Summary： Battery protection is one of the serious issues of concern when using a battery, and this article will introduce you to all the battery protection methods that you need to know to keep or use your battery safely.
Batteries are one of the crucial power backup devices in modern technology systems. They play key roles in powering electric cars, home appliances, lighting systems, and many others. The main challenge facing battery consumers is how to increase their lifespan and ensure that they enjoy their services for a longer period. Battery failure has been a thing experienced in the consumer industry, and some of the causes of these failures include the voltage and temperature effects. The battery can be said to have failed when it does not supply or retain voltages as expected, and these can be noted when there is a reduction in battery charging and discharging times and also an increase in battery cell temperatures above 700C. This article is going to look at battery protection methods, which will go a long way in reducing battery failures experienced in the consumer industry.
Cell protection is the perfect ingredient used to monitor and ensure cells work within the predetermined operating window and conditions, hence protecting consumers against losses caused by battery failure. Cell protection occurs externally to the battery cell, and the purpose is served well by the Battery Management System (BMS). The cells of the batteries can also be supplied with special safety measures such as handling instructions, user protection systems, and design safety measures.
Each cell chemistry and application requires a different level of protection. Lithium batteries require special control and protection circuits to ensure that they work within the predetermined voltages, temperature, and current limits. The protection circuits of the lithium batteries must have the ability to remove high voltages that accumulate within the microseconds of short circuits. Remember, the failure of the lithium cells can be more dangerous, and it can lead to fires and explosions.
Below are some of the important things that battery cell protection should be able to address.
The diagram below serves as an illustration of how the cell protection mechanism serves as a restriction condition to ensure that the battery works within the safe working predetermined zone.
Red zone: Cell manufacturers highlight these areas as the no-go zones, and if cells go to this area, they are marked as permanently damaged.
Theoretically, the cell can work in the remaining areas. Still, in reality, this has no error margin to safeguard the battery. Therefore, the protection devices are employed to ensure that the functionality of the cell is limited to the area marked in green.
The white area that separates the red and the green zones is referred to as the safety margin of the design.
Figure 1: Cell Protection Courtesy of Simon Mugo
From the diagram above, three protection schemes exist that provide two levels of battery cell protection from the overcurrent and overtemperature effects. If one scheme fails, the other scheme acts as a safety net.
Too much temperature leads to the failure of all the battery cells. Most battery protection circuits, hence, have to make use of thermal fuses that permanently shut up the battery if the temperature goes beyond the predetermined manufacturer’s limit.
The thermistor is not indicated in the diagram above. Thermistors are electrical components with resistance that varies with temperature. They can be classified as PTC and NTC thermistors. The PTC has a positive coefficient of temperature, with its resistance increasing gradually with the increase in temperature. The NTC thermistors have a negative coefficient of temperature, with their resistance decreasing with an increase in temperature. The purpose of the thermistors is to monitor and protect the circuit by providing temperature compensation. They terminate charging or disconnect the battery from the charging.
From the diagram above, we have a resettable fuse, which offers protection to the cell against overcurrent and overvoltage. It serves a similar function as the thermal fuse, but it has the advantage of resetting itself once the fault has been eradicated or the battery has cooled down. The resettable fuse gets triggered once a preset temperature level is achieved.
Batteries are protected against overcurrent by a current sensing gadget that is sensitive to current and reacts immediately when the upper set limit has been achieved, thus interrupting the flow of the current in the circuit. It is very difficult to measure current, and the method used measures the voltage and the resistance in the path of current. On reaching a specified current limit, the sensing device triggers the switch, which breaks the current flow. This switch can either be a relay or a semiconductor device.
Intelligent batteries come in when we have smart communication between the charger of the battery and the battery itself. This smart system is what we refer to as the intelligent charging system.
Apart from sending important signals to the battery charger, the intelligent battery charging system can switch on helpful warning lights that inform the user of the condition of the battery or send signals that relate to the battery conditions to the user. The monitoring purpose is achieved using the battery management system.
This is the method involved in dealing with cell pressure in smaller cells. It has a tiny switch used to interrupt the flow of current through the battery cell if the incoming current pressure is more than the predetermined amount.
Pressure, Temperature, and Current Switch
This is a more complex gadget that detects current surges and breaks the current path just in case there is temperature or excess pressure. This type of battery cell protection resets instead of disconnecting the cell when it gets triggered. It is good for small cylindrical cells, such as 18650.
Sometimes, batteries need to be isolated from external faults, and this can be done through system isolation. For example, in the automotive industry, when there is an accident, the attached inertia switch should be able to isolate the battery. Here, the battery has to incorporate an external switch in the current pathway, which gets triggered by a signal externally to the circuit.
