While the Faa doesn't approve, I use a no fluid, Gel-Cel battery about the size of a garden tractor battery. It was made in '96, has sat unused for as long as 18 mos. and unlike usual lead acid batteries including those approved by the Faa, cannot leak fluid. Turns the starter over nicely on both Contental and Lycoming engines. I keep an old approved battery around in case the AI won't sign off the airplane at annual time.
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