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36V Lithium Ion Battery Pack Charge&Discharge Problem

Shenzhen Sinoli Electronic Co.,Ltd | Updated: Jan 20, 2019

36V Lithium ion Battery Pack Charge&Discharge Problem

We have more and more application are using 36v Lithium ion battery as DC power to give power supply. Some battery performs good, some are not. Except what shall we take concern to use and maintenance. Today let's have a look what's the problem with 36v lithium ion battery pack in seriese and parallel connection in production aspect.

Voltage inconsistency problem. battery cell voltage inconsistency problem during production is a very normal thing, but it could give a sever damage to customer power supply systems. 36V lithium-ion batteries rated voltage is 3.6V (some products are 3.7V). The termination charging voltage at full charge is related to the anode material of the battery: the anode material is 4.2V of graphite and the anode material is 4.1V of coke. The internal resistance of different anode materials is also different. The internal resistance of coke anode is slightly larger, and its discharge curve is slightly different, as shown in Figure 1. Generally known as 4.1V lithium-ion battery and 4.2V lithium-ion battery. Most of the current batteries are 4.2V, and the termination voltage of lithium ion batteries is 2.5V~2.75V (the battery factory gives the range of working voltage or termination voltage, with slightly different parameters). Continuous discharge below the termination voltage is called over-discharge, which will damage the battery.

So, let's have a look at what happen during battery cell production process. In the production process of 36V lithium ion battery cell, there are many processes from coating to finished product. Even though the voltage, resistance and capacity of each group of power supply are the same after strict testing procedures, there will be differences in one way or another when they are used for a period of time. Like a mother's twins, they may look exactly the same when they are born, and it's hard to tell them apart as mothers. However, as two children grow up, there will be one or another difference in lithium-powered batteries. After a period of difference in use, it is difficult to apply the whole voltage control method to lithium power batteries, such as a 36V battery pack, which must be connected in series with 10 batteries. The overall charge control voltage is 42V, while the discharge control voltage is 26V. With the integrated voltage control method, there may be no problems in the initial use stage due to the excellent consistency of the batteries. After using for a period of time, the resistance and voltage of the battery fluctuate and form a state of inconsistency (inconsistency is absolute, consistency is relative). It is impossible to achieve its purpose to use the overall voltage control at this time. For example, when 10 batteries are discharged, the voltage of two batteries is 2.8V, the voltage of four batteries is 3.2V, and the voltage of four batteries is only 3.4V. Now the overall voltage is 32V. We let it continue discharging until 26V. In this way, the two 2.8V batteries are under 2.6V in an over-discharge state. Lithium batteries are scrapped after several overdischarge. On the contrary, overcharge will also occur when charging by means of integrated voltage control. For example, the above 10 batteries were charged at the current voltage state. When the overall voltage reaches 42V, the two 2.8V batteries are in a "hungry" state, and the rapid absorption of electricity will exceed 4.2V, while over-charged batteries over 4.2V will not only be scrapped due to the high voltage, but also dangerous, which is the characteristics of lithium-powered batteries.