Before learning the over-discharge of the lipo battery, let’s learn the discharge curve of the lipo battery first.
As we can see from the above figure, the voltage curve of a lipo battery during the entire discharge process can be divided into 3 stages:
- In the primary stage, the battery voltage drops rapidly, the higher the discharge rate, the faster the voltage drops.
- The battery voltage enters a stage of slower changes. This period of time is called the platform area of the battery. The lower the discharge rate, the longer the platform area lasts.
- When the battery is nearly fully discharged, the battery voltage begins to drop sharply until it reaches the discharge cut-off voltage.
The cut-off voltage refers to the lowest voltage allowed when the battery is discharged. If the voltage is lower than the discharge cut-off voltage and the discharge is continued, the battery voltage will drop rapidly, resulting in over-discharge, which may damage the electrode active material, lose its chemical reaction ability, and shorten battery life. Once the lipo battery voltage is lower than 2.7V, it may cause the battery to be scrapped.
Why does over-discharge damage the battery?
During the normal discharge process, Li+ is separated from the layered carbon material, pass through the electrolyte and the diaphragm, and is embedded between the metal oxide layers of the positive electrode. Generally, it will only cause a change in the layer spacing, and will not cause damage to the crystal structure.
When the battery is over-discharged, the copper foil of the anode as the current collector is oxidized to Cu2+, and the Cu2+ ions diffuse to the cathode driven by the concentration difference between the anode and the cathode. At the same time, with the excessive loss of Li+ in the anode, the SEI (solid electrolyte interface) layer decomposed, the layered structure of the negative electrode carbon material is irreversibly damaged. And metallic lithium is deposited on the surface of the cathode.
With the continuous deposition of lithium on the surface of the cathode, Cu2+ will also be reduced and precipitated on the cathode to form metallic dendrites. The continuous growth of dendrites eventually penetrates the separator and causes internal short circuits.
Are the lipo batteries unable to be recharged after being over-discharged?
No, as long as it is not over-discharged severely, the lipo battery can still be charged. However, the battery capacity, cycle life, self-discharge and other performance will be impaired. When using the B6 charger to charge over-discharged lipo batteries, it may prompt “LOW VOLTAGE” and will not be able to charge the batteries normally. At this time, we can select the NiMH BATT charging mode to charge the lipo batteries. When the batteries voltage returns to 3.2*nS(V)(for example, 2S lipo battery is charged to 6.4V ), exit the NiMH charging mode and reselect the Lipo battery charging mode. Then, the batteries can be charged normally. If you do not know how to charge a lipo by B6 charger, click here.
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