Why Does a 20,000mAh Power Bank Not Output 20,000mAh? Malaysia
Why Does a 20,000mAh Power Bank Not Output 20,000mAh? Malaysia
You buy a 20,000mAh power bank, connect it to a USB tester and perform a discharge test.
But the final result is much lower than 20,000mAh.
Does this mean the power bank is fake?
Not necessarily.
One of the most common mistakes in power bank testing is comparing the mAh rating of the internal battery directly with the mAh measured at the USB output.
The two measurements may be taken at different voltages.
To understand power bank capacity correctly, you need to consider voltage, mAh, Wh and conversion efficiency.
What Does 20,000mAh Actually Mean?
A power bank contains internal battery cells.
The advertised capacity is generally associated with the internal battery system at its specified nominal voltage.
For a simplified example, assume:
Capacity: 20,000mAh
Nominal cell voltage: 3.7V
First convert:
20,000mAh = 20Ah
Then calculate energy:
20Ah × 3.7V = 74Wh
The important number here is approximately:
74Wh
This represents the theoretical energy based on those example specifications.
Why Doesn't the USB Output Show 20,000mAh?
The USB output does not necessarily operate at the same voltage as the internal battery.
For a conventional 5V USB output, the power bank must convert the battery voltage to approximately 5V.
Even before considering conversion losses:
74Wh ÷ 5V = 14.8Ah
That is approximately:
14,800mAh at 5V
Therefore, comparing:
20,000mAh at approximately 3.7V
directly against:
mAh measured at 5V
is not an equal comparison.
What Happens After Conversion Loss?
Power conversion is not perfectly efficient.
Energy can be lost through:
-
DC-DC conversion
-
Internal circuitry
-
Cable resistance
-
Connector resistance
-
Heat
-
Battery internal resistance
Therefore, actual usable output will normally be lower than the theoretical calculation.
For example, if we use an illustrative 85% conversion efficiency:
74Wh × 85% = 62.9Wh
At 5V:
62.9Wh ÷ 5V ≈ 12.58Ah
or approximately:
12,580mAh
This is only an example calculation.
Actual results depend on the power bank design and test conditions.
Does 12,000–13,000mAh Mean a 20,000mAh Power Bank Is Fake?
Not automatically.
This is exactly why mAh alone can be misleading when the input and output voltages are different.
A better comparison uses energy:
Watt-hours (Wh)
instead of looking only at mAh.
You should also check the manufacturer's rated energy and rated output capacity where provided.
What Is Wh?
Wh means watt-hour.
It represents energy.
A simplified formula is:
Wh = V × Ah
For example:
20Ah × 3.7V = 74Wh
This allows measurements at different voltages to be compared more meaningfully.
Why Is Wh Better for Power Bank Testing?
Imagine two measurements:
20,000mAh at 3.7V
and
13,000mAh at 5V
At first glance, 13,000mAh looks much lower.
But calculate the energy:
20Ah × 3.7V = 74Wh
13Ah × 5V = 65Wh
Now the relationship becomes much easier to understand.
The difference includes power-conversion losses and other system losses.
This is why technicians should pay attention to Wh, not just mAh.
Can a USB Tester Measure Power Bank Capacity?
A suitable USB tester can help measure the electrical energy delivered through the USB output.
For example, the FNIRSI FNB58BT USB Tester can monitor parameters including:
-
Voltage
-
Current
-
Power
-
Accumulated capacity
-
Accumulated energy
This provides much more useful information than simply counting how many times a power bank can charge a smartphone.
Why Is Charging a Phone Not a Good Capacity Test?
You may hear someone say:
“My phone has a 5,000mAh battery, so a 20,000mAh power bank should charge it four times.”
Real-world charging does not work that simply.
Losses can occur through several stages:
Power bank battery → voltage conversion → USB cable → phone charging circuit → phone battery
The phone also consumes energy while operating.
Screen activity, applications, mobile network activity and temperature can all influence the result.
Therefore:
20,000 ÷ 5,000 = 4
does not mean you are guaranteed exactly four complete charges.
How Should You Test a Power Bank?
For more controlled testing, use:
Power bank → USB tester → suitable electronic load
This allows the power bank to discharge under more consistent conditions.
The tester can then record electrical measurements throughout the test.
For repeatable comparisons, keep the test conditions as similar as possible.
Why Does Test Current Matter?
A power bank may behave differently at different loads.
For example, testing at:
5V / 1A
is not necessarily equivalent to testing at:
5V / 3A
Higher current can increase losses and voltage drop.
Battery behaviour can also change under heavier loads.
Therefore, when comparing two power banks, try to use similar:
-
Output voltage
-
Load current
-
Cable
-
Test equipment
-
Environmental conditions
What About 9V, 12V or USB PD Output?
Modern power banks may support fast-charging outputs above 5V.
Examples can include negotiated output levels such as:
9V
12V
and other USB PD profiles depending on the device.
This makes comparing mAh even more confusing.
For example:
5,000mAh at 5V
and
5,000mAh at 9V
do not represent the same amount of energy.
That is another reason Wh is particularly useful.
Can FNB58BT Test Fast-Charging Power Banks?
The FNIRSI FNB58BT is designed for more advanced USB testing than a simple voltage-current display.
It can be useful for investigating:
-
USB-C charging
-
USB Power Delivery
-
PPS
-
Fast-charging protocols
-
Voltage and current
-
Power
-
Capacity
-
Energy
-
Charging behaviour
This makes it suitable for technicians who need to investigate both power-bank capacity and fast-charging performance.
When Should You Suspect a Power Bank Problem?
A low capacity result deserves further investigation when:
-
Delivered energy is far below reasonable expectations
-
The power bank shuts down unusually early
-
Voltage becomes unstable under an appropriate load
-
Performance has deteriorated significantly compared with previous tests
-
The unit behaves abnormally even with known-good cables and test equipment
However, always verify the test method before concluding that the power bank is defective.
New vs Old Power Bank
Battery capacity decreases with ageing and use.
A power bank that has been used for several years may not provide the same usable energy as when it was new.
Possible signs of deterioration include:
Reduced runtime
Earlier cutoff
Lower usable energy
Greater voltage drop under load
Unexpected shutdown
A controlled USB discharge test can help quantify this change.
Don't Judge Power Bank Capacity by mAh Alone
The key principle is simple:
mAh without voltage does not tell the complete energy story.
When comparing a power bank's internal capacity with USB output measurements, consider:
mAh + Voltage + Wh + Conversion Efficiency
This provides a much more technically meaningful comparison.
FNIRSI FNB58BT Power Bank Capacity Testing Malaysia
The FNIRSI FNB58BT USB Tester can help technicians monitor USB voltage, current, power, capacity and energy when evaluating power banks and other USB-powered equipment.
It is suitable for electronics technicians, mobile phone repair shops, QC personnel, power bank sellers, engineers and electronics enthusiasts.
For FNIRSI FNB58BT enquiries in Malaysia, send us a photo or specification of the power bank, charger or USB device you want to test.
MTM Precision Sdn Bhd
Showroom & Service Centre:
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia.
Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my
Product enquiries are welcome from Selangor, Kuala Lumpur, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak.
07 Oct 2026
