ZB2L3 battery capacity tester, designed to measure the discharge capacity of batteries. It can test various battery types, including 18650 lithium-ion batteries, with a voltage range of 1.2-12V. 

Here's how it generally works:

The tester discharges the battery through an external load. 

It measures the discharge current, voltage, and capacity (in Ah) of the battery. 

The test continues until the battery voltage reaches a set cut-off point. 

The tester displays the actual capacity of the battery. 

Additional information: 

The tester can support a maximum capacity of 9999Ah (9999000mAh).

It can automatically identify and adjust the battery termination voltage.

Some models include a micro USB port for power.