The reason the voltage falls after shutting off the FEH is the "surface charge" dissapates. It can take a few hours to dissipate unless the headlights are left on a few minutes. A 12.5 voltage is about a 80% charge.

You voltage seems on the low side, but extreme heat may lower the charging voltage. My 2005 FEH 12V battery with the ICE/HV running is around 14.2 to 14.6V in San Diego's mild climate. Check the voltage before and after driving and see if it is in the 14V range.

A voltage of 13.5 will take days to fully charge the battery. 13.2V is considered a float (ie: prevents a sitting battery from self discharging), not a charging voltage. A 12V battery needs between 13.8V and 14.4V to recharge in a reasonable time.

Just for reference, I've left the FEH (and my TCH) sit for as long as 30 days and had no problems starting when returning. Both still have the factory batteries.

Edit: This morning after sitting in the garage overnight, the FEH measured 14.27V after starting the ICE. After returning home from driving some errands the battery was 13.68V with the FEH still running. Your readings of 13.5 - 13.8V look normal for the FEH after it has warmed up. I suspect your old battery had a weak cell that prevented it from charging properly. Once that happens the battery will sulfate, charging less and less as time goes by.

 

I assume you mean an x-gauge. Depending on your SG version, some of the older ones don't work with the x-guage, like we'd like. My first one had this problem, even tho it was updated to the latest. I swapped with my wife's newer one in case I needed an x-gauge, but I use a DashDaq so this isn't a problem.

What version is your software? http://www.scangauge.com/support/pdfs/SGMan5_1.pdf (Page 24 tells how to read determine your SW version).

 

My current SG is 3.15
I think at one time I had the 3.15* but it got replaced.

 

So it sat in the garage 3 days of not moving. Voltmeter still read ~12.25. Not bad. So wife drove to work & back, it sat 1hr before I left. I now have volt on the SG showing. After starting, ICE on it showed 12.7V. the 5mi drive to work it never went above 13.1. I only got that high at a stop light with AC & Radio off. Granted it's 95 now with Heat Index of 105. Stll sound normal? Should I take it in?

Added:
just got home after it sat for 9hrs, 81F outside. Volt no reads 13.3-13.5 on the drive home, radio & AC made little difference. I think I saw a max of 13.7.