Battery capacity is a measurement of how much electrical energy a battery can store and deliver before it needs recharging. It tells you, in concrete terms, how long a battery will power your device. A smartphone battery rated at 5,000 mAh, for example, can theoretically supply 5,000 milliamps of current for one hour, or 1,000 milliamps for five hours. The higher the number, the longer the battery lasts.
mAh vs. Wh: Two Ways to Measure Capacity
You’ll see battery capacity expressed in two main units, and which one you encounter depends on the device. Milliamp-hours (mAh) shows up on smartphones, earbuds, and portable chargers. Watt-hours (Wh) appears on laptops, power tools, and electric vehicles. Both describe capacity, but they measure slightly different things.
mAh tells you how long a battery can supply current without factoring in voltage. It’s a simple, current-focused number. Wh goes a step further by including voltage, which means it represents the total energy stored in the battery. That distinction matters because two batteries with the same mAh rating but different voltages hold different amounts of actual energy. A 5,000 mAh battery at 3.7 volts stores 18.5 Wh, while a 5,000 mAh battery at 7.4 volts stores 37 Wh, or twice as much energy.
The conversion formula is straightforward: multiply mAh by voltage, then divide by 1,000 to get Wh. For very large batteries like those in electric vehicles and home energy storage systems, capacity scales up to kilowatt-hours (kWh) or even megawatt-hours (MWh).
What Happens Inside the Battery
In a lithium-ion battery (the type in most modern devices), capacity comes down to how many lithium ions the battery’s internal materials can absorb and release. The battery has two electrode layers: an anode and a cathode, separated by a liquid electrolyte. When you use the battery, lithium ions travel from the anode to the cathode through the electrolyte, and that movement generates a flow of electrons through your device’s circuit. Plug the device in to charge, and the process reverses: ions flow back to the anode, restoring the battery’s stored energy.
A battery’s capacity is essentially limited by how much lithium its electrode materials can hold. Over time, some of that lithium gets trapped in chemical side reactions and can no longer shuttle back and forth. That’s why batteries gradually lose capacity with age.
Rated Capacity vs. What You Actually Get
The number printed on a battery label is its nominal capacity, measured under specific, controlled conditions at the factory. In real-world use, you’ll almost never hit that exact figure. The practical capacity of a battery is generally below its theoretical maximum because not all of the active material inside gets fully utilized during each charge and discharge cycle.
Several factors push actual capacity above or below the rated number. Temperature is one of the biggest. Discharging speed is another. And the battery’s age plays a growing role over time.
How Discharge Speed Affects Capacity
Battery engineers use something called “C-rate” to describe how fast a battery is charged or discharged relative to its size. A 1C rate means the battery delivers its full rated capacity in one hour. A 0.5C rate stretches that to two hours, and a 2C rate compresses it to 30 minutes.
Here’s the catch: draining a battery faster reduces the usable capacity. At a 2C rate, internal resistance converts some energy into heat, and you might only get about 95% of the rated capacity or less. Drain it slowly at 0.5C, and you can actually exceed 100% of the rated figure. This is why manufacturers of alkaline and lead-acid batteries often rate capacity at a very slow 0.05C discharge (a 20-hour drain), making the numbers look as good as possible. If you use the battery harder than that test rate, you’ll get less runtime than the label suggests.
Temperature’s Impact on Capacity
Cold weather is notorious for killing battery performance. In freezing conditions, the chemical reactions inside slow down, and the battery delivers less energy per charge. Heat creates a different problem. While a warm battery may perform fine in the short term, high temperatures accelerate permanent capacity loss. Research published in the National Library of Medicine found that the rate of capacity degradation roughly tripled when batteries were stored at 70°C compared to normal conditions. At extreme temperatures around 100°C, one study measured nearly 39% capacity loss in just the first two charge cycles.
For everyday use, this means leaving your phone on a hot car dashboard or regularly charging in direct sunlight does real, lasting damage to how much charge your battery can hold.
How Your Device Estimates Remaining Capacity
The battery percentage on your phone or laptop comes from a method called coulomb counting. The battery management system continuously tracks how much current flows in and out of the battery over time. By adding up the total charge delivered (measured in amp-hours) and comparing it to the battery’s known full capacity, the system calculates what percentage remains.
This approach is simple and works well for short-term estimates, but it has a notable weakness: small measurement errors accumulate over time. Sensor inaccuracies, temperature shifts, and aging all introduce drift, which is why your phone sometimes jumps from 15% to dead without warning, or sits at 1% for an oddly long time. Devices periodically recalibrate by letting the battery charge fully or discharge deeply, which resets the baseline and corrects accumulated errors.
How Capacity Degrades Over Time
Every lithium-ion battery loses capacity with use. The industry standard considers a battery to have reached end of life when it retains only 80% of its original capacity. For a phone that shipped with 5,000 mAh, that means end of life arrives when the battery can only hold 4,000 mAh. Most smartphone batteries hit this threshold somewhere between 500 and 1,000 full charge cycles, depending on usage patterns.
How deeply you discharge the battery on each cycle has a significant effect on how quickly it degrades. Routinely draining a battery to near-zero accelerates aging. Shallower cycles, where you recharge before the battery drops very low, put less stress on the electrode materials and extend total lifespan. This is why many electric vehicles and energy storage systems are programmed to operate within a limited range of their full capacity rather than cycling all the way from 100% to 0%.
Picking the Right Capacity for Your Needs
When comparing batteries or devices, keep a few things in mind. For devices that share the same voltage (like most smartphones, which run around 3.7 to 3.85 volts), mAh is a perfectly fair comparison. A 5,500 mAh phone battery holds more charge than a 4,000 mAh one. But when comparing across different device types or voltages, Wh is the more accurate number because it accounts for the energy difference that voltage creates.
Portable chargers (power banks) deserve extra skepticism. A power bank rated at 10,000 mAh won’t deliver 10,000 mAh to your phone, because energy is lost in voltage conversion and heat during the transfer. Expect roughly 60% to 70% of the rated capacity to actually reach your device. So a 10,000 mAh power bank will realistically charge a 4,000 mAh phone about one and a half times, not two and a half.