The rules are all in the US-EU bilateral agreement and its TIPS , although the mentioned 2007 EASA decision also refers.
The ‘critical’ defintion is not evident: it is referring only to those parts defined as such during the investigation for the TC, and which the TCDS refers to as such either directly or in the limitations section of the ICA, SM, AMM or TLM. No matter how critical you think a magneto is (or even the one dual magneto on Peter’s TB20) it is not a “critical” part for this purpose since it is not listed or referred to as such in the TCDS or the aircraft’s maintenance manual.
However, the windows in our pressurized 210 are listed as an airworthiness limitation limited to a life of 10000hrs (and the turbine disks and shafts and others on jet and turboprop engines and…) so if I replace them with a PMA alternate, then EASA would not accept them unless their PMA certification was validated.
Each PMA part comes with specific applicability limitations as part of the approval and each PMA manufacturer is obliged to inform their customers of such restrictions.
The Concorde batteries last for years past where they “fail” the load test, and this is one great use for them
That UPS happened to contain two 12V batteries in series, and you can mod any UPS in this way.
This UPS is on Justine’s PC, and I can confidently say she is the only gurl in the world whose PC has a UPS powered by an ex aircraft battery 
FWIW (sample size = 1)
My Concorde RG24-12 failed after 3 years and 10 months.
Operated roughly 400 hrs with 350 starts.
First warning only about 2 wks ago, Skytec starter had a very short “pausing” before pulling through (engine starts after 1/2 prop turn). Two starts later, the starter current “exceeded” the battery capacity, and the voltage collapsed – but started the engine !
Fortunately on the way back home.
Cap test: 5% (!)
Plane lives in a hangar, flies 1-2 times per months. No charging in between.
Maintenance Shop 1 says: first Concorde we remove from a ship since we started installing them in 2015!
Maintenance Shop 2 says: we see younger Concordes beginning to fail…
Ordered a new Concorde, since the new Gill 7242 has no reputation, but long lead times.
The one after that should be a Li-Ion one as per the other thread…
I am “OK” with Gill (G243) in the SR22. They are a bit less expensive than the Concordes (I wouldn’t use the word cheaper), and the first three ones ran between 6 and 7 years before either dying or being replaced out of caution. They indeed give very little advance warning.
I also don’t do any “in between charging” and fly more or less frequently.
My last 2 or 3 Concordes lasted about 6-7 years. I give them the reconditioning charge (with the CTEK charger) at each Annual.
After being removed, they seem to work for another 5-10 years for a UPS, wired (bodged) externally.
I’d really like to put an odyssey on the firewall of my PA28. Would help W&B too as its a bit nose heavy.
My maintenance outfit won’t have any of it.
