A magnetic power bank's headline capacity is not the number of milliamp-hours your phone will receive. Cell capacity (often quoted at 3.7 V nominal) and rated capacity (quoted at a 5 V output under a stated load) measure different things, and a wireless charging path adds another loss layer on top of both. Before you buy, specify or test one of these units, the useful question is: under which test conditions was each figure obtained, and does my use case match them?
This guide covers what capacity figures on a magnetic power bank actually mean, how consumer test programmes measure them, where Qi2 and Qi2 25W change the picture, and the conditions under which a published number stops describing your situation. Brand-specific claims are deliberately excluded; work from the datasheet wording and the standard the product is certified to.
Scenario and user goal
Typical setting: a household or a small office buys a magnetically attaching battery pack for phones and earbuds. The buyer sees “5,000 mAh” or “10,000 mAh” and expects a specific number of full phone charges. In practice the unit delivers fewer, sometimes markedly fewer, because the phone's own battery sits at roughly 3.85–3.9 V, the pack must step up to 5 V or higher for wired output, and the wireless path converts energy to a magnetic field and back. Each conversion converts some energy into heat.
The goal is therefore not “find the biggest mAh number” but “estimate usable delivered energy for my scenario, and verify it with a test that states its conditions”.
Operating context: what physically happens inside the pack
- Cell energy. Cell capacity multiplied by cell voltage gives watt-hours. A 10,000 mAh pack built from 3.7 V cells holds roughly 37 Wh; a pack using slightly higher-voltage cells can show the same mAh while storing more energy.
- Conversion to the output rail. Boosting to 5 V, plus regulator and protection losses, is why the rated capacity is always lower than the cell figure. Rated capacity is a guaranteed minimum for the stated output and load.
- Wireless transmission. On the magnetic path the coil alignment, gap, foreign-object detection and thermal behaviour all influence how much of the delivered 5 V-equivalent energy reaches the phone. Heat generated in the coils and in the phone is energy that never reaches the phone battery.
How published capacity figures are actually obtained
Three distinct measurement conventions appear on the same product page, and mixing them is the most common specification error.
| Figure | What it measures | Typical stated conditions |
|---|---|---|
| Cell / typical capacity (mAh) | Charge stored in the internal cells | Cell nominal voltage (often 3.7 V), room temperature |
| Rated capacity (mAh) | Charge delivered at the output port | A stated output voltage and current (for example 5 V / 3 A), after conversion losses |
| Wireless output power (W) | Charging rate on the coil, not stored energy | Standard profile (for example Qi2 15 W), with a compatible receiver |
None of these rows tells you “how many phone charges” on its own. A fourth figure is needed: the energy actually delivered in the target scenario, which is one step further removed again.
Evidence from independent consumer testing
Hong Kong's Consumer Council tested 15 power banks in a European laboratory, referencing IEC 62133-2 for safety and measuring conversion efficiency by fully charging each sample and then measuring the usable energy at 5 V in a 20 °C room, comparing it with the energy declared for the internal cells. All samples declared 10,000 mAh cells, with declared energy between about 36 Wh and 38.7 Wh; measured usable energy ranged from about 28.5 Wh to 34.6 Wh, corresponding to conversion efficiencies from roughly 79% to 95%. Three samples reached 90% or above.
Two further findings matter for anyone writing a capacity test:
- Temperature changes output. At 0 °C, usable energy was 1.8% to 27% lower than at room temperature, with twelve samples more than 15% down.
- Storage matters too. The programme also compared output after storage for 28 days at 40 °C against room-temperature performance, and ran 40 °C output tests.
Those are wired-path measurements at 5 V. A magnetic pack has to be characterised separately, because the coupling and heat path differ. Reporting a wireless pack's “usable energy” using the wired procedure would overstate what the coil delivers.
Standard lineage and what certification does and does not say
Qi v2.0, launched in 2023, introduced magnetic attachment with 15 W charging in the Magnetic Power Profile. The Wireless Power Consortium published Qi v2.2.1, marketed as Qi2 25W, in July 2025; WPC describes it as delivering nearly 70% more charging power than Qi2 15 W, with the claim that a certified pairing can take a smartphone battery from 0 to 50% in about 30 minutes. Certification is granted only after independent authorised laboratory testing of safety and interoperability, and the WPC maintains a public certified-products database.
Three limits follow directly from that scope:
- Certification is interoperability and safety testing, not a capacity guarantee. The Qi2 logo does not state how many watt-hours your phone will receive.
- The 15 W or 25 W profile is a maximum profile, not a constant rate. Actual power depends on the receiver, its thermal state and the alignment the magnets achieve.
- Battery safety standards are separate. IEC 62133-2 (portable sealed secondary cells and batteries) and GB 31241 (the Chinese safety specification for lithium-ion cells and batteries used in portable electronic equipment) address safety requirements; they are not efficiency or rated-capacity test methods.
Designing a capacity test that fits a real use case
The following is a practical test outline, not a standard. It is written so the result can be compared with the manufacturer's rated-capacity claim rather than replacing it.
1. Record the declared figures first
Note cell capacity and voltage, rated capacity with its output voltage and current, wireless profile and wattage, and any declared conversion efficiency. If the datasheet gives only mAh, convert to watt-hours so figures become comparable across packs.
2. Fix an ambient temperature and state it
Room temperature is the usual reference; the Consumer Council data shows why a 0 °C result must not be presented as typical. Log temperature during the run, since a warm coil throttles faster.
3. Define the load and the receiver
For wired output, use the stated output condition. For the magnetic path, name the receiving device and its supported profile, and keep the same device across compared samples.
4. Measure delivered energy, not just runtime
An inline USB power meter gives voltage, current and accumulated watt-hours on the wired path. For wireless, a meter on the receiver side (or a measured phone-battery delta with the screen off) is needed; phone-side percentage alone is too coarse.
5. Report both paths separately
Wireless and wired results should never be merged into one efficiency number. The gap between them is itself the useful information: it tells you what the magnetic convenience costs in delivered energy.
Fit conditions: when a magnetic pack's numbers line up with the job
- Top-up use. Short, repeated charging during a commute or meetings uses a small part of the curve where alignment is easy and heat has little time to build. Here the headline capacity is almost irrelevant; profile compatibility matters more.
- Pocket or bag carry with a phone that supports magnetic attachment. Magnets set the gap and alignment, so day-to-day results are consistent with the spec.
- Wireless earbud cases and small wearables. Low power draw means the wireless loss is small in absolute terms.
- Known comparison basis. When the pack's rated capacity is declared at a stated 5 V output, you can convert to watt-hours, apply a plausible efficiency band and sanity-check the “number of charges” expectation.
Trade-offs
- Sealed wireless design versus wired output. A sealed pack with only a coil removes port wear but gives away efficiency; a pack adding a USB-C port recovers most of that loss but reintroduces the port as a wear point.
- Thinness versus thermal headroom. Slim packs have less surface area to shed coil heat, so sustained wireless output falls sooner, and delivered energy drops for long sessions even though the cell capacity is unchanged.
- Higher wattage versus declared energy. A 25 W profile charges faster but is not a larger battery; the same cell capacity gives fewer effective minutes under heavy use.
- Cell voltage choices. Packs built on higher-voltage cells can declare more watt-hours for the same mAh, which is legitimate but makes mAh-only comparisons misleading.
Implementation constraints
- Do not state a wireless efficiency figure sourced from a wired test. Label which path was measured.
- State the receiver. A phone limited to a lower profile caps the test at that profile regardless of the pack.
- Watch foreign-object detection. Metal near the coil, cards with magnetic strips and keys should be kept clear; the pack may pause charging, which shows up as missing energy in a test log.
- Account for case thickness. A thick or non-magnetic case increases the gap between coils and reduces coupling.
- Know your regulatory frame. Air transport rules for lithium batteries are set by the relevant aviation authorities and depend on watt-hour rating, so the watt-hour figure, not the mAh figure, is what gets checked.
Evidence and case methodology
Any published evaluation of a magnetic power bank should be able to answer five questions: which figure is being reported (cell, rated, or delivered); at what ambient temperature; through which path (wireless or wired); with which receiver and profile; and whether the measurement comes from a declared standard, an independent laboratory programme, or an illustrative estimate made for this article. Where a scenario is illustrative, it should be labelled as such rather than presented as a measured result.
Applied this way, the capacity test becomes a specification-matching exercise: match the test conditions to the use case, then decide whether the pack's declared numbers describe your situation. If you are comparing pack form factors rather than test procedures, the same discipline applies to how a pack attaches and stays attached in daily use.