How to charge multiple strings of lithium-ion batteries

Lithium-ion batteries have the advantages of high energy density, small size, light weight, etc. In the mobile phone, notebook computer and other markets, it has completely replaced other batteries and has a 100% occupancy rate; at present, lithium-ion batteries are rapidly expanding into power tools, In the electric bicycle and other applications, its broad market prospects are increasingly recognized by the industry. However, in order to promote the application and development of lithium-ion batteries faster than nickel-metal hydride, nickel-cadmium, and lead-acid batteries, we must continuously improve its safety and service life. This article will discuss a new charging solution from the perspective of the charger to improve the safety of lithium-ion batteries, extend battery life, and reduce the cost of the charger.
In the process of using the battery, we often hear the battery industry say: "The battery is less badly used, and more is being burnt." This sentence we can understand that the wrong charging conditions or methods will be more likely to damage the battery and reduce the battery life. Taking 18650 cobalt acid lithium ion battery as an example, (1) charging over temperature, around 70 °C: electrolyte interface (SEI) mode begins to decompose and heat; 120 °C or so: electrolyte, positive electrode begins to thermally decompose, causing gassing and temperature Rise rapidly; again to around 260 °C: the battery exploded. (2) Charging overvoltage, with an overpressure of 5.5V, lithium metal precipitates, the solvent is oxidized, the temperature rises, a vicious cycle occurs, and the battery catches fire and explodes. It can be seen that the harsh charging method has a serious impact on the battery characteristics.
Therefore, regarding how to charge, we will discuss the following important issues together.

(1) Why do I need a precharge function?
The battery operating voltage is from 2.5V (carbon negative battery: 3V) (electricity: 0%) to 4.2V (electricity: 100%). When the voltage is less than 2.5V, the discharge is terminated and the current loss of the internal protection circuit is also reduced to ** (discharge loop is closed). Of course, in practical applications, due to different internal materials and better protection of the battery, the discharge termination voltage can range from 2.5V to 3.0V. When the voltage exceeds 4.2V, the charging circuit is terminated to protect the battery safety; when the operating voltage of the single battery drops below 3.0V, we can consider the over-discharge state and terminate the discharge circuit to protect the battery. When the battery is not in use, the battery should be charged to 20% of the battery, and then stored in moisture. Since lithium-ion batteries have a high energy ratio, overcharging and over-discharging should be strictly avoided in battery use. Over-discharge can lead to difficulty in recovery of active substances. At this time, if you directly enter the rapid power-on mode (high current), it will cause damage to the battery, affecting the service life and may bring invisible danger. It is necessary to charge to 2.5V or 3.0V with a small current (C/10) and then convert it to fast charge.
Although lithium-ion batteries currently have protective plates in their applications, under normal circumstances, the probability of over-discharge will be small, but without pre-charging, the following situations may still bring hidden dangers:
1. The protection board is invalid.
2. Self-discharge rate for long-term placement (5%~10%/month).
Therefore, the small current pre-charging function can effectively solve the charging problem of the over-discharge battery.

(2) Is the charging current as large as possible?
Taking a single-cell lithium-ion battery as an example, its charging method includes constant current and constant voltage charging processes. The constant voltage is usually 4.2V (taking LiCoO2 battery as an example), and the constant current setting is 0.1C ~ 1C. Although charging with a large current will shorten the charging time, it will also shorten the battery life cycle and reduce the capacity, so we need to select the appropriate constant current value for charging.
The following is a curve of different current charging and battery capacity of a 4.2V/900mAH LiCoO2 battery (see Figure 1). We can see that after about 500 charging and discharging cycles, the battery capacity of small current charging is significantly larger than that of high current charging. battery capacity.



(3) What is the voltage accuracy requirement for constant voltage charging?
As a high-energy-density battery, overcharging has a great hazard to lithium-ion batteries. It may swell and leak, and it will often explode. Over-charge will easily cause the electrolyte inside the battery to react quickly and cause the battery. The service life is shortened, so accurate constant voltage charging value is of great significance to lithium ion batteries.
In order to fully charge the battery, the accuracy of the constant voltage and the termination voltage should be within 1%. Taking a lithium cobalt oxide battery as an example, ** as close as possible to 4.2V, but not exceeding 4.2V, this high-precision voltage charging method can reduce the dissolution of cobalt and stabilize the layered structure of LiCoO2, making it a package. Do not change phase change, improve cycle performance, and maintain high capacity, see Figure 2.


Figure 2 LiCoO2 chemical properties (Source: RWTH Aachen University)

As can be seen from Figure 3, even a slight overpressure can cause two phenomena to change: (a) the initial capacity of the battery increases; (b) the battery cycle life decreases.

In the case of a multi-cell lithium-ion battery connected in series, in order to ensure the battery capacity and life of the obtained **, sometimes the accuracy of 1% can not meet the requirements of the application, so sometimes the accuracy is required to be within 0.5%. Therefore, the accuracy control of the charging voltage is a key to the lithium ion battery charger.
At present, there is such a misunderstanding about the charging voltage of lithium batteries. It is considered that with the battery protection board, it is not necessary to care about the voltage accuracy, which is not desirable. Because the battery protection board is used to protect against possible accidents in a timely manner, it considers more safety factors than performance factors. For example, taking a 4.2V battery as an example, the overvoltage protection parameter of the protection board is 4.30V (some may require 4.4V). If the battery is overcharged every time, the battery capacity will be very high with 4.30V as the charging cutoff point. Fast decaying.

(4) Why do I need a charging timer?
I once had a charger manufacturer telling me that the charger user who used to come back and returned the charger said that the charger was broken. The reason was that the battery was charged for one day, the battery was not fully charged, and the charger did not turn the light. It was always a red light. When the manufacturer actually measures the charger, it finds that it is normal and meets the factory requirements. What is the problem? This is mainly because the charger does not take into account that the self-discharge of the battery becomes larger after aging. If the current setting of the off-charge is too small, the aging battery will not reach the set point of charging completion, which causes the user to misjudge that the charger is broken.
The role of the charge timer is to prevent the damaged or excessively circulated lithium battery. In the cut-off section of the charge, the self-discharge is too large, making it difficult for the battery to enter the EOC state (higher than the judgment current), on the one hand, bringing the user The battery is not full of misjudgment, on the other hand, it may also cause overheating and even danger of the battery due to excessive charging.