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What Is Battery Capacity in Amp Hours?

Battery capacity in amp hours (Ah) is the amount of current a battery can supply over time. A 100 Ah battery can, in theory, supply 5 amps for 20 hours, or 10 amps for 10 hours. The Victron BMV-712 Smart manual we opened describes capacity in exactly these terms. It also explains a limit that matters for real use: the usable capacity drops when the battery is discharged quickly. This article covers what the rating means, how the C rate fits in, and how the battery monitor setting uses the number.

Quick answer

• Battery capacity in amp hours is current multiplied by time. A 100 Ah battery discharged at a steady 5 A is empty after about 20 hours.
• The C rate expresses discharge speed relative to capacity. For a 100 Ah battery, 1C means 100 A and C5 means 20 A.
• Faster discharge gives less usable capacity. The Victron manual uses Peukert’s law and a Peukert exponent to account for this.
• On the BMV-712 Smart, the capacity setting is entered in Ah, with a range of 1 to 9999 Ah in the technical data.

What an amp-hour rating means

An amp-hour rating is a product of two numbers: how much current the battery gives, and how long it gives it. The Victron battery capacity chapter puts it this way: battery capacity “is expressed in Amp hour (Ah) and indicates how much current a battery can supply over time.” The chapter’s example is a 100 Ah battery discharged with a constant current of 5 A. That battery is totally discharged in 20 hours.

The same arithmetic works in other directions. If a load draws 2 A from that battery, the time before the battery is empty is roughly 50 hours, using the rating alone. This is a simple calculation. It assumes the battery delivers its full rating at that current, which the next sections show is not always true.

The C rate

The C rate describes how fast a battery is being discharged compared with its capacity. The Victron chapter defines 1C as the one-hour rate: a current that empties the whole battery in one hour. For a 100 Ah battery, 1C is 100 A. The chapter then gives two more examples. A 5C rate for that battery would be 500 A for 12 minutes, which is one fifth of an hour. A C5 rate would be 20 A for 5 hours.

Battery labels often quote capacity at a specific rate, such as a 20-hour rate. That is why the same battery can be printed with different numbers. When you compare two ratings, check the hour count they are based on. The Victron manual notes that battery capacity ratings can be found in the battery datasheet.

Why fast discharge gives less capacity

The Victron manual states the core rule directly: “The capacity of a battery depends on the rate of discharge. The faster the rate of discharge, the less capacity will be available.” It gives a worked example. When a 100 Ah battery is discharged completely in two hours, its capacity is greatly reduced, and it may give only about 56 Ah. The chapter says this happens because of the higher rate of discharge.

The relationship is described by Peukert’s law, and the exponent in that law is the number a battery monitor can adjust. On the BMV-712, the Peukert exponent can be set from 1.00 to 1.50. The manual explains that a higher exponent makes the effective capacity shrink faster as the discharge current rises. An ideal battery would have an exponent of 1.00 and the same capacity at any current.

The manual also warns that the exponent is only a rough approximation. At very high currents, the battery may give even less capacity than a fixed exponent predicts. The chapter advises against changing the default value except for lithium batteries.

Where capacity appears in the battery monitor

In the all-features chapter, the battery capacity setting is described as “the battery capacity in Amp-hours (Ah).” The manual says the maximum depends on firmware. A capacity of 32500 Ah is supported from firmware version v4.19 onward, while earlier versions support a maximum of 9999 Ah. Most ordinary batteries are far below either limit, but the note matters if you run an older firmware version.

The technical data chapter confirms the same input range for the BMV-712 Smart. Battery capacity is listed as 1 to 9999 Ah. The amp-hour reading is listed with a resolution and accuracy of plus or minus 0.1 Ah, when a 500 A shunt is used. Those figures describe the monitor’s measurement, not the battery’s capacity.

Turning a rating into a usable estimate

1. Find the rated capacity in amp-hours and the hour count it is based on. Use the datasheet, not only the label.
2. Estimate the average current the load draws, and compare it with the battery’s rated discharge rate.
3. Expect less usable capacity at a fast rate. Do not assume the full rating is available for a high-current load.
4. Enter the capacity in the monitor, and check that the Peukert exponent matches the battery chemistry or the datasheet.

Batteries can deliver high currents at their terminals. The Victron manual has a separate battery safety section, which this article does not summarize. Read it before connecting or working on any battery system.

To learn how voltage readings fit into a battery check, see How to Check Battery Charge With a Multimeter. A multimeter can show voltage, but it does not show amp-hour capacity by itself. For a worked battery test with a meter, read How to Test a Battery Pack With a Multimeter.

How we chose

This article answers one question: what an amp-hour rating means and how capacity changes with discharge rate. We used three chapters of one manual, the Victron BMV-712 Smart manual, because it covers capacity, C rates and the monitor setting for a named model. General statements about battery chemistry appear only where the manual makes them. All numbers in the examples come from the manual or from simple arithmetic on those numbers.

What we could not verify

The Victron chapters we opened do not give capacity ratings for any specific battery, and they do not compare lead-acid and lithium capacity curves. They describe Peukert’s law and the default exponent, but they do not provide a full discharge curve for a given battery. The example of 56 Ah for a two hour discharge comes from a 100 Ah example in the manual, not from a measured test of any battery. We did not check a battery datasheet for this article.

FAQ

Is amp-hour capacity the same as watt-hours?

No. Amp-hours measure charge, which is current multiplied by time. Watt-hours also include voltage. The Victron manual uses amp-hours for capacity and state of charge, so this article stays with that unit. If you need energy in watt-hours, you also need the battery’s nominal voltage, which the manual does not give for every battery.

What does 1C mean on a battery label?

1C is a discharge rate that would empty the battery in one hour. For a 100 Ah battery, that is 100 A. The Victron chapter gives this definition, along with 5C and C5 examples that show how the notation works.

Why does my battery run out sooner than its amp-hour rating suggests?

The Victron manual says the capacity available depends on the rate of discharge. A fast discharge leaves less usable capacity than the rating at a slow rate. Check the hour count on the rating and compare it with your load current.

What is the Peukert exponent?

It is the number in Peukert’s law that sets how fast effective capacity shrinks as the current rises. The BMV-712 can be set from 1.00 to 1.50. The all-features chapter gives a default of 1.25 for lead-acid batteries and 1.01 for lithium batteries when the value is unknown.

What is the highest capacity the BMV-712 accepts?

The technical data lists 1 to 9999 Ah. The all-features chapter notes that firmware v4.19 and later support 32500 Ah. Check your firmware version before you enter a value above 9999 Ah.

Bottom line

Battery capacity in amp hours is the current a battery can supply over time, and the rating only holds at the rate it was measured. Faster discharge gives less usable capacity, so use the rated hour count and your real load current together before you size a battery.

Sources

• Victron Energy, “Manual – BMV-712 Smart Battery Monitor,” chapter 8 “Battery capacity and Peukert exponent,” https://www.victronenergy.com/media/pg/BMV-712_Smart/en/battery-capacity-and-peukert-exponent.html, opened 2026-10-27.
• Victron Energy, “Manual – BMV-712 Smart Battery Monitor,” chapter 7.2.1 “Battery capacity” and chapter 7.2.6 “Peukert exponent” in “All features and settings,” https://www.victronenergy.com/media/pg/BMV-712_Smart/en/all-features-and-settings.html, opened 2026-10-27.
• Victron Energy, “Manual – BMV-712 Smart Battery Monitor,” chapter 11.1 “Technical data,” https://www.victronenergy.com/media/pg/BMV-712_Smart/en/technical-data.html, opened 2026-10-27.