You’re correct, AGM batteries have been used with ordinary vehicles’ charging systems for demanding competition and military applications for several decades and outperforming other conventional 12v batteries . A few folks have apparently managed to out engineer the battery manufacturers and identify vehicle charging issues that everyone has apparently missed all these years. A complementary philosophy is buying cheap Costco batteries that need to be replaced every 2 years as the “best” solution for having the freshest and strongest battery.
This was the battery I ran in my Jeep:
Designed to handle the increased use of onboard accessories in today’s vehicles, ODYSSEY Performance Series batteries combine long service life, high reliability and deep cycle capabilities.
odysseybattery.ca
Specs say "12V" (they could be more specific) but the average flooded lead acid battery will have a resting voltage of 12.6V. My Odyssey Performance series would charge for the most part from my alternator's output to 12.6V, maybe 12.8V on a good day. The alternator's charging voltage was regulated by the vehicle's ECU based on temperature, RPM. Unfortunately, there was a constant draw on the battery from added equipment, the primary reason I upgraded to something better. Topping it off with an AGM charger overnight would net a resting voltage of 13.2 to 13.3V which would gradually drop over a few weeks closer to that 12.6V resting voltage the vehicle's charging system could achieve. Had there been no parasitic drains from added electronics, I would have expected less voltage loss over the same period.
Could I have just left the vehicle to charge the battery? Sure. Would it have performed at its best and maintained that extra small buffer of capacity? Not so much, but it would still start the Jeep when showing either side of 11V on my voltmeter. The OEM battery tapped out when discharged to 12.0V and would only generate a click. For my use case, connecting a charger overnight once a month wasn't a huge inconvenience for a little extra amperage and avoiding that partial state of charge loop the vehicle's charging system alone would keep the upgraded battery in. The Odyssey didn't seem to mind (and they say it is good for 400 discharge cycles- try that with a standard battery), but the Northstar I ran before it would get stubborn about charging to optimal capacity if left to the charging system alone for months and months. It eventually died an early death because I had mostly left it to the vehicle to maintain its charge for its first three years. Sadly, I only read up and found out why I only got a portion of service life out of mine compared to other users after it was too late.
There are plenty of affordable AGMs that won't necessarily need a top up charge, but those batteries will perform closer to what you can expect a standard flooded lead acid battery to do i.e. no extra capacity above what is stated and a similar service life. Periodic charging is a small price to pay should you want or need a little more redundancy in a slightly stronger battery. Avoiding that partial state of charge will help you coax the maximum number of cycles out of that battery you paid extra for to extend that service life. As for competition use, the competition guys will charge banks of high end batteries to have at the ready beforehand and swap them out to maintain maximum voltage available for their amplifiers. Military and industrial users are more likely to look at the battery as something consumable to be replaced proactively at an interval rather than something that can be maintained to last a decade with proper care. If the consumer wants to do the same, it's their money.
If I am dropping close to $500 for a battery, I don't mind an extra step once a month if it means optimal performance and not having to replace it as often. If that price point is hard to justify and periodic charging is too much hassle, then by all means just get the biggest capacity you can find in a flooded lead acid battery from a supplier with the best warranty and replace it proactively as required. In the Jeep world, dual battery systems were not unheard of for overlanding setups, but I have yet to see someone do a dual battery setup in the Land Cruiser. Staying tuned on that front.
