How do lithium-ion batteries protect from short circuit

In lithium batteries, a short circuit (SC) is a rare occurrence. It happens due to incorrect actions of users, failures in the battery management system (BMS). An internal short circuit occurs for three reasons:

• Mechanical damage. As a result of deformation or puncture of the battery cell, the membrane (separator) separating the positive and negative electrodes is partially ruptured.
• Electrochemical damage. Due to improper operation (for example, charging at sub-zero temperatures), lithium deposition and dendritic growth occur in battery cells. Over time, lithium metal deposits penetrate the membrane, causing a short circuit.
• Thermal damage. High temperature causes large-scale shrinkage and destruction of the separating membrane, which leads to short circuit.

When an internal short circuit occurs, a large current flows through the battery cell, causing it to heat up and also heats up neighboring cells. As a result, thermal runaway of the lithium-ion battery may occur with complete failure.

INTERNAL SHORT CIRCUIT STAGES

An intra-element short circuit usually develops gradually, going through three stages:

• Initial stage. The voltage drop caused by an internal fault occurs slowly. The heat generated is small, it is promptly removed by the cooling system, so the temperature of the lithium-ion battery remains almost unchanged. This stage lasts a long time and is difficult to detect.
• Middle stage. The voltage drops significantly, and the generated heat does not have time to be removed, which leads to heat accumulation. The temperature of the lithium-ion battery increases significantly. This stage does not last long, but is easier to detect because the symptoms are pronounced.
• Final stage. The voltage in problem cells rapidly drops to 0 V, and a lot of heat is instantly released over a large area. An exothermic chain reaction (thermal runaway) occurs. This stage is fleeting and cannot be stopped.

DETECTION METHODS

To avoid runaway thermal runaway, five methods have been developed for early detection of short circuits inside lithium-ion batteries.

1. Determination of deviation of measured data

First, a model is created to predict the state of a lithium-ion battery, and then the measured current values of voltage and temperature during charging or discharging are compared and analyzed with the predicted values. Since at the initial stage of an internal short circuit the voltage and temperature values do not change significantly, this method is ineffective. It is also not suitable for monitoring batteries connected in parallel.

2. Detection of abnormal voltage recovery

If an abnormal voltage recovery occurs during charging or discharging batteries with a ceramic membrane, this indicates an internal short circuit. This method helps determine the problem only with a specific type of lithium battery and only with a series connection.

3. Determination of the degree of self-discharge

A short circuit in the battery inevitably causes abnormal self-discharge, which can be detected, for example, by comparing the voltage before and after idle time. However, this method is not suitable for detecting short circuits during battery operation, as well as in parallel connection.

4. Determining Cell Consistency

If the voltage, capacity, state of charge and other parameters of any battery cell deviate greatly from the normal parameters of other cells, then an internal short circuit has occurred in it. Since at the initial stage of an internal short circuit the voltage and capacitance do not change significantly, this method is ineffective. It is also not suitable for parallel connected batteries.

5. Definition of special circuits

Voltage and current detection is performed in a symmetrical ring circuit topology. When a change in the symmetry of the circuit parameters is detected, the location of the battery cell in which the internal short circuit occurs is precisely determined. This method is accurate and works when batteries are connected in parallel. Its disadvantages are the high cost of equipment and the impact on the dynamic consistency of the battery.

PROTECTIVE MEASURES

The risks of lithium-ion battery short circuits are reduced in several ways:

• improvement of materials, manufacturing technology of battery cells;
• improvement of the design and protection system of the lithium-ion battery;
• correct operation.

Let’s discuss the first two points in more detail.

MATERIALS, PRODUCTION TECHNOLOGIES OF LITHIUM BATTERIES

Manufacturers are improving the materials of the membrane (separator), electrolyte, and coatings of positive and negative electrodes. They are also improving production processes to reduce waste.

• The use of ceramic separator with high temperature resistance and low self-discharge rate, as well as flame retardant electrolyte, help suppress dendrite growth, reduce the risk of internal short circuit.
• Coating the positive and negative electrodes of battery cells with a low-conductivity compound or PTC material can reduce internal short-circuit current and heat generation.
• Removing impurities from the electrolyte and membrane of the battery cell helps prevent irreversible adverse reactions, reducing the risk of short circuit caused by breakdown of the membrane by metal particles.
• The use of technology to determine the integrity of the internal structure of the battery cell, the accuracy of processing and alignment of pole parts avoids the potential risk of internal short circuit.

Let’s discuss the first two points in more detail.

SMART DESIGN

Using the BMS software, a smart warning and safety management strategy is set. This helps to monitor the condition of lithium-ion battery cells, promptly determine the location of the problematic element, and promptly eliminate potential threats. The risk of short circuit due to high loads is reduced due to redundancy and equalization of charging or discharging modes.

In the design of lithium-ion batteries, manufacturers include fuses for the battery cells, modules and the battery itself, as well as vehicle load fuses. This multi-level control provides good protection.

Intelligently designed lithium-ion battery cooling system improves heat dissipation and prevents thermal runaway. A system that preheats the battery to a suitable operating temperature before charging in the cold avoids the formation of dendrites.

RESULTS

Manufacturers use various methods to detect internal short circuits in battery cells. They are introducing multi-level protection against short circuits and working to reduce manufacturing defects. However, users are advised to protect lithium batteries from damage, overcharging, excessively deep discharge, hypothermia, and overheating.