Lithium-ion battery self-discharge detection plays an important role in evaluating the capacity, cycle characteristics and service life of a single lithium-ion battery or battery pack. Rapid detection of the spontaneous consumption of stored power of lithium-ion batteries in open circuit conditions is currently a research hotspot in this field. The open-circuit voltage method was used to measure the voltage drop of lithium-ion batteries per unit time, namely the self-discharge K value, to characterize the degree of self-discharge. Additionally, a self-discharge K value detection system for lithium-ion batteries was designed. The system was composed of upper and lower computer computers, connected by Type-C mode, which allowed for the detection of the self-discharge rate of lithium-ion batteries. A ternary lithium-ion battery was used as the test object, and a 14-day standing experiment was conducted to detect the self-discharge K value during the standing process. The test results show that the system not only shortens the self-discharge detection time of lithium-ion batteries, but also accurately detects the self-discharge rate of lithium-ion batteries. The self-discharge K value of the lithium-ion batteries was detected under five states of charge: 20%, 40%, 60%, 80%, and 100%, and at four temperature conditions: 20 ℃, 50 ℃, -20 ℃, and -40 ℃. According to the K-value test results, the influence of state of charge and self-discharge ambient temperature on the self-discharge rate of lithium-ion batteries can be analyzed.