There is disclosed a method of reducing the conductivity of deposits formed from the combustion of a fuel comprising an iron-containing compound, said method comprising adding a manganese-containing compound to the fuel. The free-radically polymerizable fluorinated polymeric intermediate comprises the reaction product of i) at least one multi-functional free-radically polymerizable material having a fluorine content of at least 25 wt-%, and ii) optionally other multi-functional free-radically polymerizable material. The invention also includes apparatus providing the means for implementing the steps of making a balanced disk pack from a disk pack. The battery unit includes a battery charge indicator, which can be a visible signal.