Sourcing power banks from China

Chemistry and format. Lithium-ion cylindrical cells (18650, 21700) are robust, well characterised and thicker; lithium-polymer pouch cells are thin and shape-flexible and are what makes a slim bank possible. LiFePO4 appears in larger packs for its cycle life and thermal behaviour. The format constrains the industrial design, so decide it before the case.

The cell manufacturer, by name, written into the specification. Cell quality is the single biggest determinant of safety, real capacity and how the product behaves after a hundred cycles, and it is invisible from outside. A factory that names its cell supplier and shows the cell's documentation is a different counterparty from one that says "A-grade cells".

New, not reclaimed. Cells recovered from scrapped packs and re-sorted are cheap, look identical, and are the most common explanation for a price that seems impossible. They fail early, they fail unevenly across a pack, and they are the failure mode that ends in a thermal event. Write "new, first-grade cells, not recovered or re-sorted" into the specification and ask for the cell purchase documentation.

The protection circuit. Over-charge, over-discharge, over-current, short-circuit and temperature cut-off are not optional and are not all present at every price point. Ask what protection IC is used and what the cut-off thresholds are — a factory that can answer runs a real design, and one that cannot is assembling somebody else's.

Then the rest of the specification: output ports and their individual and combined power, protocol support (USB Power Delivery, and PPS if you are claiming it), pass-through charging, input port and speed, display type, enclosure material and flame rating, and the marking on the product itself.

Rated capacity versus cell capacity. The number on the box is usually the cell pack's capacity at its nominal 3.7 V. What the user gets is delivered at 5 V, so the arithmetic above applies before any losses. Many markets expect the deliverable figure to be stated as well, and a listing that only shows the larger number is the one that generates returns.

Conversion loss is real and is not a single number. Boost conversion, cable losses, the load, the temperature and the state of charge all move it, which is why this page publishes the arithmetic and not an efficiency percentage. Measure it on your own sample at the load you care about rather than accepting a figure.

Test it properly, on more than one unit. A defined load, from full to cut-off, recorded — and repeated on several units from the same batch, because consistency is the property a cheap cell fails first. A single flattering unit tells you nothing about a run.

Cycle behaviour. Capacity after a defined number of cycles is where reclaimed and low-grade cells separate from good ones, and it is the number a factory that is confident in its cells will happily discuss.

And do not let the marketing capacity into your own claims unchecked. In most markets the party placing the product on the market is answerable for what the packaging says, and "the factory printed it" is not a defence.

UN 38.3 is the transport test, and it applies to the cell, the battery pack and often the finished article. No compliant UN 38.3 documentation, no air freight — and a supplier who offers a report belonging to a different model has offered you nothing. Check the model number and the applicant on the report itself.

A safety data sheet accompanies the shipment. It is prepared for the specific battery and is part of the dangerous-goods paperwork rather than a marketing document.

Destination certification is separate and depends on your market: the EU and UK expect a technical file and the corresponding marking, the United States has its own regime for the radio and safety aspects present, and Japan and Korea require their own approvals for lithium-ion products. Establish which apply to YOUR market before you order, because certification is the step most often discovered late and the one that cannot be compressed.

And remember who carries the obligation. Placing the product on the market under your own brand generally makes you the manufacturer for regulatory purposes, so the technical file, the traceability and the recall duty attach to you, not to the factory whose certificates you were shown.

Transport thresholds — watt-hour limits, packing instructions, state-of-charge rules — are deliberately not published on this page. They live in the guides on watt-hour conversion and shipping lithium batteries by air, because one set of numbers in one place is how a set of numbers stays right.

Read the business licence and the 经营范围 first. A manufacturer's registered scope names production; a reseller's names only sales, wholesale or trade. It is the cheapest check available and it settles a question that a website cannot.

Ask whether the pack is assembled in-house. Many "power bank factories" buy finished cell packs and assemble; that is a legitimate model, but it means the cell decision is being made by somebody you are not talking to, and it changes who can answer a failure analysis.

Ask what testing happens on the line, not in the lab. Capacity test per unit or per sample, ageing, insulation resistance, temperature during charge, and what happens to units that fail. A factory that can describe its own reject rate and what it does with rejects is telling you it measures.

Then buy two of the shortlisted product, test one to the limits you care about and take the other apart. The cell markings, the joint quality, the protection board and the internal layout will tell you more in an afternoon than any amount of correspondence.

Every supplier listed on this site is a manufacturer — no trading companies, not even free ones — and the registered-scope check above is one of the things confirmed before a listing goes live.