Obviously, replace the battery... Obviously give the battery a constant
current charge before installing... Obviously, use baking soda and
copious water flush on the battery box, belly of the plane, etc.,
before installing the new battery...

Go to the auto store and get a voltage meter that plugs into the
cigarette lighter... Plug it into the lighter receptacle, or otherwise
get it hooked up... Turn the master on (all radios off) and notice the
voltage... Now start the engine (alternator switch is off if possible)
Once the engine is running and/or the alternator switch turned on
immediately note the voltage... If it is higher than 15 volts
immediately shut the alternator off, or stop the engine, as you have an
overvoltage condition... If the voltage is high do not run the engine
for ten minutes, but get a mechanic to help you... Yes, ten minutes of
overvoltage can ruin a battery...

If the charge voltage is normal after starting the engine up, mid to
high 14 volt range, then assume the battery just spontaneously went
shorted as one of those strange things that happen... Murphy never
sleeps...

denny