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Effect of voltage uniformity on the series use of lithium ion batteries

Abstract: The cycle life of 18650 single cell battery under different charging voltage and over-discharge current was tested. The SOC of battery pack is caused by the quantitative difference, and the inconsistency with the voltage of battery pack is due to the self-discharge series use of single battery, resulting in the cycle process of single battery over-discharge and the cycle life of decayed battery pack. A three-electrode system was used to measure the potential relationship between the positive and negative electrodes of the real battery during charging and discharging.

Lithium ion battery; uniformity; Overcharging; Discharge; In series

The consistency of lithium-ion batteries is the user's requirement for batteries. In the field of multi-series combination, the inconsistency of batteries will affect the performance of batteries, and even bring potential safety hazards. There are many factors that lead to inconsistent use of multiple series of batteries, including the primary battery itself, such as inconsistent capacity, inconsistent self-discharge rate, inconsistent internal resistance, inconsistent service life, etc. There are also external reasons, such as different battery cooling environment, internal resistance between different battery connections, and even uneven discharge caused by the detection circuit. This paper mainly discusses the influence of voltage inconsistency on the use of multiple batteries. Inconsistent battery voltage is a common problem in practical applications, and voltage is also the most important monitoring parameter in battery management system. In practical applications, the main reasons for the battery voltage inconsistency are the uneven capacity and battery self-discharge, while the battery voltage inconsistency affects the use of batteries in series.

The battery used in this paper is 18650 cylindrical power lithium battery.

A battery for overcharge cycle life test uses 2C constant current charging, and the current is less than 0.05 degrees at different voltages, and then the 10-point current is transferred to a battery for 2.5VC overcharge cycle life test. 2C constant current charging, 4.2V constant voltage current is less than 0.05C, and then 1C, 10C constant discharge to 1.0,2.5V. The cycle life test of 5S1P combination adopts 2c constant current to 21.0v, 21.0v constant voltage to 0.05c, and 10c discharge to 12.5v. During the test, single battery overcharge and overcharge protection shall not be carried out.

For the three-electrode test of the battery, the reference electrode uses a nickel wire with a diameter of 1mm. The circular surface of the nickel wire is wrapped with metal lithium sheet, which is ground on the glass surface to make the fresh metal lithium tightly wrapped on the nickel wire as the metal lithium reference electrode. After the 18650 battery is discharged, remove the battery cover with pliers. Then insert the reference electrode into the central hole of the electrode group and inject the electrolyte into the central hole. Make a plastic cover of PP material with holes in advance and seal it with Parafilm tape. Finally, the three-electrode system was measured in a drying room with humidity less than 1%.

Test content:

1.1 Cycle life of single battery under different charging voltage

1.2 Overdischarge cycle life of single battery

1.3 Effect of uniform voltage on cycle life of battery pack

1.4 Charge and discharge of real battery three-electrode system

In this paper, the cycle performance, voltage and initial SOC difference of lithium ion battery in 18650 under different charging voltage and discharge current were measured. When the battery self-discharge caused by artificial simulation led to inconsistent series battery voltage and overcharge protection did not have a part, the battery cycle life, due to single battery discharge, led to battery failure. Through the test of the actual three-electrode system of the battery, the change relationship of the negative electrode potential at the end of the discharge was obtained.

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