Why smartphone battery life is far more complex than just having a big battery

All-day battery life is a major checkmark for any new smartphone purchase. Here at Android Authority we go to great lengths to test the battery life of every handset we review. Still, it's all too easy to turn to a spec sheet, look at the mAh listing and make all sorts of assumptions based on a single number. This sometimes leads to us being bamboozled by the lackluster battery life from a seemingly huge 4,000mAh cell or surprised at how long a tiny battery inside phones like the Pixel 3a or iPhone SE can last.

A phone's mAh rating is a rather meaningless statistic when taken on its own. Battery life is actually a very complicated picture, with a wide range of contributing hardware and software variables. In this article, we'll be digging into why you shouldn't trust mAh as your sole guide to a phone with the longest battery life.

Buyers guide: Smartphones with the best battery life

What does mAh mean on a battery anyway?

The mAh abbreviation you find on battery spec sheets stands for milliampere-hour. This is a unit of electric charge which is equal to supplying one milliamp of current (3.6 coulombs of charge) constantly for one hour. So a 1mAh battery can provide 1mA of current for one hour, while a 1,000mAh battery provides 1mA of current for 1,000 hours. However, a 1,000mAh battery providing 2mAh of current will only last 500 hours. Of course, smartphones don't last hundreds or thousands of hours because they draw much more than 1mA of current from the battery. The more current drawn by your phone the shorter the length of time the battery lasts.

How long a mAh lasts depends entirely on how much current is drawn by the phone.

Everything being equal, a phone with a larger battery capacity will last longer than a smaller one. However, this is seldom the case, as internal hardware and therefore power consumption varies so widely. Phone A may consume 10%, 20%, or even 30% more current than Phone B.

The unique hardware and software inside each smartphone means that no two are ever alike. This is why simply knowing a battery's mAh capacity doesn't give you any really useful information about the expected battery life at all.

Read more: The definitive guide to everything that affects smartphone battery life

Battery life vs mAh: Tested

Before we dive into the "why" a bit deeper, let's do some testing! We ran a batch of phones with various specs and battery capacities through our in-house Speed Test G benchmark and recorded how long it takes to drain the battery. Speed Test G is a pretty brutal stress test, so these results represent pretty much the minimum screen on time that you can expect from these smartphones. The results are arranged in ascending battery capacity.

There's no clear and obvious trend from our results. We might expect to see a roughly steady increase in battery life-time as capacity increased but that isn't the case.

While our phones with gargantuan 5,000mAh and 6,000mAh cells offer some of the longest battery life-times under our extreme stress test, it's actually the 3,700mAh Google Pixel 3a XL that lasts the longest here. This is all thanks to its lower-end specs, so don't dismiss smaller batteries out of hand! Similarly, there's just a few minutes difference between the 3,700mAh Pixel 4 XL and the 5,000mAh Asus Zenfone 6, highlighting that capacity alone is not a guarantee of long or short battery life.

Our stress test doesn't identify a direct correlation between capacity and battery life.

Perhaps the most notable trend from our results is that so many phones actually fall between the 3 hours 30 minutes and 4 hour mark under very heavy stress. This appears to be the sweet spot that most manufacturers target, albeit with much lighter processing workloads in mind. The 2,800mAh Google Pixel 4, 4,000mAh Galaxy S20 (Exynos model), and the 4,510mAh OnePlus 8 Pro all fall within a just few minutes of each other.

Clearly, the different hardware specifications and software features strain their batteries in very different ways. But what exactly are those differences?

It all depends what your phone is powering

The battery powers all the hardware inside your smartphone, from the processor to display and any other features packed into the handset. That much is obvious, but different pieces of hardware suck down more or less juice. Low and mid-range processors, for example, consume less power than their flagship counterparts. Generally speaking, higher performance requires more power. This is often why affordable handsets often have longer battery life than premium phones for any given battery capacity.

But as we've seen, even high-end flagships can have very different levels of power consumption. See the situation with Samsung's Exynos and Snapdragon Galaxy S20 flagships. Manufacturers can also under or overclock chipsets, and even alter the CPU scheduler to hit their preferred performance and power points.

Related: The best portable battery chargers

There are some good examples of extra hardware sucking down the battery juice too. The Google Pixel 4 and its Soli radar system is a prime example of a feature that drains much more battery than a handset without it. Including a time-of-flight camera focusing system, more powerful stereo speakers, or a 4K display all impact battery life too. Even something seemingly as small as charging up the S Pen on recent Galaxy Note phones adds up. These features make handsets unique, but they come with a price.

Manufacturers balance high performance and demanding hardware against all-day battery life, capacity, and costs.

The trend in faster refresh rate displays plays a big part as to why modern phones consume so much power. This appears to be the reason that the Samsung Galaxy S20 series phones stick to 60Hz out of the box, despite technically supporting 120Hz. The Pixel 4's 90Hz mode is also linked to display brightness in an attempt to save battery life. The reason being that the faster display content refreshed, the more power the display and phone's processor consume.

Want more examples? Did you know that the 4,300mAh OnePlus 8 with a 90Hz display gets better battery life than the 120Hz OnePlus 8 Pro with its larger 4,510mAh battery? The two phones have otherwise virtually identical specs, highlighting just how much of an effect displays and refresh rates have on battery life.

Battery life isn't just a hardware consideration either. Smartphone software can also affect battery life by killing off background apps to reduce CPU usage and wake-ups. For example, Huawei's EMUI is notoriously more aggressive in this regard than Samsung's One UI.

5G is power intensive

Another recent trend complicating the matter of mAh vs battery life is the launch of 5G. 5G modems and radio components require more power than previous generation 4G equivalents, meaning that your battery won't go as far if you're on a 5G network. Complicating things further, different 5G modems and chipsets drain different levels of power.

Related: Best 5G phones you can buy and all the 5G phones coming soon

Mid-range chipsets with integrated 5G modems, such as the Exynos 980 and Snapdragon 765G, should consume slightly less power than the premium tier external modems used in flagship smartphones. This may be partially why phones like the LG Velvet and reportedly even the Google Pixel 5 are set to ditch Qualcomm's power-hungry flagship tier chipset, the Snapdragon 865. Again though, these SoCs have slower peak speeds, so it's another power versus performance trade-off.

The move to 5G hardware has certainly increased the necessity for larger battery capacities. However, whether this is an important purchasing consideration depends on whether you're actually on a 5G or 4G network. If you're sticking with a 4G tariff for now, then power consumption from these components won't be as high and battery life should be more in line with previous generations. Again though, this all depends on other hardware. According to Redmi general manager Lu Weibing, the move from 4G to 5G typically consumes at least 20% more power. So you'll need a roughly 20% larger battery capacity to achieve the same battery life as an equivalent 4G phone.

Looking out for the best battery life

The key takeaway from all of this is that building a smartphone with all-day battery life isn't as simple as picking the biggest battery possible. Manufacturers have to weigh up cost, space, and the hardware they intend to power. The more feature-rich a handset, the tricker the balancing act. Most manufacturers try to target a balance of hardware and battery capacity that will get you through a full day of typical use and sometimes that doesn't require a huge cell.

Our stress test doesn't identify a direct correlation between capacity and battery life because there isn't one. Bigger batteries obviously provide more power to play with, but the underlying hardware choices made by manufacturers have just as big of an impact on actual battery life results.

There are far more pieces to the battery life puzzle than just pure capacity.

Mid-range smartphones with less energy-hungry technology, such as the Pixel 3a, tend to get away with smaller batteries while still providing all-day battery life. In the premium segment, manufacturers use larger batteries capacities (and the phones are bigger too) to power more demanding technologies, like 5G, high refresh rate displays, or extra gaming performance.

Of course, how you use your phone adds a whole other layer to the battery life debate. Casual Facebook browsers are bound to end up with much more battery left at the end of the day than mobile gamers.

Want longer battery life? See these habits to maximize battery life

That's Why Smartphones Keep Getting Bigger