What Is mAh and Why Is It Bad for Measuring Battery Capacity?

tl;dr: If you’re comparing smartphone batteries, the mAh values will usually work. Otherwise, look for a battery’s watt-hour (Wh) value to get a more universal capacity rating. 

This is the FixHub Portable Power Station’s battery. This chonky battery can fully charge a smartphone with a “5,000 mAh” battery 2–3 times. And yet, the battery is only rated at 5,200 mAh. How does that add up? Why are there quotes around the first mAh rating? And what does mAh even mean?

mAh is short for milliamp-hour. There are two uses of “mAh”—there’s the sloppy colloquial use where it represents energy capacity, then there’s the technically correct use where it represents charge capacity. 

mAh as energy capacity

Energy capacity describes how much juice a battery contains. It’s what you’d normally think of when you hear “capacity”. You can use capacity to figure out which battery is bigger, or how many times you can use one battery to charge another. 

You’ll see “mAh” being used commonly to describe capacity. My iPhone 12 Pro Max is on its third “3,687 mAh” battery. The rechargeable Eneloop AA battery in my mouse has “2,000 mAh minimum”. My PC’s uninterruptible power supply (UPS) uses a “9,000 mAh” sealed lead-acid battery. Based on the mAh ratings, I should expect better iPhone battery life with two AA batteries (4,000 mAh vs. 3,687 mAh). Similarly, if I replace the expensive UPS battery (9,000 mAh) with 5 AA batteries (10,000 mAh), I should get better longevity! But, this is simply not true. In reality, I wouldn’t be able to make it through the day with the AA battery iPhone, and my AA battery UPS would keel over the moment I lose power.

The unspoken assumption

Here’s the kicker—mAh alone isn’t enough to represent energy capacity. You have to account for an underlying assumption: the nominal voltage. A battery’s nominal voltage describes the “average” voltage during its operation. It’s a defining characteristic for a battery and is normally printed on the packaging. The mAh value combined with the nominal voltage gives you the battery’s true capacity rating.

It’s sorta like shoe sizes. If I, being a guy, told you I wore size 10.5 shoes, you’d assume I’m talking about Men’s size 10.5. If you bought me some boots in Women’s size 10.5, I’d be really sad because they’d be too small! In this example, the shoe size would be the mAh value, and the Men’s/Women’s characteristic would be the nominal voltage. 

If you’re comparing single batteries of the same type (alkaline, Li-ion, lead-acid, etc.), they’ll all have identical nominal voltages—and mAh would work to compare capacity. When the nominal voltages between two batteries are different, the mAh values are incomparable to each other. This happens:

1. When you compare mAh across different “types” of batteries, aka battery chemistries. 
2. When the battery is actually multiple batteries connected together.

Scenario 1: Different battery types

I have an alkaline AA battery, a rechargeable (NiMH) AA battery, and a Li-ion (18650) battery, all rated at “2,400 mAh”. Even though the mAh ratings are the same, these batteries have different energy capacities. The batteries have drastically different chemical compositions, which changes their nominal voltages. Alkaline: 1.5 V; rechargeable NiMH: 1.25 V; Li-ion: 3.6 V. The “2,400 mAh” Li-ion battery contains almost 3x the energy capacity of a “2,400 mAh” rechargeable AA battery!

Scenario 2: Multi-battery packs

When you wire multiple batteries together to form a pack, you can get all sorts of mAh values.

For example: the FixHub’s battery pack contains six identical Li-ion cells connected in 3S2P (three cells in series, and two of those sets in parallel). The resulting pack has a 5,200 mAh rating and a nominal voltage of 10.8 V. If you wired the six Li-ion cells differently, say in 2S3P (two cells in series, and three of those sets in parallel), you’d get a battery pack with a 7,800 mAh rating, albeit at 7.2 V. If you connected all the cells in a line (6S), you’d end up with a 2,600 mAh rating at 21.6 V. The nominal voltage and the mAh rating varies depending on the wiring configuration.

2,600 mAh, 5,200 mAh, 7,800 mAh—which of these mAh values represents the battery pack capacity as a whole? None of them, actually. Does connecting the batteries in 3S2P or 2S3P result in one having more capacity than another? Nope. They all use the same six batteries, and the pack will always have the same total capacity, no matter how you wire the batteries up. But, using mAh as capacity would make you think otherwise. 

How battery bank makers list capacity

Most battery bank manufacturers realize how confusing and useless it would be to list their actual battery bank’s mAh and nominal voltages. Instead, they use the colloquial mAh: treat the battery bank like a single, giant Li-ion battery, with a 3.6 V nominal voltage. That way, consumers can figure out how many times they can recharge their Li-ion powered gadgets (which usually have a single 3.6 V Li-ion battery) by comparing the “mAh” ratings.*

*Reality is never that simple. Conversion losses drastically reduce the actual usable mAh, sometimes by as much as 40%!

To calculate the battery pack’s colloquial mAh when you’re given an individual cell’s mAh, multiply the individual cell’s mAh rating with the total number of cells. 

The FixHub’s battery pack contains six 2,600 mAh 3.6 V Li-ion cells. The colloquial mAh would therefore be 6×2,600=”15,600 mAh”.

This colloquial mAh value would make it easy to understand that you can charge your 5,000 mAh phone battery with the FixHub Power Station approximately three times. But it would be technically incorrect. 

mAh as charge capacity (what mAh is meant to be)

mAh is a technical expression of charge capacity. Batteries are essentially containers of chemicals that can store electric charge. The SI unit to express stored electric charge is a coulomb (charge delivered by 1 amp for 1 second), but mAh (charge delivered by 1 milliamp for one hour) is the common unit for batteries. In fact, 1 mAh = 3.6 coulombs! 

Charge capacity is different from energy capacity. You’ll rarely encounter charge capacity mAh in everyday life, unless you’re designing battery-powered gadgets or interfacing directly with the battery cells.

If you are designing battery-powered gadgets, you can use charge capacity to estimate device runtimes. LEDs are a good use case: they get brighter based on how much current you supply. If you were designing a bike light with a 5,000 mAh battery, it can supply the LED 5,000 mA for an hour (bright!) Or, it can supply the LED 2,500 mA for two hours (less bright, but longer). You can make these calculations by knowing only the current requirements. mAh can also be used when designing a battery pack. If you’re building something that requires high surge current (power tools, for example), you’ll want high mAh, which is accomplished by wiring more batteries in parallel. 

Wh: a better way to rate energy capacity

Using mAh to represent energy capacity works fine within a small circle of products—mainly devices that use single-cell Li-ion batteries, along with their accessories. When you mix in categories that use different battery chemistries or multi-batteries—laptops, drones, RC toys, power tools, etc.—mAh is no longer a reliable metric for comparing capacity. Instead, use the watt-hour (Wh) unit of measurement, which builds the nominal voltage into the value, resulting in a proper capacity rating. Watt-hours are agnostic to battery chemistry or multi-battery configurations. A 50 Wh battery bank has the same energy capacity as a 50 Wh laptop battery, which has 5x the capacity of a 10 Wh smartphone battery. 

Whether the mAh value is colloquial (representing energy capacity) or technical (representing charge capacity), you can convert it into watt-hours:

• Divide the mAh by 1,000 to get Ah (amp-hours)
• Multiply the Ah with the battery nominal voltage

The FixHub’s battery is 5,200 mAh with a 10.8 V nominal voltage. It would therefore have 5,200/1000×10.8=56.16 Wh. 

Technically incorrect, but fine day-to-day

Using mAh colloquially to represent energy capacity is fine, as long as you’re aware of its assumptions and limitations. The tech industry is slowly embracing Wh to represent capacity. Until then, if you’re comparing mAh values between two batteries and one seems to be unnaturally low or high, you’ve got the power to set the record straight: convert to watt-hours.