AGM Battery Voltage Chart: How to Read a 12V AGM's State of Charge

EcoFlow- 2026/11/10

Table of contents

A 12V AGM battery reads about 12.8V to 13.0V at rest when full and about 12.2V at 50% charge. Makers advise against dropping below 11.6V. Readings hold up only after the battery rests with no load or charger attached. Fullriver asks for at least 8 hours of rest, and Lifeline asks for at least 4.

This AGM battery voltage chart also covers charging setpoints and the right way to measure. A short section shows how to keep lights and Wi-Fi running while a battery is out of service.

12V AGM Battery Voltage Chart (State of Charge)

Resting voltage, also called open circuit voltage, tracks how full an AGM battery is. It is the reading taken after the battery has rested with nothing connected. The chart below is one manufacturer's table, and other brands may differ slightly.

Small gaps matter here. A reading near 12.4V falls between 60% and 70% charge, while 12.8V is close to full.

Fullriver Resting Voltage by State of Charge

Fullriver publishes these values for a 12V AGM at 80°F, measured after at least 8 hours of rest. Each 10% step is only about 0.11V to 0.15V apart, so a meter that shows two decimal places helps.

State of charge	Open circuit voltage
100%	12.83V
90%	12.72V
80%	12.60V
70%	12.47V
60%	12.34V
50%	12.20V
40%	12.06V
30%	11.91V
20%	11.76V
10%	11.61V

Lifeline Benchmarks

Lifeline gives fewer data points, and it uses a shorter rest period. Three numbers stand out:

Full charge: 13.0V or higher, which equals 2.17V per cell, after at least 4 hours of rest.
Half charge: About 12.2V marks roughly 50%. This matches Fullriver's 12.20V.
Lower limit: Lifeline advises avoiding anything below 11.6V. Fullriver's 10% mark of 11.61V sits right at that line.

Why the Two Makers Disagree

The fully charged AGM battery voltage differs by maker. Fullriver lists 12.83V, while Lifeline lists 13.0V or higher. That gap is at least 0.17V.

This is why the manual for the battery in use comes first. Any AGM voltage chart reflects one brand's design and test conditions, so it works best as a close guide and not an exact match.

Other battery chemistries follow very different curves. Compared with a general battery voltage chart, AGM numbers sit in a narrow range.

When an AGM bank is out of service for testing or charging, the devices it powered still need power.

Keeping Lights and Wi-Fi On While a Battery Is Out of Service

Testing, equalizing a flooded bank, or replacing a bad AGM can take a battery offline for hours or days. A separate power station can carry a few small loads in the meantime. It works as a stand-in for small devices, not a fix for the AGM.

A Stand-In for Small Loads

When an AGM battery is pulled for testing or replacement, the lights and router it fed go dark too. An EcoFlow portable power station can cover those small loads on its own. That leaves time to charge, test, or replace the AGM properly. It has its own LFP battery and display, so no voltage chart is needed to read it.

A work light, a phone, and a router add up to a small load. A 1kWh unit covers that for many hours. That is usually more than enough for a full charge-and-test cycle on an AGM.

EcoFlow DELTA 3 Classic (1024Wh)

•Fast AC Recharging: Reaches 0 to 80% in 45 minutes from a standard wall outlet, with 4 charging methods in total. • 1,800W Output: 3,600W surge, and up to 2,600W with X-Boost for compatible devices such as some microwaves and window AC units. Check each device's wattage first. • 10ms Auto-Switch: Switches to battery power within 10ms for devices plugged directly into the unit, such as a Wi-Fi router or NAS.

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Runtime Estimate

The figures below are an estimate. The 20W, 10W, and 10W loads are placeholders, so real runtime depends on what each device actually draws.

Load (example)	Watts used	EcoFlow DELTA 3 Classic (870Wh usable est.)
Work light + phone + router (placeholders 20W + 10W + 10W)	about 40W	about 22 hours

The unit bridges small loads only. It does not repair an AGM, and it is not a stand-in for a full battery bank. Larger loads, such as a well pump or an air conditioner, call for a different plan.

LFP batteries use a different voltage curve entirely. Anyone switching chemistries should review the LiFePO4 voltage chart before reusing any AGM thresholds.

Charging voltages are a separate set of numbers from resting voltages.

AGM Charging Voltages: Bulk, Absorption, and Float

While charging, an AGM's voltage is higher than its resting voltage. Chargers hold specific setpoints in each stage. These vary by maker and temperature.

Setpoints by Maker

The AGM battery charging voltage is highest during bulk and absorption, then steps down for float. Float is the lower, steady setting that keeps a full battery topped off.

Stage	Fullriver 12V at 77°F	Lifeline 12V
Bulk/Absorption	14.7V	14.2 to 14.4V
Float	13.65V (13.7V nominal)	13.2 to 13.4V

Fullriver's absorption setpoint sits 0.3V to 0.5V above Lifeline's range. Its float setpoint is also higher. A charger preset for one brand may not suit the other, so the maker's numbers should guide the settings.

Temperature Compensation

Fullriver specifies a correction of -4mV per °C per cell, which is -2mV per °F per cell. A 12V battery has six cells, so that adds up to about 24mV per °C.

The effect shows in Fullriver's own figures:

Cold battery: At 32°F, absorption rises to 15.30V. Cold cells accept charge less readily, so the setpoint moves up.
Warm battery: At 104°F, absorption drops to 14.34V. Heat raises the risk of overcharging, so the setpoint moves down.
Equalizing: Lifeline does not advise equalizing for AGM batteries. That step suits flooded banks, not sealed AGMs.

Charge Rate

On the AGM battery charge rate, Lifeline's fast-charge guideline is about 25% of rated Ah. For a 100Ah battery, that works out to about 25A.

Charge controllers set these voltages in solar systems. Many types of solar charge controllers include an AGM preset. Even then, the preset is best compared against the battery maker's numbers.

The chart only works if the reading is taken the right way.

How to Measure AGM Battery Voltage Accurately

Measure at rest, with loads and chargers disconnected. A digital multimeter gives the clearest reading. Note the temperature too, since published charts assume set conditions.

Readings match the AGM battery voltage chart only when taken at rest.

Rest Time Before Testing

Fullriver: At least 8 hours after charging.
Lifeline: At least 4 hours after charging.

When the maker is unknown, the longer wait is the safer choice.

Steps for a Clean Reading

Disconnect loads and chargers. Remove anything attached to the battery. Even a small draw can pull the reading down.
Set the meter to DC volts. Pick a range that shows two decimal places if the meter allows it.
Connect the probes. Touch the red probe to the positive terminal and the black probe to the negative terminal.
Read and compare. Match the display against the AGM battery full charge voltage and the state-of-charge table in the maker's manual.

Reading the Result

Surface charge: A reading taken right after charging runs high. Waiting out the rest period lets it settle.
Load: A connected load pulls the reading low. This can make a healthy battery look half empty.
Weak battery: A battery that will not hold voltage after a full charge may need replacing. Have it load-tested before deciding.

Regular checks suit any deep cycle battery, and resting voltage is the simplest one to track.

Conclusion

A 12V AGM at rest reads about 12.8V to 13.0V when full and about 12.2V at 50%, depending on the maker. The best habit is simple: measure after resting, use the battery's own chart, and recharge before it drops below about 50%. This AGM battery voltage chart gives a starting point, but the manual comes first. While an AGM is out of service, an EcoFlow DELTA 3 Classic can keep lights and Wi-Fi on for small loads.

FAQs

What voltage is a fully charged 12V AGM battery?

A full 12V AGM reads about 12.8V to 13.0V at rest, depending on the maker. Fullriver lists 12.83V, while Lifeline lists 13.0V or higher. Readings hold only after the battery rests with no load or charger attached. Always confirm in the manual.

What voltage is 50% on a 12V AGM battery?

A 12V AGM reads about 12.2V at rest when it sits near 50% charge. Fullriver lists 12.20V at 50%, and Lifeline gives about 12.2V. Lifeline also advises keeping average depth of discharge at or below 50%. Recharging sooner leaves more reserve.

What is the charging voltage for an AGM battery?

It depends on the brand. Fullriver lists 14.7V for absorption and about 13.7V for float. Lifeline lists 14.2V to 14.4V for absorption and 13.2V to 13.4V for float. Temperature also shifts the setpoints, so the maker's data sheet applies.

How long should an AGM battery rest before checking voltage?

Fullriver says at least 8 hours after charging, and Lifeline says at least 4 hours. A longer rest is the safer choice, since a surface charge right after charging reads high. Disconnect all loads and chargers before taking the reading.

How low can an AGM battery go?

Lifeline advises avoiding anything below 11.6V and keeping average depth of discharge at or below 50%. At rest, 11.6V sits near Fullriver's 10% mark of 11.61V, so recharging well before that point helps protect the battery.