Peter wrote:
Does CS-STAN cover battery replacements with a different type?
Not the present issue, but the proposed new issue will. A condition is that the battery manufacturer declares that the new battery complies with ETSO-C173A.
Given all candidate vendors are American, I wonder what exactly that means?
What is the application process, and is “AML TSO” possible?
Peter, it actually mentions ETSO-C173a or equivalent.
You know the next Q 
Some interesting views above but I don’t think that anyone will convert a 12VDC aircraft to a 24VDC aircraft its a lot of work to do just for one aircraft . Cessna did this as part of the C150 sorting out program that became the C152 but that was for a lot of units and IMO was a very good move if only to reduce the chances of students flattening the battery during starting ( to make it practically impossible you have to fit a slickstart )
Comments about car batteries are in invalid as like a someone above I got eight years out of a OM car battery and could get a very cheap replacement that would be lucky to see three winters before failure, if you want the OM quality battery and the eight year life the price is comparable with an aircraft battery.
As said above large aircraft use three phase 400Hz 110 or 115 V depending on how you mesure the voltage.
