Paul asked: Do you really put grease on a clean battery post before attaching the cable? In the Cleaning a Car Battery section, it says to prevent corrosion, you should put grease on the clean terminal post before you reattach the cable. Won’t this prevent a good electrical connection?
The grease that should be applied is white lithium grease. It is readily available at automotive stores. It will not interfere with the electrical connection, but will help prevent future corrosion by displacing the air in the surrounding space.
The idea is to apply the grease to the terminal after you have connected and tightened the battery cable. This will create a layer of grease between the conducting surfaces and the surrounding air and moisture, and thus, prevent corrosion. Always apply it to a clean terminal for best results.
It is also wise to install a pair of felt washers underneath the terminals. These washers are available in automotive stores, right near the batteries. The help prevent high-resistance shorts across the battery case (when batteries get really dirty, the layer of oil and dirt will sometimes be conductive enough to connect the two terminals electrically)