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Batteries

AGM Battery State of Charge

When using or charging an AGM battery, you often need to know how full the battery is – or in technical terms, its State of Charge (SoC). The SoC is one of the most important metrics for AGM batteries, as it tells you how much energy do you have left and when you should recharge.

A quick and easy way to find this out is to measure the battery voltage with a basic multimeter and check the corresponding SoC from a chart connecting the two. But there is one problem: many of the AGM voltage–State-of-Charge charts floating around are inaccurate and confuse the no-load and under-load voltages – a mistake that may cost you your AGM battery.

In this article, I will give you more reliable no-load and under-load voltage charts for AGM batteries for State of Charge estimation. The charts are based on manufacturer data and my own measurements. I will also explain how to measure the battery voltage to get a reliable estimate of the state of charge.

If you are not familiar with the battery terminology used here, you can jump to the end, where I explain the basic concepts around AGM battery State of Charge.

Having unwittingly run a few AGM batteries much emptier than necessary, I know how important an accurate voltage-to-SoC chart is: running AGM batteries empty damages them permanently and should be avoided by monitoring the SoC and stopping before it’s too late. I hope the info I have collected here will help you save your batteries!

AGM No-Load Voltage

The simplest way to determine the State of Charge of an AGM battery is to measure the battery no-load or open-circuit voltage – this is the voltage across the battery poles when no load is connected.

Open-circuit voltage table

The table below gives you the typical open-circuit voltages of a 12-volt AGM battery at each State of Charge from 100% down to 0%. These numbers are averaged from my own measurements on Victron and Exide AGM batteries and data published by Mastervolt, Trojan and UPG for their AGM batteries. For 6-volt or 24-volt AGM batteries, divide or multiply the numbers by two.

AGM battery
State of Charge
(SoC)
Open-circuit
voltage*
Notes
100%12.7…13.0 Vvaries
80%12.5…12.7 V
60%12.3…12.5 Vrecharge
recommended
40%12.0…12.2 V
20%11.8…12.0 V
10%11.7…11.9 Vstop discharge
0%11.4…11.6 Vpermanent
damage
*after 12h rest

Open-circuit voltage chart

The open-circuit voltages for Exide, Mastervolt, Trojan and UPG AGM batteries are shown in more detail in the graph below. The graph also includes a mean line that you can use if data for your AGM brand is not available.

Notes on the data

Before using the data in the table and graph, note:

  • The voltages are ranges: the relation of SoC to open-circuit voltage depends on battery model, condition and use history
  • The values presume a rest time without charge or discharge – Mastervolt specifies 12 h

The voltage variation in the table is due to many factors: differences between AGM brands, individual battery use history and wear and also temperature to some extent. UPG actually gives a whopping 0.5V uncertainty to the voltages at all States of Charge, but I averaged this down based on my own and other manufacturer data.

In any case, remember that the open-circuit voltage will give you only a rough indication of the State of Charge: as a rule of thumb, add a +-10% uncertainty to the SoC value you estimate.

Voltage points to remember

The essential in the open-circuit voltage chart can be boiled down to a few easy-to-remember voltage points:

  • A fully charged AGM battery – 100% SoC – should read somewhere from 12.7V to 13.0V without load.
  • A half-full unit, 50% SoC, should be around 12.1V to 12.3V, i.e. slightly above nominal voltage
  • A practically flat AGM battery at 10% SoC reads 11.7V to 11.9V without load

The most important point in the table:

If your AGM battery reads 11.8V without load, it is practically empty – do not discharge further

Trying to squeeze out the last 10% damages the battery permanently, and is not worth it outside emergencies. Instead, you should recharge such a battery ASAP.

AGM Under-Load Voltage

A Victron Super Cycle 100Ah AGM battery drained at around 0.1C rate shows 12.37V under-load voltage, which indicates a State of Charge around 60%.

The State of Charge of an AGM battery can also be estimated from its under-load voltage, that is, the voltage while connected to a load.

The under-load voltage is advantageous in that it stabilizes fast and allows State of Charge estimation online while the battery is used – no hours of waiting needed. The downside is that there is no single under-load voltage for an AGM battery – the under-load voltage depends on the load level.

Under-load voltage table

12V AGM

State of
Charge
Under-load
voltage*,
@ 0.01C
discharge
Under-load
voltage**,
@ 0.1C
discharge
Notes
100%12.9V12.7V
80%12.6…12.7V12.5V
60%12.4…12.5V12.3…12.4Vrecharge
recommended
40%12.1…12.3V11.9…12.1V
20%11.8…12.0V11.6…11.8V
10%11.6…11.8V11.2…11.3Vcut-off; stop
discharge
0%11.3…11.5V10.2V
*after 20 min
loading
**after 10 min
loading

The table above shows the typical under-load voltages for a 12-volt AGM battery at different States of Charge. The voltages in the table are based only on my own measurements, because manufacturer data on under-load voltages is typically limited to the final discharge voltage (FDV, see below). Values are given for two load levels: a light discharge rate 0.01C and a medium rate 0.1C. For 6V and 24V AGM batteries, divide or multiply the tabulated voltages by two.

Notes on the table:

  • To estimate the State of Charge, you must know the load current at least roughly and translate it to a C-rate
  • The load should be steady before measurement for around 20 min for 0.01C light loads and 10 min for 0.1C medium loads

Final discharge voltage

Final discharge voltage (FDV) is the under-load voltage at which an AGM battery is practically empty; it corresponds to 0…10% SoC. This voltage is sometimes called the end voltage, and discharge should be stopped at this voltage at latest.

The chart below displays the final discharge voltages for 12V AGM batteries at different discharge rates. I have compiled the chart from the recommendations of Mastervolt, Mighty Max, UPG and Victron, and also included a typical or mean value for other brands.

Final discharge voltage is the most important under-load voltage – it is something you (or your load controller) must know, or otherwise your AGM batteries will have a very short life.

Typical final discharge voltages for 12V AGM batteries are also given in a table format below.

AGM BATTERY

Discharge
rate
Final
discharge
voltage
Notes
0.01C11.7Vclose to
no-load
0.1C10.7…11.1V
0.3C10.3…10.8V
0.5C9.8…10.3V
1C9.5…10.0V
3C7.8…9.4Vlarge
variation

Cut-off voltage

Cut-off voltage or low-voltage cut-off is a concept related to final discharge voltage (FDV). Cut-off voltage usually refers to the voltage at which the battery discharge is stopped voluntarily, while at FDV the battery is basically empty and the discharge must stop.

In normal cycle use, the cut-off voltage should be set much higher than the final discharge voltage specified by the manufacturer. Not discharging all the way down to FDV extends your battery life considerably.

Notes on AGM voltage measurement

Meter accuracy

The voltage of an AGM battery can be measured by a simple multimeter. A very basic 0.1V meter accuracy is enough, as the connection between this voltage and the State of Charge (SoC) is not very precise in any case.

Wait to stabilize

The main thing to remember, though, is to make sure that the AGM battery voltage is stabilized before you take a reading. After charging or discharging, the battery voltage will take some time to reach its proper open-circuit voltage. Mastervolt specifies a 12-hour rest period, but my own measurement showed at least 50 mV change still between 12 and 22 hours after disconnecting charger from a 12-V 100Ah Exide Marine AGM – see graph below.

Stabilization time for the open-circuit voltage of an AGM battery after disconnecting from charger. Exide 12V 100Ah Marine AGM.

Note that also under-load voltages take some time to stabilize if the load level changes.

Stabilization times

Apart from the 12h for the open-circuit voltage, I have not found any manufacturer data on how long you should wait for AGM battery voltage to stabilize.

I can still give some guidelines from my personal experience. In general, I have found the voltage to stabilize fast for heavy loads, but very slow for light loads and no load:

  • The open-circuit voltage (no load) is the slowest state to stabilize, taking up to a day to creep to within 0.1 V of the final voltage; Mastervolt specifies 12 hours before taking a reading
  • At a slow discharge at 0.01…0.02C, the voltage stabilizes in 10 minutes
  • Moderate discharge rate of 0.1C steadies the voltage in under 5 minutes (after which a steady decline)

No time to wait?

Waiting 12 hours to get a proper measurement of the open-circuit voltage is obviously not always practical.

Fortunately most of the stabilization happens in the first 10 minutes after disconnecting a charger or a load. So if you are in a hurry, there is a shortcut to be taken: see where the voltage goes in the first 10 minutes, and extrapolate 0.1V to 0.2V to the same direction to get an estimate of the stabilized voltage.

Not waiting for the real stabilized voltage reading will add a bit to the uncertainty in estimating the State of Charge. However, it’s often the best you can do.

State of Charge (SoC) explained

The State of Charge basically means how much charge the battery contains relative to its maximum capacity; it means how full the battery is.

State of Charge is usually expressed as a percentage: a 100% SoC means a full battery, a state of charge of 50% tells you the battery is half full, and around 10% is practically flat.

Typical AGM SoC

In general, AGM batteries stay in the best condition when kept as close to 100% SoC as possible all the time. A typical cycle use case would see an AGM battery SoC will go between 100% and 70%, for example, in an alternating charge–discharge sequence.

Discharging to 50% SoC or below is called a deep cycle – such cycles wear the battery, and should be avoided if possible. Discharging below 10% is very harmful to an AGM battery, and makes sense only in emergencies.

Why does SoC matter?

State of Charge is the key metric in AGM battery operation. You need to know the SoC to:

  • Tell the remaining runtime of your boat or RV house power system
  • Know when to stop discharge to avoid battery damage
  • Know when your AGM batteries have reached full charge

State of Charge vs. Depth of Discharge (DoD)

Battery Depth of Discharge (DoD) is a metric parallel to the State of Charge. Simply put, the Depth of Discharge tells how much of full charge you have drained out of the battery, while State of Charge tells how much is left.

The relation of Depth of Discharge (DoD) to the State of Charge (SoC) can be expressed as

DoD = 100% – SoC [%]

Thus a fully charged battery at 100% SoC has 0% Depth of Discharge, and a practically flat one at 10% SoC has 90% Depth of Discharge.

AGM battery State of Charge Q&A

What voltage is a fully charged AGM battery?

An fully charged 12-volt AGM battery should typically have a voltage of 12.7V to 13.0V when not connected to a load or a charger – such a voltage is called the open-circuit voltage. Similarly, 6V and 24V AGM batteries should have voltages of 6.35…6.5V and 25.4…26V, respectively. Note that these readings assume the battery has rested at least 12h after charging. For more info, see the open-circuit voltage chart above.

If a fully charged 12-volt AGM battery is connected to a load, its voltage may be anywhere between 11.75V and 12.9V depending on how much current is being drawn from the battery. At high discharge rates the voltage also drops fast, making the measurement ambiguous.

What is the resting voltage of an AGM battery?

The resting, i.e. open-circuit voltage of a fully charged 12-volt AGM battery is 12.7V to 13.0V – see above.

What voltage should a 12V AGM battery be?

A 12-volt AGM battery typically reads 12.7V to 13.0V when full and unloaded, but may have any voltage from 13V to 8V if drained or actively discharged. For best battery life, an AGM battery should be kept fully charged and close to 13V no-load voltage.

For more information on AGM open-circuit voltage, see section above; for more on AGM battery storage charging, see my other article on AGM battery charging.

Is there a minimum voltage for an AGM battery?

There is no absolute minimum voltage that an AGM battery can have, but some lower limits can be given:

  • A healthy 12-volt AGM battery should have an open-circuit voltage of at least 11.7V. The expected open-circuit voltage depends on the State of Charge of the unit; for more numbers on this, see above.
  • When loaded, A healthy 12-volt AGM battery may show voltages anywhere between 8V and 13V. What voltage you should expect depends both on the State of Charge and the discharge rate – see above.

At what voltage is an AGM battery flat?

A completely flat or empty AGM battery at 0% State of Charge typically has an open-circuit (no-load) voltage between 11.4V and 11.6V. If the battery is loaded, the voltage will depend a lot on how much current is being drawn – for the numbers, see above.

Note that discharging an AGM battery down to 0% State of Charge will damage the battery permanently. Outside emergencies, you should stop discharge at 10% SoC, at which point the battery will have a no-load voltage between 11.7V and 11.9V. The corresponding under-load voltages are called final discharge voltages and given above.