The "20,000mAh" Power Banks Couldn't Charge a Phone Even One and a Half Times
Power Bank Capacity Test Guide - Why Power Banks That Passed Inspection Get "Only Half the Capacity" Reviews After Sale, Cell Capacity vs Rated Capacity, Converting mAh to Wh, Electronic Load Discharge Tests Compared With Efficiency, Cycle Life and Self-Discharge Tests, Approximate Criteria, Gaps in Factory Reports, Supplier Cell Substitution Such as Recycled Cells and Missing Cells, Spec Sheet Items, Stage-by-Stage Checks and a Checklist

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An online seller imported 3,000 units of a 20,000mAh power bank from an electronics factory in Shenzhen. The approval sample charged a phone three or four times with no trouble, and the pre-shipment inspection found no problems with appearance, port output, indicator lights or packaging. The product page said "High capacity 20,000mAh, charges the latest phones 4 times."

In the second week of sales, the tone of the reviews changed. "Can't even charge my phone one and a half times." "My USB meter shows only 7,000mAh." When a few units from remaining stock went to a lab for a discharge test, output energy came to about 38Wh. With genuine 20,000mAh cells inside, the product should have delivered around 60Wh. A teardown showed the cells weighed about 190g in total, roughly two-thirds of a normal unit, and the cell wrappers showed signs of having been peeled off and rewrapped. The factory had put proper cells in the approval samples and cheap recycled cells in production.

Power bank capacity failures are tricky because a visual inspection reveals nothing. The cells sit inside a plastic case, and a phone plugged in will still charge, so checking the ports tells an inspector nothing about capacity. On top of that, even a good unit delivers less than the mAh printed on it, which makes it hard to tell whether a customer complaint is within the normal range or a real defect. This article starts with how to read capacity markings, then covers test types and criteria, common supplier tricks, and how to build capacity testing into your spec sheet and inspection checklist.

An answer of "Yes, it's 20,000mAh" or plugging in a phone once is not enough. Set cell capacity, rated capacity (at 5V output), Wh, conversion efficiency, cell maker and cell count in numbers in the spec sheet, and run discharge tests on samples drawn from production lots. That is how you prevent capacity claims after sale.

1. Why Even Good Units Deliver Less Than the Label

One thing to clear up first. The large "20,000mAh" on a power bank is usually the capacity of the cells inside, measured at the cell voltage of 3.6-3.7V. Power going to a phone is boosted to USB 5V (9V or more for fast charging), so the same energy becomes a smaller mAh number. The boost circuit, the cable and the phone's own charging circuit also lose some energy as heat.

Here is the math. 20,000mAh × 3.7V = 74Wh. At 85% conversion efficiency, usable energy is about 63Wh, which works out to about 12,600mAh at 5V. Even a good unit should read around 12,000mAh on a USB meter. That is why products sold in China print a rated capacity at 5V (额定容量) next to the cell capacity (电池容量).

The problem is when a unit falls well below that normal range. The causes are usually these five:

• False capacity labeling (虚标): Cells worth 10,000mAh go in and the label says 20,000mAh. This is the most common trick, and the most blatant.
• Recycled or low-grade cells: Used cells pulled from scrapped laptops or power tools (拆机电芯) are rewrapped, or cells that failed capacity grading are used. They perform passably at first, then drop sharply after a few dozen cycles.
• Fewer cells: Three cells go into a space designed for four, and the gap is filled with foam or plastic.
• Low conversion efficiency: Swapping the boost IC, inductor and MOSFETs for cheaper parts can drop efficiency into the 70s and increase heat.
• Self-discharge in storage: If cell quality is poor or the state of charge at shipment is too low, units over-discharge during months in a warehouse, losing capacity or refusing to charge at all.

Capacity needs to be checked separately from safety certification. Safety certifications such as KC look at overcharge, short circuit and fire risk; they do not confirm that the labeled capacity is accurate. Certification in general is covered in Episode 10 and lithium battery shipping rules in Episode 90. This article focuses on whether customers actually get the capacity on the label.

2. Test Types - Discharge Capacity Test and Supporting Tests Compared

If you just say "Please test the capacity," the factory may plug in a USB meter for a few minutes or send the cell maker's cell report and call it done. Results for the same product change with discharge current, output voltage, cut-off condition and ambient temperature, so set the method and conditions first.

Get Results in Wh So You Can Compare

When a factory says "12,000mAh output," it means very different things depending on whether that is at 5V or 3.7V and whether it was measured at 1A or 3A. Ask for output discharge results in Wh. Wh compares across voltages, so a single number lets you check cell capacity, conversion efficiency and airline limits. In the spec sheet, also write the discharge voltage and current, cut-off condition, ambient temperature (e.g. 25±5°C) and sample size.

Screen by Cell Weight First

There is a simple check you can do on site before sending anything to a lab. Lithium-ion cells hold a fairly predictable amount of energy per unit of weight, roughly 200-260Wh per kilogram for normal cells. A 20,000mAh unit rated at 74Wh needs close to 300g of cells alone, so if the whole product weighs 250g, the labeled capacity cannot be right. An 18650 cell weighs about 45-48g and a 21700 cell about 68-70g, so counting cells gives you the answer quickly.

3. Setting the Criteria

Below are approximate ranges commonly referenced in the industry. Requirements differ by sales channel and destination market, so use this table only as a starting point and set actual criteria based on your product configuration and the lab's advice.

Wh is the marking most often overlooked. Most airlines allow power banks of 100Wh or less in the cabin without approval, require airline approval for 100-160Wh, and do not allow them in checked baggage. If a unit above 27,000mAh (about 100Wh) has no Wh marking or a wrong one, customers may have to leave it behind at airport security, and that complaint comes back to the seller.

4. Why You Shouldn't Take Factory Reports at Face Value

Ask a power bank factory for capacity data and you will usually get the cell maker's cell report or a certification test report. These documents have several gaps.

5. Common Supplier Tricks

Once cells are inside the case they can't be seen, and cell wrappers can be replaced at will, so substitution is easy. Here are the types we have actually encountered in consultations.

The most common is the case at the top of this article: good cells only in the approval samples. Appearance, indicator lights and output ports are identical, so an inspector can't tell by picking one up or plugging in a phone. Keep a few approval samples aside and discharge-test them alongside production samples under the same conditions, and you will catch many of these cases. The same principle applies to plastic material substitution, covered in Episode 125.

6. The Damage Capacity Defects Leave Behind

Capacity defects don't show at shipment and appear only after customers have used the product for a few days, but the damage spreads wider than you might expect.

• Reviews and channel penalties: A "capacity scam" review lingers longer than a star rating, and repeated complaints can get a listing taken down on marketplaces.
• Labeling and advertising issues: A labeled capacity far from the real one can be treated as false labeling.
• Risk of turning into a safety issue: Recycled cells don't just have less capacity; their higher internal resistance and cell-to-cell variation also raise the risk of heat and swelling.
• The whole stock is suspect: Cells in the same lot likely came from the same source, so picking out only some units is hard.

For product liability and recalls, see Episode 53; for electronics aging tests, see Episode 133.

7. Items to Put in the Spec Sheet

A single line saying "20,000mAh" gets interpreted differently by every factory. Put the items below in a table and attach it to the spec sheet. For how to write a spec sheet, see Episode 59.

Asking for cell shipping records may seem excessive, but it does more than testing. A discharge test checks only a few samples; cell shipping records show whether the specified cells actually arrived in quantities matching production. For the procedure when cells or circuit parts change, see Episode 119.

8. Building Capacity Tests Into Your QC Checklist

Sample Stage

When you receive approval samples, keep a few aside and send one to a third-party lab for an output discharge test, separate from the factory's report. If possible, tear one down and photograph the cell maker, model, weight and count. That record becomes the baseline for comparing production units.

Production Stage

During production, check the lot numbers and quantities of incoming cells, and require the factory to measure cell voltage and internal resistance before assembly and remove cells with large variation. Get finished-product discharge test records for each lot. If the results show the same number every time and there are no photos of the measurement screen, start by questioning whether the tests were actually done. For production schedule management, see Episode 102.

Pre-Shipment Inspection Stage

The inspector picks packed units personally, weighs them and tears down one or two to confirm the cells match the approval sample. On site, run a short discharge check with a USB meter and an electronic load, and send leftover samples to a lab for a full discharge test. For pre-shipment inspection, see Episode 100.

9. Sample Request Wording for the Factory

Putting the wording below in your purchase order or spec sheet takes cell selection out of the factory's discretion and makes it easier to assign responsibility if capacity claims arise.

"本产品须使用与确认样相同品牌、型号的全新A品21700电芯4颗，单颗容量5000mAh，禁止使用拆机电芯、二手电芯或重新包膜电芯。电池容量标示为20000mAh 3.7V（74Wh），额定容量12000mAh（5V⎓2A）。充满电静置1小时后以5V 2A恒流放电，输出能量不得低于62Wh，环境温度25±5℃。未经我方书面同意不得更换电芯品牌、型号、主要电路元件或生产工厂，如需更换须事先书面通知并重新测试。每批须提交注明品号、批号、测试条件、Wh结果及照片的放电测试记录，以及电芯出货凭证和电芯批号。测试样品由我方验货员随机抽取并可拆机检查。如有不合格，须加抽复测；复测仍不合格的批次暂停出货。"
(This product must use four new Grade A 21700 cells of the same brand and model as the approval sample, 5,000mAh each; disassembled, used or rewrapped cells are prohibited. Cell capacity is labeled 20,000mAh 3.7V (74Wh) and rated capacity 12,000mAh (5V 2A). After full charge and 1 hour rest, output energy at 5V 2A constant current discharge must be at least 62Wh, at an ambient temperature of 25±5°C. The cell brand, model, key circuit components or production factory may not be changed without our written consent; any change requires prior written notice and retesting. For each lot, submit discharge test records showing part number, lot number, test conditions, Wh results and photos, together with cell shipping records and cell lot numbers. Test samples are drawn at random by our inspector and may be torn down. If a sample fails, additional samples must be retested; any lot that still fails will have its shipment held.)

10. Three Real Cases

These are anonymized versions of cases we often see in consultations. Amounts and figures are examples.

Case 1 - 20,000mAh Power Bank

This is the case from the top of the article. The seller took the test results and teardown photos to the factory, but the spec sheet only listed "20,000mAh," colors and port configuration, with no cell maker or output Wh criterion, so the factory held its ground, saying "the cell capacity label is correct." In the end the factory admitted using cells different from the approval sample and refunded part of the cost, but refunds, scrapped stock and product page revisions still left a loss of about 26 million won. From the next order on, the seller puts the cell maker, model and output Wh criterion in the spec sheet and tears down samples from every lot.

Case 2 - 10,000mAh Promotional Power Bank

A promotional goods company that ordered 2,000 logo-printed power banks as corporate gifts was told after delivery that the units "get hot enough to burn your hand while charging and die quickly." Testing showed cell capacity matched the label, but conversion efficiency was only 68%. The factory had swapped the boost IC and inductor for other parts to hit its cost target. Check only the cells and ignore the circuit, and this is what happens. The company now locks the key parts list in the spec sheet and requires at least 85% efficiency.

Case 3 - Wireless Charging Power Bank

A brand importing wireless charging power banks from a factory in Dongguan released stock that had sat in a warehouse for over six months and got a flood of returns saying "it won't charge out of the box." State of charge at shipment had been below 10%, and a circuit with high standby current had over-discharged the units in storage. The brand added a 30-50% state of charge at shipment and a 28-day self-discharge test to the spec sheet, and changed its warehouse rules so that long-stored stock gets a top-up charge every three months. For inventory management, see Episode 43.

11. Common Mistakes

• Trusting the "20,000mAh" label without setting the cell maker and output Wh in numbers
• Judging by whether a phone charges when plugged in once
• Accepting the cell maker's cell report or a safety certification report as a finished-product capacity report
• Handling customer complaints without knowing the normal range (about 60% at 5V), or dismissing a real defect as normal
• Checking only the cells and not the conversion efficiency and circuit parts
• Not setting the state of charge at shipment and long-term storage conditions

12. Power Bank Capacity Control Checklist

Specification Stage

• Cell format, count, maker and model are specified
• Cell capacity, rated capacity (5V) and Wh markings are specified
• Discharge voltage, current, cut-off condition, temperature and sample size are written down
• Criteria for conversion efficiency, cycle life and self-discharge are written down
• Sales claims (number of charges, fast charging) are confirmed by calculation

Sample and Production Stage

• A few approval samples are kept aside, and one has been torn down and recorded
• Approval samples were discharge-tested separately at a third-party lab
• Discharge test records with photos were received for each lot
• Cell shipping records and cell lot numbers were checked

Pre-Shipment Inspection Stage

• Production units were weighed and are within tolerance
• Random samples were torn down and the cells match the approval sample
• An on-site discharge check was done and leftover samples were sent to a lab
• State of charge at shipment is within the specified range

Wrapping Up - Check Wh and Cells, Not One Plug-In Test

Power banks all look much the same from the outside. The difference shows only when a customer on the first day of a trip finds the phone dying sooner than expected. Writing the cell maker, output Wh and conversion efficiency into the spec sheet and checking discharge test results for every lot doesn't cost much. Put a few hundred thousand won in testing next to tens of millions in refunds and scrapped stock, and it is clear which is cheaper.

There is something you can do right now. Ask the factory currently producing for you to send the maker and model of the cells in use, cell shipping records, and output discharge test records for recent lots in Wh. If the answer is a single line saying "Yes, it's 20,000mAh," draw up a capacity control sheet and send it before your next order.

StartMade supports you on the ground with spec sheets covering cell specs and capacity criteria, third-party testing of approval samples, cell checks on production lots, and pre-shipment inspection. If you've already received capacity claims, we start with testing and tearing down remaining stock; if you're preparing a new product, we start with cell selection and setting criteria.