Cars run off the alternator, not the battery. The battery is to provide starting power and power when the engine isn't running. Cars haven't run off of the battery since generators. Way back in 1969 I started my car and then disconnected the battery. It ran fine. (Don't do this today. The battery needs to be in the circuit to prevent damage to other electrical components.)

An AGM's advantage is that it can be placed in any position, even upside down, and never needs to have water added. Other than that, it's just another wet cell battery. I use 2 in my travel trailer, which has a power supply to properly charge them. At 100% on a normal day the voltmeter reads 12.7. A regular charger will get them close to full, enough so that I wouldn't worry about missing .1V. I have used a regular charger on my 4 (2 in the trailer and 2 backup in the garage) and they top off at 12.7.

The AGMs I have used or seen are deep cycle, not starting batteries. There may be AGM starting batteries but I don't see the advantage to them. A normal wet cell, maintenance free or regular, is more than sufficient if you properly maintain it.

Battery voltage is affected by temperature, another thing to consider.

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No disagreements from me. I went back and edited my post to better explain my story. My AGM starter battery, with lack of any diode mod, caused me terrible intermittent electrical issues. Issues that I couldn't find anyone else on the internet referencing. This seemed like a decent thread to drop the info into. Maybe someone will find it useful in the future.

That's really strange. I don't know why an AGM battery would cause intermittent electrical issues. It's really just a regular lead-acid battery but instead of liquid electrolyte it's wrapped in a fiberglass mat and the case is sealed. There's no electronics involved like there is with LiFePo4 batteries. Maybe there's a bad / broken internal connection somewhere? I've seen all kinds of strange electrical issues and the best way to resolve them is to swap parts with known good parts, like you did.

Regardless of what the voltage shows on a battery, the best test is a load test. Harbor Freight sells a very simple battery load tester. I have found it to be invaluable in determining the actual strength of a battery. If you still have the AGM you might want to try that, unless they did that at the auto parts store. Still, a good tester to add to your workbench.
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I did have the battery tested at an Autozone, and the CCA it was putting out was 15-20% under the battery's rating. Which (I assume) tracks linearly with it being 15-20% undercharged, on account of the alternator's voltage output being too low, with my lack of diode mod.

I can only speculate that there was some transient voltage/amperage dip happening that caused the ECU to not be fed enough power, only for the briefest moment. I assume with a diode mod, and >14v in the system while running, I would not have had the issue

Anyhow I plan to use a DC to DC charger and turn that 24f AGM battery into a leisure battery for camping, etc.

Anyone here use a voltage booster pro Texas slim? It adds either 0.4v-0.5v on low setting or 0.8v-1.0v to your alternator charge. I got 2 Weize AGM batteries wired in parallel (dual purpose type for starting and rv).
My 2003 4runner v6 charges at 14.1v to 14.9v At idle (voltage climbs higher for a while then falls to max idle, usually around 13.2 volts).
If you're concern is idle charge on a AGM like many truckers or 4x4 off road campers like me you want that AGM charging with that 14.1 volt minimum.
Instead of buying a custom alternator, keep your 130amp alternator and buy a simple Texas slim pro. Pull your 7.5amp mini fuse for your alternator and replace it with this thing. See photo. I'll solve your issue on LOW setting making your idle 14.1 instead of that weak 13.1 volts that ruins it. The good news is even when your drive it doesn't send the charge into the danger zone. The high setting would mess your AGM up charging it at 15-16 volts depending on your alt charge in motion.
I bought one just as a backup. It solves having to lug around a AGM charger. Its not unsafe and is dependable. I bought 2 (1 to use 1 for a backup)

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