There two benefits and two detractors to using FF (or any oil based undercoating designed to wick) on batteries:
- The FF will keep the post connection lubed so that reduces the likely hood of stripping the hardware. this would be common among all of the sprays or greases, but FF will wick into the threads and get to places in the hardware where you need it.
- The FF will wick int the wire connection and prevent corrosion that you can't see within the cable itself. This is the reason I use it.
- FF or any oil coating will make a mess. My trucks are coated underneath and in many places under hood, making it impossible to do anything without getting covered in grimy, greasy, filth.
- I have had some door seals swell when in contact with FF. So it does affect rubber and foam components. Will it weaken a battery housing or damage wire insulation? I doubt it, but you never know.