How to Test USB Charger Wattage with FNIRSI FNB58BT Malaysia

A USB charger may be labelled 20W, 33W, 45W, 65W or 100W, but how do you check how much power it is actually delivering?

Simply connecting a smartphone and looking for a “Fast Charging” message does not tell you the actual wattage.

A USB tester such as the FNIRSI FNB58BT can help measure voltage, current and power while also providing useful information for investigating modern fast-charging systems.

However, there is one important point:

A 65W charger will not automatically output 65W just because you connect a tester.

The connected device or load must request and support the appropriate charging mode.

What Does 65W on a USB Charger Mean?

When a charger is rated at 65W, this normally refers to its maximum specified output capability under particular operating conditions.

It does not mean:

The charger always outputs 65W.

Actual power depends on factors such as:

Connected device
Charging protocol
Requested voltage
Current demand
USB-C cable
Charger port
Battery condition
Number of devices connected

A smartphone may only draw a fraction of the charger's maximum power.

How Is USB Charger Wattage Calculated?

The basic formula is:

Power (W) = Voltage (V) × Current (A)

For example:

5V × 2A = 10W

9V × 2A = 18W

9V × 3A = 27W

20V × 3A = 60W

The FNB58BT can display the electrical parameters during testing, making it easier to see the actual power being transferred.

Step 1: Connect the USB Tester

The basic measurement arrangement is:

USB Charger → FNIRSI FNB58BT → Device or Load

Make sure all equipment used in the test is suitable for the expected voltage, current and power.

The tester is inserted into the power path so that it can monitor what is happening between the charger and connected equipment.

Step 2: Check the Voltage

Start with the voltage reading.

Traditional USB power commonly starts around 5V, but modern fast-charging systems may negotiate higher voltages.

If you expected a higher-voltage charging mode but the system remains near 5V, investigate whether:

The device supports the required charging mode
The charger supports the required protocol
The cable is suitable
Protocol negotiation has occurred correctly

Do not assume that higher voltage should appear automatically.

Step 3: Check the Current

Next, observe the current.

For example, if the tester shows:

and

the approximate power is:

18W

If it shows:

and

the approximate power is:

27W

This immediately provides more useful information than simply seeing a charging icon on the device.

Step 4: Check Actual Power

The FNB58BT can display power directly during USB testing.

This helps answer:

How much power is actually being transferred right now?

For example, a charger marked 65W may currently be delivering only:

12W

18W

30W

or another value.

That does not automatically mean the charger has a problem.

You need to understand why the connected device is requesting that particular amount of power.

Why Does My 65W Charger Show Only 20W?

There are several possible reasons.

Device Limitation

The connected device may not support 65W charging.

For example, connecting a lower-power smartphone to a 65W laptop charger does not force the phone to accept 65W.

The phone controls how much power it requests within the supported charging system.

Charging Protocol

The charger and device must support compatible charging methods.

These may include technologies such as:

USB Power Delivery
PPS
Quick Charge
Manufacturer-specific fast charging

A high-wattage charger does not guarantee maximum charging speed with every device.

USB-C Cable

The cable can also affect charging performance.

An unsuitable cable may prevent the system from reaching the expected power level.

When troubleshooting, compare the original cable with a known-good cable while keeping the charger and device unchanged.

Battery Percentage

Charging power often changes as the battery fills.

A device may accept relatively high power when its battery is low and reduce charging power as it approaches full charge.

Therefore, testing a phone at 15% battery and 95% battery may produce very different results.

Temperature

Battery and device temperature can also influence charging.

If the device becomes too warm, its charging-management system may reduce power to protect the battery and electronics.

Can You Test a 65W Charger Using a Phone?

You can measure what the charger delivers to that phone.

But this does not necessarily test the charger's full 65W capability.

For example:

65W charger + phone limited to 20W

may produce approximately 20W even if the charger is completely healthy.

To test the charger's maximum output capability properly, you need a suitable test setup capable of requesting and loading the required output.

This may involve appropriate protocol triggering and an electronic load.

Why an Electronic Load Can Be Useful

A suitable electronic load allows technicians to apply a controlled electrical load.

Compared with a smartphone, this can make charger testing more repeatable.

A smartphone constantly changes its charging demand according to:

Battery percentage
Temperature
Operating conditions
Charging algorithm

An electronic load provides more controlled testing conditions when properly configured.

FNB58BT and Fast-Charging Protocol Testing

The FNIRSI FNB58BT is more useful than a basic voltage-current meter when working with modern USB chargers because it also provides functions related to fast-charging protocol investigation.

This can help technicians determine whether a charger supports particular charging technologies before performing further tests.

That is especially useful when comparing several chargers that have similar wattage ratings but different protocol support.

Example: Comparing Three USB Chargers

Suppose you have:

Charger A – 20W

Charger B – 45W

Charger C – 65W

You connect the same phone and cable to each charger.

The measured results might not increase proportionally with the printed charger wattage.

Why?

Because the phone may have its own charging limit.

For example, once the phone reaches its maximum supported charging mode, moving from a 45W charger to a 65W charger may provide little or no additional charging power.

This is normal behaviour.

Testing Multi-Port USB Chargers

Multi-port chargers require additional attention.

A charger may advertise a high total wattage, but power distribution can change depending on:

Which port is used
Number of connected devices
USB-A versus USB-C
Port combination
Charging protocols
Manufacturer's power-allocation design

For example, a USB-C port may provide higher power when used alone but lower power when another port is also active.

Therefore, test the actual port configuration you intend to use.

100W Charger Does Not Mean Every Port Is 100W

This is another common misunderstanding.

If a multi-port charger is advertised as 100W, check whether this refers to:

Maximum output from one specific port

Combined output across multiple ports.

Always check the charger's output specification.

A USB tester can then be used to verify actual behaviour under the required test conditions.

Can FNB58BT Tell If a Charger Is Fake?

It can provide useful measurement evidence, but avoid making conclusions from only one reading.

If a charger claims 65W but your phone only draws 18W, that does not prove the charger is fake.

A proper evaluation should consider:

Output profiles

Protocol support

Cable capability

Load capability

Device charging limit

Actual voltage and current under controlled conditions

Only then can the result be interpreted properly.

Useful for Charger Comparison

The FNB58BT can be useful when comparing:

Original vs third-party charger
20W vs 30W charger
45W vs 65W charger
USB-A vs USB-C charging
Different USB-C ports
Different charging cables
Power-bank outputs

The key is to keep the test conditions consistent.

Change one variable at a time.

Who Should Use the FNIRSI FNB58BT?

The FNB58BT is particularly relevant for:

Mobile phone repair technicians
Electronics repair shops
Charger and cable sellers
Power bank sellers
Electronics technicians
QC personnel
Engineers
Electronics enthusiasts

For basic “is there USB power?” testing, a simple USB meter may be sufficient.

For USB-C, PD, fast-charging and charger troubleshooting, a more advanced tester becomes much more useful.

FNIRSI FNB58BT USB Charger Tester Malaysia

The FNIRSI FNB58BT USB Tester can help measure and investigate USB voltage, current, power, charging protocols, USB-C cables and charging behaviour.

If you are trying to test a 20W, 33W, 45W, 65W or higher-power USB charger, send MTM Precision a photo of the charger specification and tell us what device or load you intend 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.