mAh is not energy: convert to Wh first
Milliamp-hours (mAh) only describe charge at a given voltage. To compare a battery case with a phone, convert both to watt-hours (Wh): multiply mAh by the nominal voltage and divide by 1,000.
- Battery case cell: 2,500 mAh rated, 2,100 mAh measured × 3.7 V = 7.8 Wh usable.
- iPhone 17 battery: 3,692 mAh at a slightly higher nominal voltage, roughly 14 Wh.
Even with zero losses, a 2,500 mAh battery case could not fill an iPhone 17 from empty. And the 2,500 mAh on the box is the rated capacity: on our bench the cell delivers 2,100 mAh, which is the number that counts.
The two losses inside every battery case
Loss 1: voltage conversion
A lithium-polymer cell sits at about 3.7 V. Charging a phone needs a higher, tightly regulated voltage, so a converter steps it up. Good converters are efficient, but every conversion turns part of the energy into heat.
Loss 2: the wireless coil
A Qi coil transfers energy through a magnetic field across a small gap. Alignment, distance and temperature all matter. This is the larger of the two losses, and it is the same whether the coil sits in a battery case or in a magnetic battery pack.
Tip: wired pass-through over USB-C skips the coil. If you need the most energy out of a battery case, plug the cable into the case rather than relying on wireless.
Worked example: Slim 2500 on an iPhone 17
| Step | Value |
|---|---|
| Energy the cell actually delivers (2,100 mAh) | 7.8 Wh |
| Energy that reaches the phone battery | about 4.5 Wh |
| Charge added to an iPhone 17 | ≈+32% |
| Charge added to an iPhone 17 Pro Max | ≈+28% |
| End-to-end efficiency | about 58% |
Roughly four tenths of the delivered energy is lost as heat in the converter and the coil. That figure is normal for wireless charging. It is why we publish the rated capacity, the measured capacity and the charge added for every Slim 2500 model.
Why bigger phones get a lower percentage
The battery case delivers about the same amount of energy to any phone of its size. A Pro Max has a much larger battery, so the same energy fills a smaller share of it. That is why our Pro Max version carries a 3,000 mAh cell (2,600 mAh measured), and it still adds about 28% rather than 32%. When you compare battery cases, always check which iPhone model the percentage refers to.
How to read capacity claims on any battery case
- Look for Wh as well as mAh. Watt-hours are what the airline and the laws of physics care about.
- Ask for the delivered charge on a named iPhone model, with the test conditions.
- Check capacity against weight. Energy scales with the mass of the cell. If a 10,000 mAh case weighs about the same as a 2,500 mAh one, one of the two numbers is wrong.
- Ask for the UN 38.3 test summary. It names the cell and its rated capacity. If the seller cannot provide it, the number on the box is a guess.
What this means for magnetic battery packs
The same physics applies. Our 5,000 mAh magnetic battery pack (4,500 mAh measured) adds about 70% to an iPhone 17. Bigger cells add more, but they also add weight and heat, which is the real trade-off when you choose between a slim battery case and a larger magnetic battery pack. See our magnetic battery pack vs battery case comparison.
Frequently asked questions
Why doesn't a 2,500 mAh battery case fully charge a 3,692 mAh iPhone?
Because the cell's measured capacity is 2,100 mAh, the phone battery stores far more energy, and the converter and the wireless coil lose part of the energy as heat. About 32% is a realistic result.
Is wired charging from a battery case more efficient?
Yes. USB-C pass-through skips the wireless coil, which is the largest single loss, so more of the stored energy reaches the phone.
Is a higher mAh battery case always better?
Not always. More capacity means more weight, more thickness and more heat. Choose the smallest capacity that gets you through your typical day.
How many watt-hours is a 2,500 mAh battery case?
About 9.3 Wh rated at 3.7 V (7.8 Wh measured). That is far below the 100 Wh limit for carry-on baggage.