Batteries: What's the difference between Wh and mAh?

Most people are familiar with the mAh label because we see it on phones, tablets, power banks, and similar electronics. It's a quick way to determine and compare whether a phone (or other device) has enough battery capacity for our needs. But it's not as simple as it seems at first glance. Charging systems are changing, and so are battery compositions, so it's good to know what mAh is, what Wh is, and how to distinguish between the two.

What is mAh in batteries? What is Wh in batteries?

The units of measurement mAh and Wh express battery capacity. Smartphones today have batteries with an average capacity of 5000 mAh, and we are approaching 6000 mAh. A battery with a capacity of 5000 mAh can provide a current of 5000 milliamperes for one hour, after which it needs to be charged (5000 milliamperes x 1 hour = 5000 milliampere hours or mAh).

If we look at laptops, for example, the ASUS ExpertBook P5 laptop we tested recently – it has a 63 Wh battery. Such a battery can sustain 63 watts of power for one hour before it runs out (63 watts x 1 hour = 63 watt-hours or Wh). Usually at least one of these units is written directly on the battery, but in some devices, access to the battery is impossible without completely disassembling the device. Think about the last time you were able to open the back of your phone and see what was wrong inside. With laptops, this is easier, but with most devices (whether for security reasons, water resistance, etc.) it is more difficult. The rare exception is devices like the Samsung Galaxy Tab Active5, which allow us free access to all internal components, but these are devices that are from the very beginning a barrier to repairability.

That's why we rely on the manufacturer for the numbers on batteries these days. They give us a rough idea of how long the battery will last, depending on the load it's under at the time. You can double or halve the current draw, and the battery will last twice as long or less.

All of this is true under optimal conditions. For example, battery life is reduced in low temperatures, and overheating and internal battery resistance can also affect battery life.

The milliampere hour mostly works, but it can also confuse the user

Let's first look back in time. We've always been used to using the symbols ampere-hours and milliampere-hours, as they're found on AA batteries, toy batteries, and even lead-acid batteries in cars. It wouldn't be the first time that a particular symbol has been used, even though it may be inappropriate or outdated in some scenarios. We still use a magnifying glass as a search symbol today, even though we don't use the physical tool much anymore. Or a floppy disk as a storage symbol, and an envelope as an email symbol.

It sounds like the mAh unit of measurement has taken a toll on us. Nothing personal against it, but it has one problem – it doesn’t take into account the voltage of the battery when representing its capacity. Two batteries may have the same capacity of 5000 mAh, but they can store very different amounts of energy. For example, if one has a voltage of 12 V and the other has a voltage of 3 V, and both have the same capacity, the first battery can store four times as much energy. Both can provide a current of 5000 milliamps for one hour, but the 12-V battery can do so at four times the voltage.

Comparing batteries by mAh can be crystal clear provided that the nominal voltages are the same. And in most cases, they are. Phones with lithium-ion batteries use individual cells with a voltage of 3.6 to 3.8 volts. Before lithium-ion batteries, most used series-connected 1.2-volt NiMH (nickel-metal hydride) or Ni-Cd (nickel-cadmium) batteries.

Today's smartphones split the battery into two cells, mainly for fast charging, space constraints, and heat distribution. If we look at the Samsung Galaxy Z Fold6 foldable phone, it uses a two-cell battery. Not even for the needs of fast charging, as its maximum charging power is only 25 W, but for space and better heat transfer. The Vivo X100 Pro also has a two-cell structure and on the official website you will find under the specifications that it has two built-in batteries with a typical capacity of 2700 mAh (7.78 V), but most often the phone is advertised as having a 5400 mAh battery.

A good example is Apple's spatial computing glasses, the Apple Vision Pro, which initially confused users with a 3166 mAh battery, which is much smaller than most smartphones on the market. But when you see that the battery operates at a higher voltage, it becomes clear that concerns about poor battery life were misplaced.

The milliampere-hour unit causes more confusion when batteries with different chemistries are mixed in. A different chemical structure also means a different battery voltage rating, which means that comparisons with lithium-ion cells with a nominal voltage of 3.6 to 3.8 V are no longer so simple. You have probably heard of LiFePO4 batteries, which boast better long-term energy storage, but also have a lower energy density and lower voltage (around 3.2 V).

This is where the problem comes in. We've had charging stations and portable batteries on the market for some time now that use this new battery structure. They boast tens of thousands of milliamp hours of battery capacity, and if we take that number and directly compare it to the capacity of the batteries in a phone, we'll come to the wrong conclusion.

If a charging station has a capacity of 50,000 mAh, you would expect it to be able to charge your phone with a 5000 mAh battery ten times. But because the voltage of the battery cells is lower, the amount of energy stored is also lower and therefore incomparable to devices with traditional lithium-ion batteries. This does not mean that manufacturers are lying, but the fact is that you need to be careful, because the mAh unit of measurement does not take into account voltage.

Is it for measuring battery capacity better to use Wh?

If we don't want battery capacity to be measured in absurd numbers in the coming years, which is of course grist for the mill for advertisers who will wave their high numbers in front of our noses, using the watt-hour (Wh) unit of measurement may be a better solution.

This might be a little confusing at first, but luckily Wh is already used in some popular devices, such as cars (kWh), laptops and handhelds (ASUS ROG Ally, Steam Deck…). The main advantage of watt-hours is that they take into account the battery voltage, making them a more practical and accurate unit of measurement. Some manufacturers already display this in addition to the typical capacity in mAh. Taking the Vivo X100 Pro example above again, the manufacturer also lists a typical capacity of 21.01 Wh among the battery specifications.

If you would like to calculate Wh from mAh yourself, use the formula Wh = (mAh x V) : 1000 or Wh = Ah x V if you are using the ampere-hour (Ah) unit.

Using Wh instead of mAh means you can compare the capacity of two batteries regardless of their nominal voltage, type, or how the battery cells are connected. This makes it easier to compare across device types. You can use Wh to compare the capacity of (any) batteries, whether they're in a phone, laptop, charging station, or car.

In all likelihood, the use of Wh is inevitable

Given that Wh is used in some of the biggest areas, such as electric cars and laptops, it's surprising that phone manufacturers insist on milliampere hours. But it's likely that in the coming years we'll either replace mAh in phone batteries with Wh, or use both units of measurement and be aware of the differences.