Battery Equivalents and Replacements

What Does Ah Mean on a Battery?

maxell lr44In order to describe features and specifications of the batteries, certain measurement units are used, including the unit for capacity - Ampere Hour or abbreviated Ah.

Other most common units are voltage (V), Cold Cranking Amps (CCA, given in Amps), Cranking Amps (CA, given in Amps) also known as Marine Cranking Amps (MCA), Reserve Capacity (RC, given in minutes), dimensions (given in inches, centimeters, millimeters), etc.

But, often the very first parameter of the battery that is being checked is nonetheless the battery's - capacity.

Published: December 9, 2021.

What Is an Amp Hour?

An Ampere Hour or as it is often abbreviated as an Amp Hour, is a unit of electric charge, equal to the electric current multiplied by time, which in battery world means the ability of a battery to provide a certain current for a certain time and is labeled as "Ah".

For large lead-acid and similar batteries, battery capacity is a value given in Ah (Amp Hours) when the battery is discharged for 20h down to the voltage that is considered as 100% DoD (Depth of Discharge).

Battery capacity can be given in milli-Amp-hours (mAh), kilo-Amp-hours (kAh), or using any other SI prefix.

Time of discharge, discharge current, cut-off voltage, temperature and similar battery parameters play important role in determining the battery capacity since the real-life batteries are far from being ideal batteries - among other things, they feature internal resistance and capacity loss when being discharged using stronger currents at lower temperatures.

For example:

Duracell LR44 battery (Datasheet PDF) actual capacity depends among other things on the cut-off-voltage:

- 130 mAh, 6k8Ω, down to 1.2V,

- 170 mAh, 6k8Ω, down to 0.9V.

If this battery is used to power devices that accept the batteries with the lowest voltage of 1.2V, then this battery feature capacity of 130 mAh, when being discharged over 6k8Ω.

But, when the same battery is being discharged over the same load but this time down to 0.9 volts, its actual capacity is 170 mAh.

digital multimeter

To measure such small currents, the best thing to do is to use a digital ammeter or digital multimeter and set it to the required range.

Large batteries, for example, BCI Group 31 batteries have nominal (20h) capacities around 100Ah. But, if the batteries are discharged faster, the capacity drops.

For example, Odyssey 31M-PC2150 is one of the best lead-acid AGM group 31 batteries featuring a nominal capacity of 100Ah.

odyssey 31m pc2150

But, as the battery's current is being increased, actual battery capacity is decreased:

- 20h discharge, 5A: 100Ah capacity,

- 10h discharge, 9.2A: 92Ah capacity,

- 5h discharge, 17.1A: 85.5Ah capacity,

- 1h discharge, 71.5A: 71.5Ah capacity,

- 30m discharge, 127A: 63.5Ah capacity,

- 15m discharge, 215.5A: 52.75Ah capacity,

- 5m discharge, 438.5A: 36.5Ah capacity, etc.

As one can see, the actual 1h capacity is 71.5Ah which is 71.5% of the 20h capacity (100Ah) - this may look like a large capacity loss, but the Odyssey 31M-PC2150 battery is one of the best performers in its class and it comes with a rather "nice" price tag.

In order to measure extra strong currents, digital caliper/clamp ammeters are recommended.

digital clamp meter

These ammeters allow the user to directly measure the electric current in the cable without the need to interrupt the electric current.

In order to get the feeling of what the battery can or cannot do, the best thing to do is to check the battery's constant current discharge charts, if available.

Battery Capacity (Ah) Frequently Asked Questions (FAQ):

Here are some of the most common questions about battery capacity and battery performances in general:

Does a higher Ah battery give more power?

Generally, yes, the battery with a larger capacity gives more power. 

But, larger capacity means that the battery is able to store and provide more energy. If the battery is designed for low-discharge rates (low power), such battery should be used as such.

Is a higher Ah battery better?

Generally, yes, the higher the Ah the better. But, if the battery is part of an electric system, for example, a car, too large battery means that the battery charger (alternator) has to work more than initially intended, shortening the alternator's runtime.

Is a 4.0 Ah battery more powerful than a 2.0 Ah?

4.0Ah battery of the same chemistry is able to store a double amount of energy when compared with the 2.0Ah battery.

Again, if the 4.0Ah battery is a low drain battery, and the 2.0Ah battery is a high-drain battery, it is very possible that the 2.0Ah battery is able to provide more power than the 4.0Ah battery.

What is the difference between a 2.0 Ah battery and a 4.0 Ah battery?

The difference between 4.0Ah and 2.0Ah battery is exactly 2.0Ah.

Seriously, the 4.0Ah battery features double capacity when compared with the 2.0Ah battery, and if the batteries are of the same chemistry and construction, the 4.0Ah battery is able to provide twice the current (and capacity) of the 2.0Ah battery when they are being discharged for the same period of time.

How long does a 4.0 Ah battery last?

It depends on the discharge current - if the lead-acid battery is being discharged with 0.2A for 20 hours, the battery will last 20 hours.

In order to find the actual discharge time, check the constant current discharge tables of the battery in question.

Do higher Ah batteries last longer?

Generally, yes, the batteries with higher Ah last longer.

Can I use a 5Ah battery instead of 2Ah?

If both batteries are of the same chemistry, construction and voltage, yes, a 5Ah battery will last longer than a 2Ah battery.

But, if the 2Ah battery is a high-drain battery and the 5Ah battery is a low-drain battery, and the 5Ah battery is used instead of a 2Ah battery, 5Ah battery will perhaps last longer in terms of minutes/hours, but in terms of supported charging/discharging cycles, it will not last longer than 2Ah battery.

Long Story Short: Battery capacity (given in Ah) shows the battery's ability to provide current under certain conditions.

For lead-acid batteries, this means for the period of 20 hours, but many manufacturers also provide constant current discharge tables.

For other batteries, this period may be shorter or even longer.

For smaller batteries, LR44 for example, capacity is given in mAh while the battery is being discharged continuously over the same resistor.

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