With unknown vehicle history I had wanted to replace the battery in mine. I grabbed a
Group 78 Motomaster AGM on sale for $150, full 5 yr non prorated warranty, and 5yr roadside boost assistance coverage as well. Didn't return the core so I could have a spare 12v battery to abuse to power my telescope mount until it dies, then I'll replace that with an
AGM 100Ah Deep Cycle for that specific application
I know this is a pretty old thread but I wanted to post a follow-up, and highlight some info I never had the slightest clue about when I bought this battery.
So, first of all it turns out this battery actually reached a point where it started acting up in Nov/Dec of 2017. I took it in to Canadian Tire for a load test and they declared it bad and replaced it with a new one, no questions asked and just a couple months shy of the 5 year mark.
Secondly, I now have a
new-to-me BMW X5 that I picked up at the end of Feb this year. In that vehicle I have noticed the occasional message on the infotainment screen alerting me to certain battery related conditions that sent me off doing a little research, where I learned that AGM batteries have different charging requirements than standard lead acid batteries. I never knew!
Apparently BMW incorporates some circuitry and logic to monitor and control the charging cycle and voltage depending on what type of battery is installed. In fact, over the life of the battery, the vehicle computer keeps track of the age/health status of the battery and actually learns/adapts the charging parameters as the battery ages (kind of like how our GMTs do when you clean the throttle body). If one changes the battery for an exact match, one needs to "register" the new battery, but a change to the type/capacity requires a bit more in-depth "re-coding" of the computer. (For anyone interested in learning more detail, this link should get you started:
https://www.bimmerfest.com/forums/showthread.php?t=966040)
I had no clue about this when I bought an AGM to replace the OE lead acid battery in the Envoy, and to the best of my knowledge there's no way to alter the charging parameters in our vehicles. Bottom line is if you buy an AGM for your GMT, you're likely not charging it optimally, are probably overcharging it in general resulting in conversion of some of the electrolyte to gas that gets vented thereby shortening the life of it to some degree. The good news is that in light of my experience, it's likely that the effects aren't drastic. YMMV
Cheers