AGM Battery State of Charge

When an AGM battery is using or charging, it’s important to be told how full it is, or AGM battery state of charge in technical words (SoC). One of the most essential indicators for AGM batteries is the SoC, which indicates how much energy is left in the battery and when it should be recharged.

A basic multimeter may be used to check the battery voltage, and a chart connecting the two can be used to check the related SoC. However it is a flaw: many of the absorbent glass fiber mat voltage SoC graph going about are incorrect, confusing the no-load as well as under-load voltages — a blunder that might cost you your absorbent glass fiber mat battery.

Below, I’ll show you some accurate no-load and under-load voltage charts for AGM batteries so you can calculate their SoC. The graphs were created using data from the manufacturer as well as my own measurements. I’ll also show you how to get a reliable estimate of the AGM battery state of charge by measuring the voltage.

AGM Battery State of Charge (SoC) explained

The SoC relates to the amount of charge in the battery in relation to its maximum capacity; it indicates what capacity rate the battery is.

A full battery has a state of charge of 100 percent, a half-full battery has a state of charge of 50%, and a battery with a state of charge of roughly 10% is nearly flat.

Typical AGM Battery State of Charge

AGM battery performs in the first position when always keeps it as close to 100% state of charge as possible. In an alternating charge–discharge sequence, an AGM battery SoC might go between 100 percent and 70 percent in a typical cycle use case.

Discharging to 50 percent SoC or underneath is the definition of deep cycling; it is probable these cycles burn down the battery and ought to be prevented. An AGM battery is discharged under 10% is extremely hazardous to it and should only be done in an emergency.

Why Does AGM Battery State of Charge matter

In AGM battery functioning, the State of Charge is the most important metric. You’ll need to understand the SoC in order to find out how much longer your boat or RV’s power system will last and to avoid battery damage, know when to stop discharging. When your AGM batteries are fully charged, you’ll be able to tell.

The Influence of a Variety of Factors on Battery Voltage

Different charging or discharging currents affect the battery voltage under different charging or discharging situations. The greater the current, the greater the voltage in the charging situation is. The greater the current, the lower the voltage in a discharging condition. The battery voltage might be affected by different manufacturers and production materials.

At various temperatures, the voltage of the battery varies; the lower the temperature and voltage. The voltage will be lowered after a period of use.

AGM No-Load Voltage

The battery open-circuit voltage or no-load – the voltage through the battery terminal during no load is applied – is the simplest way to assess the AGM battery state of charge of an absorbent glass fiber battery.

Open-circuit voltage table

The graph underneath that shows the usual open-circuit voltages of 12V absorbent glass fiber mat batteries at each SoC, from 100 percent to 0 percent. These figures were calculated using data from Maxworld Technologies for their AGM batteries, as well as my own measurements on Maxworld Technologies AGM batteries. Divide or multiply the figures by two for 6V or 24V AGM batteries.

Open-circuit voltage chart

The graph below depicts the open-circuit voltages for Maxworld Technologies AGM batteries in greater detail. If data for your AGM brand is not acessible, the graph contains a mean line that you can use instead.

Open-circuit voltage graph

Something You Should Know about the Data

Note the following before utilizing the data in the graph and graph that the voltages are ranges: the relationship between state of charge and open-circuit voltage is dependent on the battery history of use, model and condition. Whats more, the figures are based on a period of no charge or discharge — Maxworld Technologies provides a 12-hour time frame.

The voltage fluctuation in the table is caused by a variety of factors, including differences in AGM brands, individual battery use history and attrition, and, to some extent, temperature. The voltages at all SoC are given a huge 0.5V nondeterminacy by Maxworld Technologies, however I equaled this down according to my own and other manufacturer figure.

In any event, keep in mind that the open-circuit voltage will only give you an approximate idea of the State of Charge: add a ±10% uncertainty to the SoC number you estimate as a rule of thumb.

AGM Under-Load Voltage

An AGM battery State of Charge can also be determined using its under-load voltage, which is the value while the battery is connected to a load.

The under-load voltage has the advantage of stabilizing quickly and allowing online State of Charge assessment while the battery is in operation — no hours of waiting required. The disadvantage is that an absorbent glass fiber battery does not have a sole under-load voltage; instead, the under-load voltage varies depending on the load level.

AGM under-load voltage table

The chart upward illustrates the average under-load voltages for a 12V absorbent glass fiber mat battery at various State of Charges. For the reason that manufacturer data on under-load voltages is often confined to the last discharge voltage, the voltages in the chart are solely dependent on my own measurements (FDV, see below). There are two load levels: a light discharge rate of 0.01C and a medium discharge rate of 0.1C. Divide or multiply the listed voltages by two for 6V and 24V AGM batteries.

On the table, there are some notes that to calculate the State of Charge, you must first determine the load current and convert it to a C-rate. Before measuring, the load should be stable for roughly 20 minutes for 10 minutes and 0.01C light loads for 0.1C medium loads.

Final discharge voltage

The last discharge voltage (FDV) is the under-load voltage for an absorbent glass fiber mat battery is almost depleted; it corresponds to 0.01% SoC. This voltage is also known as the end voltage, and discharge ought to come to a halt at this voltage.

The final discharge voltages for 12V AGM batteries at various discharge rates are shown in the graph below. I created the chart using Maxworld Technologies, and their recommendations, as well as a typical or mean value for other brands. The most essential under-load voltage is final discharge voltage; if you don’t know it, your AGM batteries will have a very limited life.

Cut-off voltage

The idea of cut-off voltage, often known as low-voltage cut-off, is connected to the last discharge voltage (FDV). The term “cut-off voltage” normally involves the voltage at which the battery discharge is voluntarily halted, whereas the term “FDV” involves the voltage at which the battery is essentially hollow and the discharge have to be stopped.

In common cycle operation, the cut-off voltage ought to be set substantially higher than the manufacturer’s last discharge voltage. You can extend the life of your battery by dont discharging all the way decrease to FDV.

Notes on AGM voltage measurement

Meter accuracy

A simple multimeter has to be used to check the voltage of an AGM battery. Because the relationship between this voltage and the SoC isnt very exact in any event, which is very simple 0.1V meter accuracy will suffice.

Wait to stabilize

The most important thing to remember is to obtain a reading when the AGM battery voltage has stabilized. The battery voltage will cost a little time to achieve its appropriate open-circuit voltage after charging or draining. Maxworld Technologies recommends a 12-hour rest interval, but after detaching the charger from a 12-V 100Ah Maxworld Technologies Marine AGM, I saw at least 50 mV differences between 12 and 22 hours – see graph below.

Stabilization time for the open-circuit voltage of an absorbent glass fiber battery after disconnection from charger

Stabilization times

I couldn’t find any manufacturer data on how long you should wait for absorbent glass fiber mat battery voltage to fix other than the 12 hours for open-circuit voltage.

I can still offer some recommendations based on my personal experience. Generally, I’ve noticed that the voltage fix quickly for large loads but slowly for loads of light and no load:

The open-circuit voltage (with no load) is the slowest to fix, requiring up to the whole day to get to within 0.1 V of the final voltage. Maxworld Technologies recommends that you wait 12 hours before reading.

The voltage fixes in ten minutes with a gradual discharge at 0.01 to 0.02C. In less than 5 minutes, a moderate discharge rate of 0.1C fixes the voltage (after which a steady decline) It’s evident that waiting 12 hours for a proper measurement of the open-circuit voltage isn’t always feasible.

Fortunately, the majority of the stabilization takes place within the first ten minutes of unplugging a charger or load. If you’re in a rush, you may estimate the stabilized voltage by looking at where the voltage moves in the first 10 minutes and extrapolating 0.1V to 0.2V in the same direction. The uncertainty in determining the State of Charge will increase a little if you don’t wait for the true stable voltage reading. It is, while, frequently the most appropriate you can do.


For the purpose of battery protection, the end voltage for normal energy storage system, like a residential solar system, with a current discharge of roughly 0.1C, can be adjusted to 11.80-11.90V.

Setting the end voltage to 11.95V and calculating the daily energy consumption (per cycle) to keep the majority of the energy used below 50% is recommended for a reasonable design.

If you want to know more about the AGM Battery State of Charge, you can visit our Maxworld News. 

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