Power banks from China: capacity, cells and freight
Why a 20,000 mAh power bank delivers 11,000, how to tell grade A cells from recovered ones, and which capacity can fly and which must go by sea.
A power bank rated at 20,000 mAh stores about 74 Wh at 3.7 V but delivers between 11,000 and 13,000 mAh equivalent at 5 V to the phone: the rest is lost in voltage conversion and heat. Both figures are correct — one measures the energy inside the cell, the other what reaches the device — but only the first is usually printed on the box. That asymmetry drives many “defective product” claims that are not defects at all.
How 20,000 mAh becomes 74 Wh, and then 11,000
A lithium cell’s capacity is declared in milliamp-hours at its nominal voltage, and energy is obtained by multiplying: 20,000 mAh is 20 Ah, and 20 Ah × 3.7 V = 74 Wh. That is the figure the cell actually stores, and the only one transport cares about.
Delivering through its USB port, the power bank has to step 3.7 V up to 5 V — or higher, for fast charging — and that conversion is not free. A well-designed boost converter returns 80% to 90% of the energy it draws; a cheap one far less, especially when it runs hot. With 74 Wh going in and 85% efficiency you get about 63 Wh of usable energy, which at 5 V is about 12,600 mAh; at 75%, about 11,100 mAh.
The Chinese power bank standard, GB/T 35590, requires the cell’s rated energy in watt-hours to be distinguished from the effective output capacity at 5 V, which is the figure that should be on the box.
The watt-hour is the figure that decides transport
A power bank’s transport route is set by its energy in watt-hours, not by its capacity in milliamp-hours or by its weight. Convert before asking for a freight quote: with 3.7 V cells, 100 Wh is about 27,000 mAh and 160 Wh about 43,000 mAh. A 20,000 mAh power bank, at 74 Wh, sits below the first threshold.
A power bank is classified as a lithium battery carried on its own (UN3480, packing instruction 965), not as a battery inside equipment. That distinction explains the restriction: as air cargo it is accepted only on cargo aircraft, prohibited on passenger aircraft, and requires a state of charge no higher than 30%. Sea freight has no state-of-charge limit, which is why the larger capacities move as consolidated sea freight.
| Power bank capacity | mAh equivalent (3.7 V cells) | Carry-on baggage | Air cargo |
|---|---|---|---|
| Up to 100 Wh | up to ~27,000 mAh | Accepted, subject to carrier limits | UN3480 / PI965, cargo aircraft only, state of charge ≤ 30% |
| Over 100 and up to 160 Wh | ~27,000 – 43,000 mAh | Requires airline approval and the number of units is usually capped | Requires approval from the State of origin and from the operator |
| Over 160 Wh | more than ~43,000 mAh | Not accepted | Not accepted as standard cargo |
These thresholds are not a universal number: they vary by carrier, by airline and by route, and must be confirmed for each shipment. Aviation regulation also moved in 2026: an ICAO addendum in force since 27 March limits passengers to two power banks for personal use in carry-on baggage and prohibits recharging them in flight. That is a passenger aviation rule: sea freight does not change.
Grade A, grade B and recovered cells: three things with the same name
The cell is the most expensive component in a power bank, so it is where cost is cut when the price has to come down, and the cut is almost never announced. Three origins circulate under the same label:
- Grade A. Meets the manufacturer’s specification: capacity within tolerance, stable internal resistance, low self-discharge.
- Grade B. Does not meet the specification — on capacity, internal resistance or self-discharge — but works: it starts well and loses capacity sooner.
- Recovered cell. Dismantled from a pack that has already been in service. It carries consumed cycles and an undocumented history.
“Grade A” on a quotation is a supplier’s claim, not a certified fact: no body issues a grade A certificate. What does exist is a test report on one specific cell model, plus the traceability of the batch you are being sold. A supplier who cannot give you the brand, model and batch number is offering you a cell, not a grade A cell.
How the real capacity of a batch is verified
Capacity is verified by discharging, not by looking. The procedure is that of IEC 61960-3, the capacity standard for portable lithium cells: charge the sample to specification, discharge it at a defined current, and measure the energy delivered. Two decisions separate a useful test from a formality: measure units drawn at random from the batch you are about to ship, not the sample the supplier picked, and fix current and temperature, because delivered capacity depends on both.
The second check is traceability. Ask for the cell’s brand, model and batch number, and verify that this model is the one named in the UN38.3 report for the pack. When the manufacturer buys cells on the open market, the model changes batch to batch with nobody documenting it. MeliPrep bills pre-shipment AQL inspection at USD 299 per inspector-day, with sampling to ISO 2859-1, and the discharge test is coordinated inside that same visit; published rates are on the pricing page.
Declared power versus sustained power
A power bank that advertises 65 W needs three things to sustain it: cells capable of delivering that current, a protection circuit sized for it, and enough surface to dissipate the heat. Many declare peak power — what the chip tolerates for a few seconds — and deliver considerably less as soon as the temperature rises.
The test is direct: a programmable electronic load is connected to the port and asked for the declared power continuously for at least thirty minutes, logging voltage, current and surface temperature. An honest product holds the power and stabilises temperature; one that declares the peak falls away within minutes or shuts down on thermal protection. It is also worth reading the label: 65 W split across three ports is not 65 W on one port.
What a BMS does and what gets cut when you save on it
The BMS is the circuit that keeps the cell from operating outside its safe range, and it is the first thing to disappear when cost has to be cut. Its minimum functions are five: overvoltage cut-off while charging, undervoltage cut-off while discharging, overcurrent and short-circuit limiting and cut-off, temperature cut-off by sensor, and balancing between cells in multi-cell packs.
When it is omitted, the product does not fail at once: it fails in use. Without balancing, cells in a pack drift apart and one works above the others. Without a temperature sensor, the device keeps charging when the cell is already hot — the condition in which a lithium cell degrades fastest and can enter thermal runaway. The verification is not visual: ask for the BMS datasheet with the thresholds in writing, and count how many temperature sensors the pack carries.
UN38.3 and MSDS: transport documents, not certificates of sale
The UN38.3 report and the material safety data sheet (MSDS) allow the goods to be transported; they do not certify that the product can be sold. They exempt you from no obligation in the destination country, and without them the cargo does not board the aircraft or the ship. UN38.3 is section 38.3 of the United Nations manual of tests and criteria on the transport of dangerous goods; it certifies that one specific cell and pack model passed eight tests: simulated altitude, thermal, vibration, impact, external short circuit, crushing, overcharge and forced discharge.
The report is tied to the cell and pack model, not to a brand or product family. A supplier may hold UN38.3 for the cell it bought and not for the pack it sells you. Checking that it covers exactly the reference you are buying decides whether the cargo ships.
On the product side, the standards asked for again and again are IEC 62133-2 for the safety of portable lithium cells and batteries, and IEC 62368-1 — fourth edition, published in 2023 — for equipment and power supply safety. Brazil assesses the battery against the local version of IEC 62133, NBR 62133, and requires the certificate holder to be a Brazilian legal entity, with marking in Portuguese.
The Chinese marking is also worth a look. Since 1 August 2024, a power bank without CCC certification cannot be manufactured, sold or imported in China, under market administration announcement No. 10 of 2023, which brought portable-electronics lithium batteries into the compulsory regime under standard GB 31241. The CCC number does not replace the tests that matter to you, but it indicates that the pack went through a real assessment.
Mexico and Brazil: what changes at destination
In Mexico, a power bank usually falls within NOM-001-SCFI-2018 (safety of electronic apparatus powered by mains or battery) and NOM-024-SCFI-2013 (commercial information and labelling), and the certificate is issued by a body accredited in Mexico. A model that charges wirelessly or carries Bluetooth with an app transmits by radio and additionally needs homologation before the CRT, which took over the functions of the IFT. A cable-only power bank does not fall into that second regime.
In Brazil, the electrical and battery safety layer is covered by Inmetro, and the radio layer by ANATEL. A power bank with wireless charging or Bluetooth falls clearly within ANATEL’s scope; for a cable-only model, certification bodies describe the scope in ways that do not agree, and the valid reading is the one the body issues for your reference. If the power bank pairs with an app, the brand owner additionally needs a product declaration before Bluetooth SIG: the Bluetooth word mark and logo are registered trademarks usable only with a current declaration.
In all three cases — NOM, Inmetro and ANATEL — the test is performed on the finished product, and obtaining the certificate after manufacturing means the goods wait at the factory while it is issued.
Comparison table: entry, mid-range and premium
What separates the three tiers is not the exterior finish: it is what goes inside and what you can prove in writing.
| Attribute | Entry | Mid-range | Premium |
|---|---|---|---|
| Cell | Grade B or recovered, no traceability | Grade A from a recognised manufacturer, identifiable model | Grade A with a documented batch and a no-substitution agreement |
| Declared capacity | Cell capacity at 3.7 V only | Both figures: cell and 5 V output | Both, verified by laboratory discharge |
| Power | Peak, with fall-off under sustained load | Sustained on the main port | Sustained, with a declared temperature curve |
| BMS | Basic protections, one thermal sensor | Five protections, sensor per zone | Documented thresholds and active balancing |
| Documents you can demand | UN38.3 for the pack, if it exists | UN38.3, MSDS, IEC 62133-2, IEC 62368-1 | All of the above, plus CCC and the batch discharge report |
| Main risk | Real capacity far below the declared figure | Spread between batches | Cost and lead time |
MOQ and why this category has a higher floor than accessories
In the rest of the consumer electronics catalogue the minimum starts at 100 or 500 units; in power banks it typically sits between 500 and 2,000. It is not a commercial decision by the manufacturer: it is arithmetic. Cells are bought in matched batches, so cells for 200 units cannot be obtained at a reasonable price. The BMS is a dedicated board that has to be assembled and tested, the enclosure needs tooling, and the certification test is paid per model, not per unit, so it only amortises above a certain volume.
For a first order below that threshold, the reasonable route is not to negotiate the minimum down but to start from a model the manufacturer already has certified and to limit customisation to the packaging. The product you sell has to remain inside the scope of the existing certificate, and that check belongs before the deposit — the same principle as buyer protection.
What to ask for before paying the deposit
- The cell’s brand, model and batch number, with a commitment not to substitute it without notice.
- The UN38.3 report naming the cell and pack model you are going to receive, together with the MSDS.
- The energy per unit in watt-hours, the sustained power on the main port under continuous load, and the BMS thresholds in writing.
A supplier who answers with documents is a supplier you can build a power bank line with. One who answers with a product photo and a figure on the box is not.