Your charging system may not be at fault here. AGM batteries or the benefits to using them is that some can be deep cycled for use in things like boat where the vehicle will sit unused for some time. However they require a long term (over time) charging plan in place to keep them useful like using a battery trickle charger. This is not what is happening in a car, unless you are driving several hours straight and putting the car on a battery tender.

Having worked inthe car audio biz for years on the retail and supplier side I have seen both view points. It started with the need to be able to address the parking lot player that would kill off a new battery in a few days, so the market responded by selling these batteris to that consumer target so at least they could recharge them and use them more than a few times. This is especially useful for demo vehicles and over the top SPL cometition vehicles.

However as consumer behavior goes so does the spread of misuse and inflated expectations.

For vehicles a standard battery has enough current reserve for normal operation and can be recharged easily and will hold a charge for a resonable period of time.

Keep in mind the cars charging system mat spec 120amp Alt, but that is a spec that should denote "in the perfrect operating condition" which includes temp which will impact perfromance.