Can a Car Alternator Charge an AGM Battery?

In an earlier article, we discussed how well different types of line-input chargers are suited for charging AGM batteries. The conclusion was that what is needed is a smart charger – preferably one with a dedicated AGM mode.

But what happens if you put an AGM battery into your car? There the battery is not charged from line power, but by the alternator. Can the car alternators charge AGM batteries?

In this short post, I will try to shed some light on the issue. In a nutshell, the alternator-based charging systems in most cars are intended for flooded lead-acid batteries, and will not work optimally with AGM batteries. With the default charging profile, these systems risk both over- and under-charging an AGM battery, which is much more sensitive to charge voltages and staging. The charge controllers in newer vehicles may sometimes be reprogrammed to work with an AGM battery.

We’re next going to look at the important job of the regulator, the exact problems of charging AGM batteries with the alternator, and when a car charging system may be compatible with AGM batteries.

Regulator is key

Car battery charging is actually controlled by a combination of the alternator and a regulator. The regulator’s task is to cap the output voltage of the alternator to a safe level both to prevent damage to car electronics and to avoid overcharging the battery during long high-RPM drives.

The positioning of the regulator varies: in some cars, the regulator may be integrated into the alternator and be inside its casing; in others, it may be an external component. In newer car models, the regulator is usually incorporated into the engine control unit (ECU) and has more advanced functionality.

Regardless of the position, it is the regulator or charge controller that sets the charging voltages, limits the charging current, and in more advanced systems, controls the charge staging.

Problem 1: Low absorption voltage

Many regulators have too low output voltage to properly charge an AGM battery. Typical regulator setpoints are 13.5V to 14.4V. These voltage levels are enough for most of the bulk charge stage, but a bit too low for a proper absorption charge, which would take a voltage between 14.2V and 14.9V depending on the temperature and the battery model. For more info, see my articles on AGM charge staging and voltages.

On a typical car charging system, an AGM battery will therefore charge slow, and never quite to 100%. This will cause fast capacity fade and lead to a short battery life.

Problem 2: No float stage

An another shortcoming of conventional alternator & regulator car charging systems is the lack of float charge stage.

For best performance and longest battery life, AGM batteries should be charged full with a high absorption voltage between 14.2…14.9V, with the exact voltage and time depending on temperature and model (see previous article).

After this, the charging voltage should absolutely be dropped to a lower level to prevent overcharging. This lower voltage stage is called float charge, and has recommended voltages between 13.5…13.8V, again depending on temperature and battery model (details in previous article).

This drop to float stage is missing altogether in many older car charging systems. If an AGM battery is installed to such a car, there is a high risk of battery overcharging on long drives.

No drop-in compatibility

We’ve seen that the output voltage of car regulators is often an unhappy medium for AGM batteries – too low for absorption, too high for float. Low regulator voltages around 13.5V are pretty good for floating AGMs, but will never charge them full. The higher regulator outputs of 14.4V are still a bit low for proper absorption voltage, but much too high for float.

Because of the mentioned issues, an AGM battery is not a general drop-in replacement for a conventional flooded-cell lead-acid automotive battery.

The issues you face if you substitute a flooded unit for an AGM depend on the car model and your use profile. In some happy cases, a combination of short typical journeys and a high voltage setting mean that the battery will work just fine.

In others, the over- or undercharging will lead to fast capacity fade, and you need to replace the battery prematurely.

Some newer vehicles may be smart enough to automatically detect the new battery type through load testing and adjust charging accordingly. In this case, you will face no issues – except for the possible warning messages of unauthorized battery change from the on-board computer.

Compatibility by reprogramming

In many new vehicles with an engine control unit (ECU), the battery charge profile is a matter of charge controller programming. Such cars may allow you to swap the original flooded-cell lead acid battery for an AGM model if the charge controller settings are updated accordingly.

Before purchasing an AGM battery for your car, make sure that your car’s charging system is compatible with AGM batteries, and the ECU is appropriately programmed to recognize the battery type. As so many other things in modern cars, this is something you will likely have to leave to an authorized service center.

Flooded lead-acid more tolerant

Some claim that AGM batteries actually are a drop-in replacement for flooded lead-acid units in car applications, because the optimal charging voltages for the two battery types are not all that different.

There is some truth to this claim: the optimal absorption and float voltages of flooded lead-acid and AGM batteries are indeed within 0.2V or so: typical values are 14.7V/13.6V for AGM and 14.5V/13.4V for flooded.

However, this claim misses an important point: an AGM battery is much more prone to both under- and overcharging than a regular flooded lead-acid battery. Specifically:

  • AGMs suffer much more from insufficient absorption charge
  • AGMs are more sensitive to overcharging

So while the crude charging profile of a car alternator & regulator combination is not really optimal for either battery type, AGMs suffer much more from the deficiencies.