Battery Case Capacity Explained (mAh)

What mAh Actually Means

A milliamp-hour is a unit of electrical charge. One mAh represents the amount of charge transferred by a current of one milliamp flowing for one hour. When a battery case is rated at 5,000 mAh, it means the case's battery cells can theoretically deliver a current of 5,000 milliamps (5 amps) for one hour, or 1,000 milliamps (1 amp) for five hours, or any other combination that multiplies to 5,000.

In practical terms, mAh tells you the total stored energy available in the case battery. A higher mAh number means more stored energy, which means more charge can be delivered to your phone. A 5,000 mAh case stores twice as much energy as a 2,500 mAh case, all else being equal. However, "all else being equal" is a critical qualifier, because the charging method, circuit efficiency, and voltage conversion all affect how much of that stored energy actually becomes usable charge in your phone.

Rated Capacity vs. Usable Capacity

The mAh rating on a battery case represents the theoretical maximum charge stored in the cells under ideal laboratory conditions. In real-world use, several factors reduce the amount of charge that actually reaches your phone's battery.

Voltage Conversion Losses

Battery case cells operate at 3.7V nominal voltage, but USB charging protocols require 5V output (or higher for fast charging). The case's internal boost converter steps up the voltage from 3.7V to 5V, and this conversion process is not 100 percent efficient. Typical boost converter efficiency ranges from 85 to 95 percent, meaning 5 to 15 percent of the stored energy is lost as heat during the voltage conversion.

To understand why this matters, consider the physics: energy is voltage multiplied by charge (in simplified terms). Converting 3.7V to 5V while conserving energy means the charge (mAh) output is lower than the charge input. A 5,000 mAh battery at 3.7V contains 18.5 watt-hours of energy. Delivering that energy at 5V, even with perfect efficiency, would yield only 3,700 mAh at the output. Factor in the 85 to 95 percent conversion efficiency, and the usable output drops to approximately 3,145 to 3,515 mAh at 5V.

Wired Charging Losses

Wired battery cases deliver power through a USB-C connector, and the total path efficiency from case battery to phone battery typically runs 75 to 85 percent when accounting for voltage conversion, cable resistance, and the phone's own charging circuit losses. A 5,000 mAh wired case delivers approximately 3,750 to 4,250 mAh of usable charge to your phone's battery in practice.

Wireless Charging Losses

Wireless battery cases suffer greater efficiency losses because the electromagnetic induction process adds another conversion step. Energy goes from the case battery (DC) to the transmitter coil (AC), across an air gap via magnetic field, to the receiver coil (AC), then back to DC for the phone's charging circuit. Each conversion loses energy as heat. Total wireless path efficiency typically ranges from 55 to 70 percent. A 5,000 mAh wireless case delivers approximately 2,750 to 3,500 mAh of usable charge to your phone.

The Practical Impact

This efficiency difference is substantial and should influence your purchasing decisions. If you are comparing a 5,000 mAh wired case against a 7,000 mAh wireless case, the actual delivered charge may be nearly identical: approximately 3,750 to 4,250 mAh from the wired case versus approximately 3,850 to 4,900 mAh from the wireless case. The wireless case needs 40 percent more rated capacity to deliver a similar amount of usable charge. When manufacturers tout high mAh numbers, always consider whether the case uses wired or wireless delivery before comparing across products.

Common Capacity Tiers and What They Mean

2,500 to 3,500 mAh (Slim Cases)

Slim battery cases in this range add roughly 30 to 50 percent of your phone's internal battery capacity in usable charge (after efficiency losses). For a phone with a 4,800 mAh internal battery, a 3,000 mAh wired case adds approximately 2,250 to 2,550 mAh of usable charge, which translates to roughly three to four additional hours of screen-on time. These cases are best for users who just need a safety margin to ensure they never hit zero before bedtime.

4,000 to 6,000 mAh (Mid-Range Cases)

Mid-range cases provide enough usable charge for approximately one full recharge of most modern phones. A 5,000 mAh wired case for a phone with a 5,000 mAh internal battery delivers roughly 3,750 to 4,250 mAh of usable charge, which is about 75 to 85 percent of a full phone recharge. This is the most popular capacity tier because it provides meaningful all-day insurance without the excessive bulk of maximum-capacity cases.

7,000 to 10,000 mAh (High-Capacity Cases)

High-capacity cases can provide one and a half to nearly two complete phone recharges, effectively tripling your phone's total available battery life. A 10,000 mAh wired case for a phone with a 4,800 mAh internal battery delivers approximately 7,500 to 8,500 mAh of usable charge, which is close to two full recharges. These cases are significantly heavier and bulkier, adding 90 to 150 grams and 8 to 12 millimeters of thickness, but they are unmatched for users who need absolute maximum runtime.

How to Match Capacity to Your Phone

To estimate how much runtime a battery case will add to your specific phone, follow this calculation. First, determine the case's usable capacity by applying the appropriate efficiency factor: multiply the rated mAh by 0.75 to 0.85 for wired cases, or 0.55 to 0.70 for wireless cases. Then divide the usable capacity by your phone's internal battery capacity to determine the percentage of an additional full charge the case provides.

For example, a 5,000 mAh wired case with 80 percent efficiency delivers 4,000 mAh of usable charge. If your phone has a 4,800 mAh internal battery, the case provides roughly 83 percent of an additional full charge. If your phone typically gets 8 hours of screen-on time on a full charge, the case adds approximately 6.5 hours of additional screen time.

Here is the same calculation for a wireless case: a 5,000 mAh wireless case with 65 percent efficiency delivers 3,250 mAh of usable charge. For the same 4,800 mAh phone, that is about 68 percent of an additional full charge, adding approximately 5.4 hours of additional screen time. The 1.1-hour difference between wired and wireless illustrates the real-world impact of efficiency losses.

Internal Battery Sizes of Popular Phones (2026)

Knowing your phone's internal battery capacity helps you choose the right case capacity. Here are the internal battery sizes of popular phone models as of 2026. The iPhone 17 Pro Max has approximately 4,800 mAh, the iPhone 17 has approximately 3,900 mAh, the Samsung Galaxy S26 Ultra has 5,000 mAh, the Samsung Galaxy S26 has 4,000 mAh, the Google Pixel 9 Pro has 5,060 mAh, and the OnePlus 13 has 6,000 mAh.

Notice the wide range: a phone with a 6,000 mAh internal battery needs a proportionally larger case battery to achieve the same percentage of runtime extension as a phone with a 3,900 mAh battery. A 5,000 mAh case provides 128 percent of an additional charge (before losses) for the iPhone 17, but only 83 percent for the OnePlus 13. Keep your specific phone's battery size in mind when evaluating whether a particular case capacity meets your needs.

Cycle Life and Capacity Degradation

Battery case cells degrade with use, just like your phone's internal battery. Each time the case battery goes through a full charge and discharge cycle, a small amount of its capacity is permanently lost. The rate of degradation depends on cell quality, charging practices, and temperature exposure.

A quality battery case retains approximately 90 percent of its original capacity after 200 cycles, 80 percent after 300 to 400 cycles, and 75 percent after 500 cycles. If you use the case battery to deliver a full charge once per day, you will go through roughly 365 cycles in a year. After one year of daily use, the case would retain about 80 to 85 percent of its original capacity. After two years, approximately 70 to 75 percent.

This degradation means a 5,000 mAh case that delivered 4,000 mAh of usable charge when new would deliver approximately 3,200 to 3,400 mAh after one year of daily use, and approximately 2,800 to 3,000 mAh after two years. The case still functions, but it provides noticeably less supplemental charge. For most users, this degradation timeline aligns roughly with phone upgrade cycles, so the case and phone reach the end of their useful lives around the same time.

Why You Should Not Compare mAh Across Charging Methods

One of the most common mistakes in battery case shopping is directly comparing mAh ratings between wired and wireless cases as if they represent equal amounts of delivered charge. A 7,000 mAh wireless case and a 5,000 mAh wired case may deliver nearly the same amount of charge to your phone, as explained in the efficiency sections above, but the higher number on the wireless case creates a misleading impression of superiority.

When comparing cases, either convert all ratings to estimated usable capacity using the appropriate efficiency factors, or compare only cases that use the same charging method. This apples-to-apples approach gives you an accurate picture of which case will actually deliver more charge to your phone in practice.