Battery capacity is the total amount of energy a battery can store and deliver before it needs recharging. It’s expressed as a number you’ve probably seen on your phone specs or a portable charger label, like 5,000 mAh. That number tells you how long the battery can power a device, but the real-world performance depends on several factors that make the story more nuanced than a single spec.
What mAh and Wh Actually Tell You
The two most common units for battery capacity are milliampere-hours (mAh) and watt-hours (Wh). They measure slightly different things, and knowing the difference helps you compare batteries accurately.
Milliampere-hours measure how much current a battery can supply over time. A 3,000 mAh battery can theoretically deliver 3,000 milliamps for one hour, or 1,500 milliamps for two hours, or 300 milliamps for ten hours. You’ll see mAh on phones, earbuds, portable chargers, and most small electronics. It’s a useful number when you’re comparing batteries that run at the same voltage, like two smartphones both using 3.7-volt cells.
Watt-hours measure total energy, which accounts for both current and voltage. This makes Wh a more complete and comparable number, especially when you’re looking at batteries with different voltages. You can convert between the two: multiply mAh by voltage, then divide by 1,000 to get Wh. So a 5,000 mAh battery at 3.7 volts holds 18.5 Wh. You’ll see Wh on laptop batteries, electric vehicle specs, and home energy storage systems. It’s also the unit airlines use to regulate which batteries you can bring on a plane.
Why the Number on the Label Isn’t the Whole Story
A battery rated at 5,000 mAh won’t always deliver 5,000 mAh of usable power. Several real-world factors shrink the energy you actually get.
Discharge rate is a big one. The faster you drain a battery, the less total energy it delivers. This relationship is described by a principle called Peukert’s Law: as the rate of discharge increases, the battery’s available capacity decreases. A battery that lasts 10 hours at a gentle drain might last significantly less than 5 hours if you double the power draw. High-performance tasks like gaming on your phone or running power tools pull more current and effectively reduce the usable capacity compared to lighter loads like browsing or standby.
Temperature matters just as much. A battery that provides 100% of its rated capacity at 27°C (80°F) will typically deliver only about 50% at negative 18°C (0°F). If you’ve noticed your phone dying faster on a cold winter day, that’s not a glitch. The chemical reactions inside the battery slow down in cold temperatures, temporarily reducing how much energy the battery can release. Heat can boost short-term performance but accelerates long-term degradation, so extreme temperatures in either direction work against you.
Rated Capacity vs. Usable Capacity
Not all of a battery’s capacity is meant to be used. Manufacturers and battery management systems intentionally limit how deeply you discharge a battery to protect its long-term health. This concept is called depth of discharge.
The chemistry of the battery determines how much you can safely use. Lead-acid batteries, common in cars and backup power systems, experience significantly reduced cycle life if discharged below 50%. That means a 100 Ah lead-acid battery really only gives you about 50 Ah of usable capacity if you want it to last. Lithium iron phosphate (LiFePO4) batteries can technically be discharged to 100% without long-term damage, though manufacturers typically recommend stopping at 80% depth of discharge to extend lifespan. Standard lithium-ion batteries in phones and laptops fall somewhere in between, with built-in circuits that prevent the battery from fully emptying or fully charging to protect the cells.
This is why a power bank rated at 10,000 mAh won’t charge your 3,000 mAh phone three full times. Energy is lost to voltage conversion, heat, and the battery management system holding back a reserve on both ends.
How Capacity Fades Over Time
Every rechargeable battery loses capacity with use. Each charge cycle causes tiny physical and chemical changes inside the cell. Electrode materials expand and contract, and small amounts of lithium get trapped in forms that can no longer participate in the charge-discharge reaction. Over hundreds of cycles, these changes add up.
The industry standard threshold for end of life is typically around 70 to 80% of the original capacity. When your phone battery has degraded to 80% of what it held when new, it’s generally considered past its useful life for that application, even though it still works. A phone that shipped with 4,500 mAh might be down to 3,600 mAh after two or three years of daily charging. You’ll notice shorter battery life and possibly more frequent charging before the battery technically “fails.”
Researchers have found that adjusting how a battery is charged and discharged can meaningfully extend this timeline. One approach, raising the minimum voltage threshold as the battery ages, has shown cycle lifetime extensions between 16% and 38% depending on the battery type. In practical terms, this is what adaptive charging features on modern phones and laptops are doing: adjusting charge patterns to slow degradation.
How to Compare Batteries Meaningfully
When you’re shopping for a phone, laptop, or portable charger, raw mAh numbers are only useful for comparing devices at the same voltage. A 10,000 mAh power bank sounds like it holds twice the energy of a 5,000 mAh one, and at the same voltage it does. But if you’re comparing across different product categories, like a phone battery versus a laptop battery, Wh is the fairer comparison.
Also pay attention to what the device actually draws. A phone with a 5,000 mAh battery and an efficient processor will outlast a phone with the same battery and a power-hungry screen. Capacity sets the upper limit, but power consumption determines how quickly you reach it.
For rechargeable batteries in tools, e-bikes, or home storage, look at both the capacity in Wh and the expected cycle life. A battery with slightly less capacity but a longer cycle life may deliver more total energy over its lifetime. The number of full charge-discharge cycles before hitting that 70 to 80% threshold varies widely, from a few hundred cycles for budget cells to several thousand for high-quality lithium iron phosphate chemistry.
What the Capacity Rating Assumes
Manufacturers test battery capacity under controlled conditions defined by international standards like IEC 61960, which specifies how secondary lithium batteries for portable devices should be tested and labeled. These tests use a standardized discharge rate, a specific temperature (usually around 20 to 25°C), and discharge the battery to a defined cutoff voltage. The number on the label reflects performance under those ideal conditions.
Your real-world experience will almost always fall below the rated number. You’re rarely at the perfect temperature, rarely drawing current at the exact test rate, and your battery is aging from the moment it leaves the factory. None of this means the label is dishonest. It just means battery capacity is a best-case benchmark, not a guarantee of what you’ll get on a cold morning while streaming video over cellular data.