Same as with my Hellfighter5 with battery installed inside the unit, how does one preserve battery life when this is used very little at all.. Quite honestly no more then 1 hour per year...
Long term storage of Lion batteries should not be done fully charged. More like 30%. Should also be stored in the freezer or fridge.
In your case the best is to not buy batteries at all, just run off of a D.C. power supply.
And to think, you also bought spare battery packs, for what purpose ?