On powerbank capacity. It’s a bit misleading.

Many people misunderstand how power banks are rated. The ratings are usually in mah, or “milliamp hours”. So a battery with 100mah will power a device that uses 100ma for 1 hour. A device’s current (amps) usage is in how many amps, or milliamps it uses per hour. Likewise, a battery with 100mah will power a device that uses 10ma for 10 hours. However many people do not know that the capacity of a USB powerbank is tested at the BATTERY, not at the USB port. So when people use a USB meter to test the battery at the USB port, the capacity shown on the meter is lower than stated. Here’s why.

Let’s say you have a power bank with a lithium battery in it. It is rated at 10,000mah. But this is tested at the battery, which is nominally 3.7vdc. (A nominal voltage is the average voltage of a battery. A Lithium battery can be charged safely to 4.2vdc, and discharged to 3.0vdc. Some Chinese charging boards will let the battery discharge to 2.5vdc but purists won’t like that.) That is, the average voltage is 3.7vdc. But you need 5vdc at the USB port, that is what a USB device requires. So the power bank has to boost the voltage from the battery from 3.7vdc to 5vdc. But this is not a perfect conversion, the average conversion efficiency is 80%. So when you test the capacity at the USB port you should get 8000mah, which is normal for a power bank rated at 10,000mah.

How to test a power bank via charging

First, here’s what you need to test a power bank through the micro-usb charging port. Get a USB meter where the input and output ports are micro-usb. They are under $10usd but go up from there depending on what features that you want. Fancy ones with a color screen, and multiple screens, and multiple ports, go for $30 or more.

There are 2 ways to test a power bank. Via charging it when it’s empty, or via draining it when it’s full. First you must hook up the USB meter.

Put a load on the USB port. Put a USB meter between the power bank and the load and drain it completely. If the load is higher amps, you will get less mah from the USB meter. An average load would be about 1000ma, which is 1 amp. But this requires you to buy a load to plug into an USB outlet.
Drain the power bank in some way, all the way. Now put a USB meter on the charge port. This requires that the USB meter has a male micro-usb on it normally. Now on the other side of the USB meter, put your charger. Zero out the USB meter. Now charge the device all the way. When it’s all the way done charging, look at how many milliamp hours (mah) went into the battery. That should equal 80% of your rating for mah on the device.

Testing a powerbank via discharging

The powerbank must be charged all the way first.
Find a USB meter. The meter should have a voltage range that measures from 3.0vdc to 6.0vdc. If it only goes down to 3.6vdc that will also be acceptable.
Get a load to put on the powerbank. These are about $5-10usd and often have a USB connector to hook into the USB meter.
Connect the powerbank USB port to the USB meter, and connect the USB meter to the load. The load will drain power through the meter and the meter will measure how many milliamp hours are used up.
USB loads are often adjustable. So if you use a smaller current (like 0.5amps) you will likely get a bit higher capacity from the powerbank than if you used a higher current (like 2 amps).
Expect your powerbank to have a capacity of about 80% its rated capacity. So a 10,000mah rated powerbank will only measure 8000mah. This is normal for the industry.

NOTE: USB meters with a color screen are often powered from your battery, and the color screens use more power than say, an LCD screen. The meter may not compensate for this color screen power usage but that will be fine for our purposes. A USB meter that has a separate power supply will be more accurate.

Voltage of batteries during discharge

A lithium battery has a voltage of between 2.8vdc and 4.2vdc, the “nominal”, or average, voltage is 3.7vdc. That is, when you charge your battery it should be at 4.2vdc. If you charge it much higher than that, it will damage that battery. As the battery drains, it will go down to 2.8vdc or a bit lower. Between those 2 ranges, is where you get the battery capacity. However you cannot use your USB meter on the powerbank USB port to check the actual voltage of the lithium ion battery since the purpose of the power bank is to ensure the USB port outputs 5vdc even if the lithium battery is down to 2.8vdc.

Draining the battery beyond 2.5vdc normally shortens the life of a battery and is not recommended. However, many Chinese charge controllers will allow the battery to go down to 2.5vdc.

Some example USB meters

Some USB meters are fancier than others. They can have different input and output ports, color screens, even multiple screens.

A fancy color screen USB meter with several input and output port types, and several different screens. — Above: Some meters are fancier with multiple screens, and color screens. Some have multiple input ports and output ports.

White USB meter with reverse LCD screen and 9 memory slots. — Above: White USB meter with 9 slots for saving data. This reverse LCD screen uses hardly any power.

Some examples of USB loads

Above: Simple USB meter with 2 green capacitors. The switch switches between 2 different currents.

Above: Fancy USB meter with load built in. The knobs adjust the current and other things. These sometimes have different types of input ports.

Helpful hints

Some USB meters will have multiple memory slots to remember previous measurements.
Some will have different types of input and output ports. The input port of the meter connects with the powerbank output USB A port.
Good brands of meters are: DROK, Makerhawk. I don’t know if Aukey or Anker makes USB power meters but those are good brands also.