I don't cover the battery for an engine detail, but I do generally cover the alternator and the distributor or ignition coils. Tin foil is my product of choice, it's very easy to shape around the part your protecting. Jason is correct about the engine being able to withstand water so you can drive in the rain, and if you've ever worked on cars, you know that water gets everywhere in the engine bay....but, the engineers do not take into consideration a concentrated blast of water when designing the engine, that's why I cover those sensitive areas. Also, I don't spray the plug wires with too much water, don't count out their ability to short through the wire harness. Actually, if you have ever seen it happen, it's pretty cool looking. When testing for a bad wire we used to run the engine and spritz water over the engine bay...where you saw the electricity arc was where you had the bad wire. Like I said, a very beautiful thing to look at...provided that you didn't have to pay for the new wires.