In some applications the thermistor may be the only means of communication between the battery and the external world. They may be used to terminate the charge (dt/dT) or to disconnect the battery from the charger in an over-temperature condition when the temperature cut off point is reached, or they could be used to turn on cooling fans. These components are used extensively in monitoring and protection circuits to provide a voltage analogue of temperature or in control circuits designed to provide temperature compensation. Similarly NTC thermistors have a Negative Temperature Coefficient and their resistance decreases as temperature increases. PTC thermistors have a Positive Temperature Coefficient in that their resistance increases gradually with temperature and over a limited range the resistance can be considered linearly proportional to temperature. Thermistors are circuit devices whose resistance varies with temperature. Most protection circuits therefore incorporate a thermal fuse which will permanently shut down the battery if its temperature exceeds a predetermined limit. If one fails the other one is there as a safety net.Įxcessive temperatures will cause all cells to fail eventually. The above diagram shows three protection schemes providing two levels of protection from both over-current and over temperature. Protection function to ensure safe operating conditions at all times even if one of the devices fails. The diagrams also illustrate how the multiple levels of The white area between the safe zone and the failure zone represents the design safety margin. Theoretically the cell could work in any of the remaining operating space, however this allows no margin of error and in practice protection devices limit the cells operating conditions to a smaller "safe" operating zone shown here in green. The red areas are specified by the cell manufacturers as "No go" areas where cells will most likely be subject to permanent damage. The two diagrams below illustrate how safety devices are specified to protect the cells from out of tolerance conditions by constraining the cells to a safe working zone. System isolation in case of an accident.Overheating - Exceeding the cell temperature limit.Under voltage - Exceeding preset depth of discharge (DOD) limits.Excessive current during charging or discharging.In general cell protection should address the following undesirable events or conditions: The following discussion illustrates some of the principles involved. Cell protection is therefore indispensable in Lithium batteries. Furthermore, the consequences of failure of a Lithium cell could be quite serious, possibly resulting in an explosion or fire. Lithium batteries in particular need special protection and control circuits to keep them within their predefined voltage, current and temperature operating limits.
In the case of short circuits, currents of hundreds of amps can build up in microseconds and protection circuits must be very fast acting to prevent this.ĭifferent applications and different cell chemistries require different degrees of protection. They contain large amounts of energy which, if released in an uncontrolled way through a short circuit or physical damage, can have catastrophic consequences. High power cells can be particularly dangerous. Safety measures can also be built into the cells themselves and examples are outlined in the section on Battery Safety. Cell protection can be external to the battery and this is one of the of the prime functions of the Battery Management System. The purpose of cell protection is to provide the necessary monitoring and control to protect the cells from out of tolerance ambient or operating conditions and to protect the user from the consequences of battery failures. Woodbank does not monitor or record these emails
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