1. The impact of low battery temperature on battery discharge capacity
Capacity is one of the most important parameters of lithium batteries, and its size changes with temperature. The two curves in the figure below are temperature capacity curves obtained by discharging the battery at 0.1C and 0.3C at different temperatures.
Obviously, as the temperature rises, the capacity gradually increases. The capacity at -20℃ is only about 60% of the capacity at 15℃. In addition to capacity, the open circuit voltage of the battery will also decrease with increasing temperature. We all know that the energy contained in the battery is the product of capacity and terminal voltage. When both multiples decrease, the energy in the battery must be the superposition of the two decreasing effects.
When the battery temperature is low, the activity of the positive electrode material decreases, which reduces the number of lithium ions that can move and bring discharge current, which is the fundamental reason for the decrease in capacity.
2. The influence of low battery temperature on battery internal resistance
The relationship between lithium battery temperature and resistance is shown in the figure below. Different curves represent different charging levels of the battery itself. In any charging situation, the internal resistance of the battery will increase significantly as the temperature decreases. The lower the charge, the greater the internal resistance, and this trend remains unchanged with temperature changes.
When the battery temperature is low, the diffusion and mobility of charged ions in the positive and negative electrode materials become worse, and it is difficult to pass through the passivation film between the electrode and the electrolyte. The transfer speed in the electrolyte is also reduced, and a large amount of heat is generated during the transfer process.
After the lithium ions reach the negative electrode, the diffusion inside the negative electrode material also becomes unsmooth. Throughout the process, the movement of charged ions becomes very difficult. From the outside, it means that the internal resistance of the battery cell has increased.
3. The impact of low battery temperature on battery charging and discharging efficiency
The following curve is the curve of charging efficiency changing with temperature. We can observe that the charging efficiency at -20℃ is only 65% of that at 15℃.
Low battery temperature brings about changes in the above-mentioned electrochemical properties, and the internal resistance increases significantly. During the discharge process, a large amount of electrical energy is consumed on the internal resistance and generates heat.
Lithium battery low temperature preheating technology
Faced with the restrictions on the use of lithium batteries when the battery temperature is low, technicians have found a countermeasure for charging and preheating. Although it is a stopgap measure, it has a significant effect on improving the discharge capacity and long-term life of lithium batteries.
Before charging or using lithium batteries in an environment with low battery temperature, the battery must be preheated. The way the battery management system (BMS) heats the battery can be roughly divided into two categories: external heating and internal heating.
Compared to external heating methods, internal heating avoids long-path heat conduction and the formation of local hot spots close to the heating device. As a result, internal heating can heat the battery more evenly, resulting in better heating with higher efficiency and is easier to implement.
