The heat is just right when you can hold your hand palm-down, 2 inches above the grate, for 4-5 seconds. If you can still see a red glow, it's too early to start cooking. While you're waiting for the charcoal, prepare the "batter" (see recipe below).
When coals are ready, place bratwurst on grill, 7-9 inches above the coals, and turn often until done, approximately 20-25 minutes. The brats are done when you squeeze them gently and they feel firm. True bratmeisters use their fingers instead of tongs to turn their brats, so they can feel when the brats are firm and therefore cooked. Turn brats often; otherwise they will be burnt on the outside and raw on the inside.
Flare-ups can quickly ruin your bratwurst, turning them a hellish shade of black. Flare-ups can be avoided with proper technique. Treat the brats with care. Don't puncture them or squeeze them too hard. If you see a flare-up developing, quickly move brats away from the flames if you can, immediately close grill cover and close vents for a minute or so. Some folks control flare-ups with a spray bottle of water. I find that a spray bottle is unnecessary, if you are prepared to move quickly and use proper technique..
When brats are done, hold them in the "batter" until ready to eat. Serve on a brat bun, dressed with your favorite condiments and accompanied with classic Wisconsin side dishes.
I suppose that it's called batter because of the butter.
For 10 brats:
3 bottles or cans beer
1 large onion, chopped
1 stick (or more!) butter
Mix ingredients and bring to a simmer. Hold grilled bratwurst in batter until ready to eat. Do not let sauce boil.