Acronyms!
(AGM) absorbed class mat
(SLA) sealed lead acid (sometimes maintence free)
(FLA) flooded lead acid
(VRLA) valve regulated lead acid (just the vent technology)
(GelCel) (different technology than AGM)
(RG) Recombinant Gas (why it can be sealed, but will vent of overcharged)
There's some overlap & name dropping, but the main "technology" is the AGM part (I think some use interchangeably with SLA correctly or not). The RG-25XC spec does say AGM specifically, as well as VRLA and RG for bragging. So this battery is like a Odyssey P680, an AGM battery. The main "technology" is the AGM part, which also are RG and have VRLA.
All batteries need some kind of pressure relief. VRLA means some kind of valve verses no valve at all, just a vent. AGM should not vent any gas unless it gases off due to massive overcharging for extended time (volts over 15). So the vent is closed but the valve opens when needed. AGM does have lead plates and acid, just suspended in mats verses sloshing around as a liquid or gel, so VRLA is just fancy talk.
AMG batteries need a special charger if you want automatic charging, set-it & forget-it. YOU CAN USE A PLAIN CAR CHARGER WITH CARE. You have to time it and manually disconnect the charger; you can't leave it on for too long or you'll damage the battery.
For example at 11.5 volts a AGM is considered 100% discharged. You need a charger of about 10amp for a PC680, suspect that's fine for a RG-25XC. The max charge time with a 10amp charger is 2 hours for the P680. If the battery is not fully 100% discharged, than you need to reduce charge time, may be a lot, like 10-20-30 minutes. Overcharging damages the battery. The 100% charge voltage for a AMG is around 12.84 volts ideally, but you want to see at least 12.6 volts or more after you disconnect the charge and manually measure open voltage. You need a 10 amp charger or larger. Charge times are much much less with a larger charger. You need at least 7 amps I recall for the PC680 sized battery to get full charge. Volts under 14.2-14.3 will not fully charge a Odyssey AMG battery.
The fancy chargers designed for AMG batteries have 3 stage logic. Odyssey explains this in great detail on page 12 of their tech manual:
http://www.odysseybatteries.com/files/techbook.pdf
Not all chargers for AGM are the same. They all might work OK, but some may have more logic and protections, including temp compensation. Just be aware of what your AMG needs to rapid charge (constant current), absorb charge (constant volts) and float. Don't overcharge it or connect it to a float charger in a discharged state, neither are goodness. One results in battery damage and one just does not help much.
If you use a new charger you have to watch them to see how they're programed, or ask the manufacture if its not spelled out like the odyssey tech manual describes.
Personally I'm not a big fan of leaving electical stuff connected in an unattended hanger, especially charging batteries in the plane. Its also against local fire code (and you insurance?). Extension cords and/or charging unattended batteries have burned hangers down before. If the battery has an internal failure or the charger fails wide open it could get HOT. A little 1.5 or 3 amp trickle charger may be safer, but they will not fully charge or condition a AGM battery. AMG batteries have very good shelf life. If your battery can't take a week or more sitting, than you need to replace it. It should be fully charged after a flight right. Your planes charge system needs to be at least 14.2-14.3 volts at the battery.
CHARGERS
Here is a nice charger:
http://www.thebatteryminder.com/12vbatteryminder-p-58.html or
LINK
It even has battery temp compensation if you want (fancy fancy but cool). Not used one but this does seem to be a true three stage charger. It does say not for aircraft batteries but they sell a line of
"aviation chargers" which are more expensive. A Odyssey P680 is a Harley Davidson/BMW motorcycle battery so it should be fine. Here is another 3 stage charger:
http://www.samlexamerica.com/products/productdescription.asp?ProductsID=8056
Schumacher Chargers:
LINK. These models: SC-1000A, SSC-1000A, SC-1200A, SSC-1500A and SC-2500A have a AGM selection and are in the 10-25 amp range. Can't verify how good they work charging AGM, not tried them, but they do make good chargers; I'v got one of there basic ones. These SC or SSC chargers go for about $50 to $80.
http://store.schumachermart.com/battery-chargers-automotive-speed-chargers.html
I just bought a SSC-1500A from ebay for less than $50 with shipping. I need another charger anyway.
One last one is this:
http://batterytender.com/product_info.php?products_id=2 (5 amps max is less than the 6 amps that Odyssey recommends for the P680 but I am sure it work. Looking at their tech info it sounds like they know what they are doing. They have a whole line of little battery maintainers (trickle chargers) which would not charge the battery fully but might work well for maintaining a fully charged battery.
