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AGM Battery Charging: Rates, Currents and Times

In the last part, we went through AGM battery charging voltages. In this article, we will look at the connected topics of AGM charging rates, currents and times – all different aspects of the question “How fast can you charge an AGM battery?

In a nutshell, charging rate in C-units means how much of the battery’s max capacity is restored each hour, charging current in amps is what produces the charging rate, and charging time means how long it takes to reach a full 100% charge. Typical charging rate, current and time for a standard 60Ah automotive AGM battery are 0.1C, 6A and 15h. Charging at higher rates is often possible, but comes at a cost to battery life.

All of this will be explained in more detail below. If you are not familiar with AGM battery charging stages, you may want to check the first part of this series before reading on.

AGM charging rates

Battery charging rate means how fast a battery is charged in proportion to its total capacity. AGM battery charging rates are specified for the bulk charge stage, which is a current-controlled stage. Absorption and float stages, by contrast, are voltage-controlled.

Standard rate: 0.1C

A standard bulk charging rate for AGM batteries is 0.1C. This expression means a charge rate that adds 10% of battery full capacity each hour; put another way, this rate would charge a battery from empty (0%) to full (100%) in 10 hours. While such a charge actually takes longer due to the slower absorption stage, the C-rate expression is useful in that it stays the same for all size batteries.

The standard 0.1C rate is a compromise between battery life and charging practicality: it is slow enough not wear or heat up the battery during charging, but fast enough to fill an empty battery overnight.

If charging time is not an issue, charging rates much under 0.1C can also be used. With large battery packs in particular, you may actually be forced to use rates below 0.1C due to lacking charger amperage.

Fast charging: 0.2C…0.3C

Most AGM battery manufacturers also allow faster charging rates up to 0.2C…0.3C for their AGM units. Mastervolt, Mighty Max and UPG all give a maximum charge rate of 0.3C; Victron specifies a more conservative “0.2C or less preferred”.

At the fast charging rates, the bulk charge stage will finish in only a few hours. While possible, fast charging wears down AGM batteries and is not recommended as a standard procedure. Fast charging also increases the battery temperature, and should be accounted for in charging voltage temperature compensation (link to previous part).

Due to the downsides, charging rates above 0.1C should be reserved for occasional use in time-critical recharges.

AGM battery
brand
Recommended
max. charging rate
Fullriver0.35C
Mastervolt0.3C
Mighty Max0.3C
UPG0.3C
Victron<0.2C preferred

AGM batteries can technically be charged at even higher rates, well in excess of 0.3C. However, charging at such rates will cause severe wear and heating, is not safe and never recommended.

Charging currents

Charging rate in C-units is what matters for your AGM battery. Your charger, however, only knows currents in amperes (amps, A).

How do rates and currents relate? Take it like this: the same charger amps produce different charging rates for different size batteries. A current of 10A into a 100Ah truck battery means a rate of 0.1C, but for a 50Ah car battery, it’s 0.2C.

The ampere values are relevant in that they tell you:

  • How powerful a charger you will need for your AGM battery
  • Which charger setting to use to get a 0.1C or some other charging rate

Like the charging rates, the currents are usually specified only for the bulk charge stage.

Rate [C] vs. Current [A]

Because amperes are key to AGM charging practice, it is important that you can translate the C-rates into amperes and back. The charge current required for a given charging rate is calculated as follows:

current [A] = rate [C] x battery capacity [Ah]

Let us take a few application examples:

  • To charge a typical 60Ah car battery at the normal charging rate of 0.1C takes a current of 0.1C x 60Ah = 6A
  • Fast charging a larger 100Ah AGM battery at 0.3C takes a current of 0.3C x 100Ah = 30A

The standard charging rate 0.1C to current conversion is very easy to remember:

Standard AGM charging current [A] = the battery size in amp-hours [Ah] / 10.

Maximum charging current

The maximum current you should use to charge an AGM battery is, in most cases, the current that produces a 0.3C charging rate. This is the max rate recommended by most AGM battery manufacturers.

The maximum charging current depends on the battery size, and can be obtained using the formula above. For a rough estimate of the max current in amps [A], divide the battery amp-hours [Ah] by 4: a 60Ah battery takes 15A, a 100Ah battery 25A, and a 200Ah battery 50A.

With larger-capacity AGM batteries, the maximum recommended charging current cannot be sourced by typical battery chargers, and the charger capacity will limit your currents.

How to set charging rate?

The charging rate of an AGM battery is controlled by selecting the appropriate current setting in your charger.

Most battery chargers allow you to select the bulk charge current from only a few options. Although this will not allow you to hit 0.1C or some other target rate exactly, the options are usually enough to give you at least one safe charging rate under 0.2C.

Many consumer chargers indicate the optional currents with a motorcycle and a car symbol, which correspond to low and high current settings suited for small and large AGM batteries, respectively.

Some more recent charger models have continuously adjustable current, allowing you a more precise selection of the charging rate. The smartest of the chargers may even have functionality to gauge the battery size on connection, and adjust the currents automatically.

AGM charging time

The charging time of an AGM battery is a sum of the bulk charge time and the absorption time. We take a look at these next, and then sum them up to total charging times.

First, we need a few definitions. Charging time obviously depends not only on the charging rate, but also how full the battery is when you start. The battery state is usually expressed using either of two concepts, Depth of Discharge (DoD) or State of Charge (SoC).

Depth of Discharge (DoD) expresses how much of battery capacity has been drained in use before charging. A DoD of 0% means the battery is full, or 100% charged; a DoD of 90% means that the battery is practically empty. A discharge beyond DoD 90% damages an AGM battery and should be avoided.

State of Charge (SoC) expresses how much of battery capacity is left; it is the counterpart of Depth of Discharge. A full AGM battery has 100% SoC and a flat one around 10% SoC. Discharging an AGM battery of to 0% SoC damages the unit, and should be avoided.

The two metrics are exchangeable, but the Depth of Discharge is somewhat more popular in the context of AGM battery charging.

Bulk charge time

The bulk charge time is easy to estimate from the bulk charge rate and depth of discharge (DoD). With the depth of discharge expressed in percent and the C rate having units 1/h, the bulk charge time can be expressed

tbulk [h] = (DoD/100% – 0.2) / C-rate

For example, bulk charging an AGM battery at 50% depth of discharge at the standard 0.1C rate takes (50/100 – 0.2) / 0.1 = 3 hours.

The same formula can also be expressed using charging current I in Amps and battery capacity C in Amp-hours:

tbulk [h] = (DoD/100% – 0.2) * C [Ah] / I [A]

Bulk charging a 75% discharged 100Ah battery with a small charge current of 5A takes (75/100 – 0.2) * 100Ah/5A = 11h.

Absorption time

By contrast to the bulk charge time, the absorption time of an AGM battery is a matter of definition and charger programming. While an hour or two of absorption time is enough to bring the battery to practically full capacity, some more time is usually required to ensure battery health.

Typical absorption times recommended by AGM battery manufacturers are from 4 to 8 hours. In most charging profiles, the absorption time scales with the depth of discharge: 4 hours is probably enough after a 25% discharge, but a long absorption time close to 8 hours should be used after a deep discharge.

Note that absorption times much below 4 hours are enough in very shallow cycle duty, such as in continuously charged automotive and solar power systems. If bulk charge time is only a few minutes, smart charging systems usually set the absorption time as some moderate multiple of this time.

Total charging time

The table below lists total charging times for AGM batteries with different depths of discharge and charging rates. I calculated the values using the formula above for the bulk charge time and assumed absorption times between 4 and 8 hours.

AGM charging times
Depth of
discharge
Bulk
0.05C
(standard)
rate
0.1C
(standard)
0.2C
(fast)

0.3C
(fast)
25%5h5h4h4h
50%11h8h7h6h
75%17h12h9h8h
90%22h15h12h10h

Although the real charging time will depend on the programming of your charger, this table should give you an idea of how long should you expect your AGM to charge at each bulk rate and DoD.

NB. When using a smart charger, I recommend waiting until the charger indicates the AGM battery is full and not removing it after any calculated charge time. Chargers have different profiles programmed in, and some charging cycles may take a lot longer than others.

AGM charging rate Q&A

How fast can I charge an AGM battery?

The fastest charging rate recommended by most AGM battery manufacturers is 0.3C; for the charge current this corresponds to, see above.

The 0.3C rate will charge a flat (90% discharged) AGM battery full in around 10 hours. If you skip the absorption stage, which is OK if done only occasionally, the charge time is cut to only 2 to 3 hours.

What charging rate for a 100Ah AGM battery?

The proper charging rate for an AGM battery does not depend on the battery size in amp-hours; 10Ah and 100Ah both have 0.1C standard rate and 0.2…0.3C fast rates.

The battery size does determine the proper charging current, though – for the formula, see above.

What charging rate for a 12V AGM battery?

The AGM battery nominal voltage does not affect the proper charging rate; the same charging rate rules apply to all 6V, 12V and 24V AGM batteries.

Do AGM batteries charge faster than flooded?

AGM batteries are reputed to accept charge faster than conventional flooded lead-acid batteries. However, I was not able to find a significant difference in the maximum charging rates recommended by manufacturers for the two battery types.

What amperage should I charge an AGM battery at?

The optimal charging amperage for an AGM battery scales with the battery size (Ah). For the charging amps corresponding to the standard charging rate of 0.1C, divide the battery amp-hours by 10. See more details above.

How long should I charge an AGM battery?

When using a smart charger (recommended), you should charge AGM batteries until the charger indicates that the battery is full. These chargers have intelligent control electronics that monitor when a full charge has been reached, and can also be left connected indefinitely.

When using a charger without an automatic float stage (not recommended), be sure to disconnect charger once the charging voltage has exceeded 14.5V for 4 hours or risen above 15V.