Milliamp hours, abbreviated as mAh, is a unit that measures how much electrical charge a battery can store. A battery rated at 1 mAh can deliver 1 milliampere of current for one hour before it’s fully drained. The higher the mAh number, the longer the battery can power a device before needing a recharge. You’ll see this rating on everything from AA batteries to smartphones to portable chargers.
How mAh Actually Works
One milliamp hour equals one-thousandth of an ampere hour (Ah), so 1,000 mAh = 1 Ah. Think of a battery like a water tank: the mAh rating tells you how much water is in the tank, while the current draw of your device tells you how fast the faucet is running. A bigger tank (higher mAh) means the faucet can run longer before the tank is empty.
The math is straightforward. Divide the battery’s capacity by the device’s current draw, and you get an estimate of how long it will last:
Runtime (hours) = Battery capacity (mAh) ÷ Device current draw (mA)
So a 2,000 mAh battery powering a device that draws 100 mA would theoretically last 20 hours. A 500 mAh battery running the same device would last 5 hours. In practice, you’ll want to multiply the result by about 0.85 to 0.9, since real-world inefficiencies like heat and voltage conversion eat into the total.
Typical mAh Ratings by Device
Knowing what’s “normal” for a given device helps you compare products and set realistic expectations for battery life:
- Smartphones: 2,500 mAh to 5,000 mAh for most models, with some newer phones pushing to 6,600 mAh or beyond
- Laptops: 5,000 mAh to 10,000 mAh, though laptops more commonly list capacity in watt-hours
- Tablets: Generally higher than smartphones due to larger screens, often 7,000 mAh to 11,000 mAh
- Power banks: Commonly sold in 5,000, 10,000, or 20,000 mAh sizes
- AA rechargeable batteries: Typically 1,900 to 2,800 mAh
Larger devices need higher mAh ratings not because they’re “better,” but because bigger screens, faster processors, and more demanding tasks drain current faster.
Why mAh Doesn’t Tell the Whole Story
Milliamp hours measure charge capacity, but they don’t account for voltage. Two batteries can have the same mAh rating but store very different amounts of energy if they operate at different voltages. This is where watt-hours (Wh) comes in. Watt-hours factor in both current and voltage, giving a more complete picture of total energy stored.
To convert between the two:
Wh = (mAh × Voltage) ÷ 1,000
A 5,000 mAh battery running at 3.7 volts stores 18.5 Wh of energy. That same 5,000 mAh at 7.4 volts would store 37 Wh, exactly double. This is why comparing mAh between devices that use different voltages can be misleading. Airlines, for example, restrict carry-on batteries based on watt-hours (100 Wh limit), not mAh, precisely because Wh is the better measure of stored energy.
Power Banks Deliver Less Than Advertised
If you’ve ever noticed that your 10,000 mAh power bank can’t fully charge a 4,000 mAh phone three times, you’re not imagining it. The issue is voltage conversion. Most lithium battery cells store energy at 3.7 volts, but USB output runs at 5 volts. Converting between voltages generates heat, and that heat represents lost energy.
The average power bank operates at about 85% efficiency. For a 10,000 mAh power bank, the usable capacity at 5V output drops to roughly 6,300 to 6,900 mAh. High-quality power banks push above 85% efficiency, while cheaper ones fall below it. When shopping for a portable charger, it’s reasonable to assume you’ll get about two-thirds to three-quarters of the headline mAh number in real-world use.
How Manufacturers Measure mAh
Battery makers test capacity by draining a fully charged battery at a controlled, steady rate and measuring how long it lasts. The speed of this drain is called the C-rate. A manufacturer might use a C-rate of 0.3, which means the battery is drained slowly enough to take about 3.3 hours to empty completely. Slower discharge rates make batteries look better on paper because they produce less heat and internal resistance.
This matters because real-world usage rarely matches lab conditions. Running a processor-heavy game on your phone draws current much faster than the gentle discharge rate used during testing. At higher drain rates, you’ll get noticeably less total capacity from the same battery. Testing on lithium-ion cells found that at a moderate discharge rate, capacity loss after 300 cycles was about 9.5%, but pushing to higher discharge rates increased that loss to around 13 to 17%.
How Charging Speed Relates to mAh
You can estimate charging time using a similar formula:
Charge time (hours) = Battery capacity (mAh) ÷ Charger output (mA)
A 5,000 mAh phone battery connected to a charger delivering 2,000 mA (2A) would take roughly 2.5 hours in theory. In practice it takes longer, because phones slow down the charging rate as the battery approaches full to protect its long-term health. The last 20% of a charge cycle almost always takes longer per percentage point than the first 80%.
Battery Capacity Fades Over Time
Every rechargeable battery loses some of its original mAh capacity with each charge cycle. A cycle counts as one full discharge and recharge, though partial cycles add up proportionally. After about 300 full cycles under normal conditions, a lithium-ion battery typically retains around 90% of its original capacity. After 500 cycles, most retain 80% or more, which is why many manufacturers define 500 cycles as the battery’s useful lifespan.
Several factors speed up this degradation. Consistently draining the battery to 0% or keeping it at 100% for extended periods stresses the chemistry. Heat accelerates the process too. If your three-year-old phone’s battery seems to die much faster than when it was new, it’s because the actual mAh has quietly dropped well below the number printed on the spec sheet. Your phone’s battery health setting, if it has one, shows this decline as a percentage of original capacity.