Upgrading the battery and have a few questions:

1). The 2020 (and probably all 5Gen) is a liquid acid battery. Some people when putting in an AGM battery are reporting a premature death because of alternator/AGM battery incompatibility. Do you think this is a 4Gen and lower problem? Any reports of this happening in 5Gen 4Runners?

2). Is this an alternator issue? Does the 5Gen alternator only work with certain batteries? Is there a best type of battery for the alternator that's in the 4Runner?

2). If I decide to stick with liquid acid, should my secondary battery also be a liquid acid or is it ok for the second battery for accessories to be an AGM?

3). What is the best battery type for accessories?

It's not a 4th or 5th get issue, it's an all cars issue. The voltage needed to fully charge an AGM battery is higher than that needed to charge a lead acid battery. Since the voltage isn't high enough, the AGM never gets completely charged and that's what causes premature death.

There are a variety of solutions to the low voltage problem.

It is a misconception that AGM batteries need a higher voltage to charge. From the Battery University: "AGM is making inroads into the start-stop function of cars. ... As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages)."

Thus 2.4 volts/cell is 14.4 volts for the normal 6 cell car battery, which is what the nominal car alternator puts out. Those that put voltage boosters on their alternator are thus overcharging an AGM battery which is what leads to their early failure.

Also, note that the maximum float charge should be less than 2.3 volts/cell (13.8 volts for 6 cell car battery) is below the nominal alternator voltage. Thus, after an AGM battery is fully charged, the alternator voltage will be overcharging the AGM battery.

Thanks for this.

I exchanged emails with Enersys and they said I am fine with an Odyssey battery as is. They also said that to another forum member years ago and he has had no issues last time he talked about it.

They also said that overcharging is a problem while undercharging, while not optimal, is not a serious issue: simply reduces the capacity somewhat.

Your alternator probably doesn't output over 14 volts reliably or consistently, especially with the higher current draw that an AGM will place when charging, at least at the start of the cycle. The current draw will rise as the voltage drops, and that can risk other damage. I’ve datalogged the voltage from my 5th and it’s barely 14 volts when cold, less when warm.

This thread has great info and empirical data: The NorthStar AGM Battery + Voltage Booster Upgrade | Tacoma World

I have AGM’s in most every vehicle with the exception of the 4runner. Never have had any issues with charging off the factory alternator.

-Chris

What's your oldest AGM?