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Batteries

How to Charge AGM Batteries with a Regular Charger

In an earlier post, we talked about how AGM (Absorbent Glass Mat) batteries should be charged with smart battery charger – preferably even one with a dedicated mode for AGM batteries. I also mentioned that a using a regular flooded lead-acid charger for AGM batteries is not a good idea – not at least in general.

But sometimes a conventional lead-acid battery charger is all you have at hand. Is there any way to make it work with AGMs? The answer is yes: with enough care, it can be done.

In this article, I will tell you how. In short, a regular flooded lead-acid battery charger can be used to charge AGM batteries, but the charging process requires monitoring and intervention on the part of the user. Specifically, the charging voltage should be measured periodically and the charger disconnected once either the maximum absorption voltage or time is exceeded.

We will next start with a quick how-to on charging AGM batteries with a regular battery charger – this description assumes that you are familiar with AGM charging in general. Further down I give a more expanded account of how a “regular” lead-acid battery chargers, the over-charging potential with AGM batteries, and how to deal with it.

Quick procedure

Caution: Using a charger not intended for AGM batteries involves risk of damage, injury and fire. Proceed only if you understand charging concepts and the risks involved – please also read Disclaimer.

The protocol for charging AGM batteries with a regular charger can be summarized to a few steps:

  1. Check the current setting: Verify that the initial charging current does not exceed a charging rate of 0.15C. Check the current using the gauge on the charger or a clamp meter. (If you are unsure on how to calculate the C-rate, see my previous article.)
  2. Monitor the voltage: Use a multimeter to measure the charging voltage in the absorption stage. If the optimal absorption voltage is exceeded by 0.2V, disconnect charger to prevent overcharging. To see the optimal absorption voltage for your AGM battery, check my article on the topic.
  3. Clock the absorption time: Disconnect the charger once the charging voltage has exceeded 14V for 4 hours.
  4. Disconnect charger: Remember to disconnect the battery after 20 h of charging regardless of the charging voltage.

How to tell a “regular” charger

In the context of lead-acid battery charging, a regular charger is a charger without intelligent control electronics. You may wonder whether the charger you have at hand is “regular” or “smart”.

Some tell-tale signs of a regular charger are analog current gauges and rocker switches – most regular chargers are also from the early 2000s or older. The table below includes a list which helps you to identify your charger.

CHARGER
IDENTIFICATION
Regular charger:

– Analog current gauge
– Rocker switches for voltage and current selection
Smart charger:
– Electronic (LCD, OLED) display
– Tactile switches for voltage and current selection
– Indicator LEDs
– “Smart” in the model name

How a regular charger works

Regular or conventional lead-acid battery chargers are meant specifically for flooded or wet lead-acid batteries, such as those commonly used in cars.

A regular charger is essentially a current-limited voltage source. This technical expression means that the charger tries to output a certain voltage, typically 14.4V for 12V batteries, but actually puts out something less if the battery takes substantial current.

This charger behavior results in what is called a CCCV charging cycle, which stands for Controlled-Current–Controlled-Voltage. The cycle works roughly like follows:

  1. CC stage: When starting to charge a drained battery (0%…80%), which readily takes in a lot of current, the charger works against its current limit. It puts out its full amperage, and the actual charging voltage is determined by the battery.
  2. CV stage: When the battery approaches full capacity (90% or so) and draws less and less current, the charger puts out a constant voltage.

The CC stage restores the main part of the battery capacity, and the CV stage does the finishing and conditioning that are vital to battery health.

Admittedly this account is a bit idealized, as most regular chargers actually have a smooth roll-off from CC to CV instead of a sharp switch, but the principle is still the same.

Compared to the 3-stage smart charger, the CC stage corresponds to the bulk charge stage of a smart charger, and the CV to the absorption charge, roughly speaking.

AGM battery & Regular charger – The issues

In principle, the CCCV charging cycle of a regular charger sounds fine for an AGM battery. After all, the CC stage does the same as the bulk stage of a smart charger, and the CV takes care of the absorption charge.

So what is the problem? There are actually two problems, both stemming from the fact that an AGM battery is very sensitive to overcharging, much more so than a flooded lead-acid one.

Problem 1: Poor voltage regulation

The first problem is that the charging voltage in the CV stage is poorly regulated. This means that although the voltage is supposed to be a constant, e.g. 14.4V, it may actually drift by more than 1V depending on the load.

Poor regulation of the CV stage not a major issue for flooded lead-acid batteries, but is a big deal for AGMs. Particularly problematic are voltage drifts above 15V, as such voltages can quickly overcharge an AGM battery and cause permanent damage.

Problem 2: No float stage

The second problem is that regular chargers have no float stage. Unlike smart chargers which step down to a lower float voltage after a prescribed absortion time, regular chargers will stay at the high absorption voltage indefinitely.

This too presents an overcharging hazard for AGM batteries: even though the CV stage voltage would be in the correct range, too long time at this voltage is certain to overcharge an AGM battery.

The solution

The overcharging hazards involved in using a regular charger to charge an AGM battery can be avoided through active monitoring and intervention.

The basic idea is that is that because the charger may overcharge, you must step in and prevent overcharging by disconnecting the charger in time. In time here means two things: cut on overvoltage and cut on absorption time.

1 Cut on overvoltage

First, monitor the charging voltage and disconnect immediately if the voltage if drifts above the appropriate absorption voltage range for your AGM battery. In most cases, you can take 14.8V as the top end of the range.

For best results, you should check the correct absorption voltage for your AGM battery model (I list some in a previous article) and apply a temperature compensation (see previous article) to this cutoff voltage.

2 Cut on absorption time

Second, clock the absorption time and disconnect after around 4 hours of absorption. You can count the absorption stage to start when the charging voltage exceeds 14V.

The optimal absorption time (and the voltage) are a bit of a compromise and a matter of debate, but it is always much safer to undercharge rather than overcharge. The mentioned 4h above 14V should be on the safe side and is a good rule a thumb.

Also note

As with all charging methods, it is vital you stick to the guidelines on charging currents, temperatures etc. when using a regular charger with and AGM battery. For more information, see my articles on AGM charging voltages and currents; to understand how the battery voltage is related to capacity, see my article on State of Charge (SoC).

Conclusion

We have seen that charging AGM batteries with a regular, non-smart, lead-acid battery charger is possible, but takes substantially more care than using a smart charger. Doable in a pinch, but not something I would recommend as a standard procedure.

If you have AGM batteries in your car, motorcycle, RV or boat, the smart thing to do is to get a smart charger.