As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMB OUT ON AN IFR FLIGHT PLAN, THE ENTIRE CREW NOTICED WHAT APPEARED TO BE A BURNING SMELL THROUGHOUT THE AIRCRAFT. WE CANCELLED THE IFR FLIGHT PLAN AND ABORTED THE FLIGHT, TURNED TO RTB AND SECURED ALL UNNECESSARY CONSUMERS. UPON RETURN TO BASE, THE MECHANIC DETERMINED THAT THE EMERGENCY BATTERY WAS HOT AND LEAKING ACID. THE BATTERY WAS REMOVED.