Do you know how to use lithium batteries correctly? Even if the lithium battery of an electric vehicle can be used for 2 to 3 days with a single charge, it is recommended to charge it every day. Lithium batteries should be charged as soon as possible after use, and the battery power should be as full as possible. A regular deep discharge of the battery is also conducive to "activating" the battery, which can slightly increase the battery's capacity. Explain how many years can lithium batteries be used? Why does deep discharge shorten the life of lithium batteries?
Limit A---------------protection circuit overcharge protection voltage (4.35V)
High pressure alert zone
Limit b---------------Li-ion battery charging limit voltage 4.20V
Normal use area
Limit C---------------Li-ion battery discharge termination voltage (2.75V)
Low-voltage alert zone
Limit D---------------protection circuit over-discharge protection voltage (2.3V)
Low-voltage danger zone
Strictly speaking, charging a lithium-ion battery to a voltage higher than the limit b (4.20V) is an overcharge. However, overcharge also has a degree of magnitude. Generally speaking, it is not considered as an overcharge if it is below 4.24V. Accept the micro-overcharge.
The battery specifications of lithium-ion battery cell manufacturers generally indicate that the charging limit voltage is 4.20V+/-0.04V.
The area between boundary A and boundary b (above 4.24V, lower than 4.35V) can be called medium overcharge.
Lithium-ion batteries in this voltage range will have a higher discharge capacity than normal lithium-ion batteries.
Please take a look at the experimental data of overcharge
The experimental conditions are
Nominal capacity: 550mAh.
Nominal charging limit voltage 4.20V
Charging system: charge at 1C (550mA) when the voltage is lower than the charging limit voltage. After the voltage reaches the charging limit voltage, the voltage remains unchanged, and the current is gradually reduced to 20mA and then the charging is terminated. This is the conventional CC/CV charging mode.
Discharge system: 1C (550mA), discharge termination voltage 2.75V
The above system is fully implemented with reference to the national standard.
We can try to change the charging limit voltage and deliberately overcharge this experimental product. The experimental table is listed as follows:
Number of cycles Charge limit voltage V Charge capacity mAh Discharge capacity mAh and the ratio of rated capacity%
The first time 4.20V570565 this is the rated capacity
The second time 4.30V610608108%
The third time 4.20V569564100%
The fourth time 4.35V633627112%
The third time is for comparison, showing that the rated capacity of the battery has hardly changed after one overcharge.
A conclusion can be drawn from this, overcharging can increase the capacity of the battery.
But this trend is not consistent. When I continue to overcharge this lithium-ion battery with a charging limit voltage of 4.35V, the conclusion is not so optimistic.
After the 50th time, the battery capacity is 480mAh. It is already 85% of the rated capacity. When I used this type of battery in a normal cycle experiment, its capacity was still more than 88% of the rated capacity after 150 cycles. It can be seen that charging has severely shortened the cycle life of the battery. In addition, it should be mentioned that the overcharged battery has slightly bulged, the original thickness is 3.84mm, and its thickness is 4.25mm after 50 cycles.
Because the voltage above 4.35V will not reach the lithium-ion battery that is protected The hydrogen and lithium ion gears can be selected) is the culprit of overcharge.
So what will an overcharge higher than 4.35V look like?
When I did the charging safety experiment, I removed the battery protection circuit, and then charged the lithium-ion battery cell with a voltage of 5.0V.
As a result, after 3 to 4 hours, the battery is severely swollen.
And some unqualified batteries exploded.
Under an electron microscope, the negative electrode of a lithium battery has a layered structure, and the positive electrode is a stack of angular crystals, the shape of which varies with the anode material.
The critical consequence of over-discharge is the laminar collapse of the negative plate. When recharging, the quantity and convenience of lithium ions embedded in the negative plate are limited.
Decreased capacity, increased internal resistance, and shortened life cannot be recovered.
Overcharging is even more terrible!
The battery reaches a state of full overflow. The negative electrode changes from the intercalation reaction of lithium ions to the deposition of lithium metal on the surface of the negative electrode, and the solvent is oxidized. As the temperature of the battery rises, the reactions between metallic lithium and the solvent, and lithium-intercalated carbon and the solvent occur one after another, causing the battery to catch fire and explode. With the analysis of the electrolyte, the binding agent can also react with lithium metal.
After overcharge, needle-like lithium metal crystals are everywhere on the pole piece, and a micro short circuit will occur when the diaphragm is pierced. In the worst case, self-discharge is aggravated; in the worst case, the crystal branch short-circuit current causes the battery temperature to rise sharply, and the electrolyte is analyzed and vaporized. In this case, no matter if the temperature is too high, the material will burn and explode. The outer shell was broken first, causing the air to enter and violently oxidize the lithium metal, which all ended in a combustion explosion.