Have your IA Read CAR 3 on battery requirements.
§ 3.682 Batteries. When an item of electrical equipment which is essential to the safe operation of the airplane is installed, the battery
required shall have sufficient capacity to supply the electrical power necessary for dependable operation of the connected electrical equipment.
§ 3.683 Protection against acid. If batteries are of such a type that corrosive substance may escape during servicing or flight, means such as a completely enclosed compartment shall be provided to prevent such substances from coming in contact with other parts of the airplane which are essential to safe operation. Batteries shall be accessible for servicing and inspection on the ground.
§ 3.684 Battery vents. The battery container or compartment shall be vented in such manner that gases released by the battery are carried outside the airplane.
Since this battery is not a lead/acid type, the venting requirements are not applicable. This battery does, in fact meet the power requirements of the systems of this aircraft. Now go read FAR 43, appendix A, paragraph (a). No where does it say that either changing battery types or moving a battery constitutes a major alteration.
As for the PMA vs non argument, I've never seen a PMA requirement in CAR 3 or 4. As a matter of fact, many of the parts used to build these aircraft were automotive parts to begin with. Replacing one part with a different, newer part is not illegal or unsafe. But it is important to insure the fit/form/function of the new part. In this case, compare the Odyssey to an older lead/acid battery. Are they the same voltage? Does it provide enough capacity to run the systems on the aircraft? If the answer is 'yes' to both, and it is, then you have satisfied the requirements of CAR 3. If you've read part 43, you know that a battery change is not a major alteration. If you chose to move the battery, again not a major, do your structures work in accordance with the aircraft manual (lots of CAR 3 and 4 aircraft do not have manuals) or in accordance with AC43.13 guidance. Now, do a log entry documenting your work and reflecting that it meets the requirements of CAR 3 (or 4), part 43, and AC43.13. Done.
