The goal of this work is to realize a device for measuring and displaying the capacity of lithiumion batteries. The described device will use a NodeMCU ESP8266 microcontroller. The microcontroller detects is battery a rechargeable and in case it is, the measuring capacity of the battery. The measured microcontroller data is printed on an OLED display and sent to the computers that display the data on the web page. To be able to determine how long it takes for the battery to discharge, you need to know the capacity of the battery and the consumption of the electronic circuit powered by the battery. To estimate the capacity of the battery, it is necessary to know the time changes of the structure and voltage in the circuit composed of batteries and consumers. Using the specified assembly, the battery will be discharged through the resistor. At the same time, the current through the resistor will be measured and the battery capacity will be calculated. The most important parameter when measuring battery capacity is the so called Soc or state of charge. The specified parameter tells what percentage of the charge remains in the battery. All cells in the battery should have equal voltage if the battery is "healthy". Battery capacity is expressed in ampere-hours (Ah) or milliampere-hours (mAh). Battery capacity provides information about the amount of energy stored in batteries. When powering the consumer from batteries, the battery voltage will drop more slowly on a higher capacity battery than on a lower capacity battery.