ORIGINAL: JoeAirPort

You always get some benefit from shielding of the ignition components of the plane. The leads coming of the battery and the switch harness could be shielded too as they radiate some noise too. But after all this effort I'm not sure it's really worth it. That's the main issue. The majority of your noise is radiated by the spark wire going to the plug. Also the ignition radiates lots of noise. Both of these are already shielded. Just separating the ignition, ignition battery, and switch harness from the radio components give you good isolation. But to answer your original question, theoretically the foil will shield the battery. How much noise was really coming from the battery pack? Probably not much.