Battery Protection Methods
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. Cell protection can be external to the battery and this is one of the of the prime functions of the Battery Management System.
Safety measures can also be built into the cells themselves and examples are outlined in the section on Battery Safety.
High power cells can be particularly dangerous. They contain large amounts of energy which, if released in an uncontrolled way through a short circuit or physical damage, can have catastrophic consequences. 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.
Different applications and different cell chemistries require different degrees of protection. Lithium batteries in particular need special protection and control circuits to keep them within their predefined voltage, current and temperature operating limits. Furthermore, the consequences of failure of a Lithium cell could be quite serious, possibly resulting in an explosion or fire. Cell protection is therefore indispensable in Lithium batteries. The following discussion illustrates some of the principles involved.
In general cell protection should address the following undesirable events or conditions:
- Excessive current during charging or discharging.
- Short circuit
- Over voltage - Overcharging
- Under voltage - Exceeding preset depth of discharge (DOD) limits
- High ambient temperature
- Overheating - Exceeding the cell temperature limit
- Pressure build up inside the cell
- System isolation in case of an accident
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.
The red areas are specified by the cell manufacturers as "No go" areas where cells will most likely be subject to permanent damage. 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 white area between the safe zone and the failure zone represents the design safety margin.
The diagrams also illustrate how the multiple levels of
protection function to ensure safe operating conditions at all times even if one of the devices fails.
The above diagram shows three protection schemes providing two levels of protection from both over-current and over temperature. If one fails the other one is there as a safety net.
Excessive temperatures will cause all cells to fail eventually. Most protection circuits therefore incorporate a thermal fuse which will permanently shut down the battery if its temperature exceeds a predetermined limit.
Thermistor (Not shown in the diagram)
Thermistors are circuit devices whose resistance varies with temperature. 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. Similarly NTC thermistors have a Negative Temperature Coefficient and their resistance decreases as temperature increases. 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. 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.
In some applications the thermistor may be the only means of communication between the battery and the external world.
Thermistors can also be used by the charger to determine starting environmental conditions and prevent charging if the battery temperature is too low or too high.
A resettable fuse indicated in the diagram above
provides on-battery over-current protection. It has a similar function to a thermal fuse but after opening it will reset once the fault conditions have been removed and after it has cooled down again to its normal state. It requires no manual resetting or replacement and so is very convenient for the user who may not even be aware of its operation.
The fuse is triggered when a particular temperature is reached. The temperature rise can be caused by the resistive self heating of the thermistor due to the current passing through it, or by conduction or convection from the ambient environment. Thus it can be used to protect against both over- current and over-temperature.
Also called a PPTC (Polymeric Positive Temperature Coefficient) device, the resettable fuse is a non-linear PTC thermistor based on a thin composite of semi crystalline polymer and conductive particles. Under normal operating conditions, the conductive particles provide a low resistance path allowing current to flow. Under fault conditions that cause excessive temperature, such as excessive current flow or an excessively high ambient temperature, the crystallites in the polymer undergo an abrupt phase
change within a very
narrow temperature range melting and becoming amorphous causing separation of the particles resulting in a large, non-linear increase in resistance.
The sharp increase in resistance is typically three orders of magnitude or more, reducing the current to a relatively low and safe level. It will hold in this high resistance state until the fault conditions are removed. On cooling the phase change is reversed and the PPTC resets to low resistance state (within certain post trip limits).
Devices have a de-rating at elevated temperatures which means that they will trip at a lower current if the temperature is higher. Environmental and electrical details of application must be full understood when designing in resettable fuse protection.
These devices are easily integrated into battery design by welding across cell terminals or placing on circuit board.
The "Polyswitch" is an example of such a device. (Polyswitch™ is a trademark of Raychem corp.)
Conventional fuses may be used to protect the battery from an overload, but in many situations they may not act quickly enough. This is particularly true if the battery is short circuited. Since the battery has a very low internal impedance, very high instantaneous currents can flow which can seriously damage the battery. Fuses however are very slow to operate in fault conditions and may not act quickly enough if the battery is short circuited.
Fast acting over current and overvoltage protection which can isolate the battery are usually provided by electronic means.
Over-current protection is normally provided by a current sensing device which detects when the upper current limit of the battery has been reached and interrupts the circuit. Since current is difficult to measure the usual method of current sensing is by measuring the voltage across a low ohmic value, high precision, series, sense
resistor in the current path. When the specified current limit has been reached the sensing circuit will trigger a switch which will break the current path. The switch may be a semiconductor device or even a relay. Relays are inexpensive, they can switch very high currents and provide very good isolation in case of a fault but they are very slow to operate. FET switches are normally used to provide fast acting protection but they are limited in their current carrying capability and very costly for high power applications.
Once the fault conditions have been removed, the circuit would normally be reconnected automatically, however there are particular circumstances when the circuit would be latched open. This could be to protect an unsuspecting service engineer investigating why a high voltage battery had tripped out.
The above diagram shows a scheme for over and under-voltage, as well as temperature protection. In this case it also shows interaction with the charger. Batteries can be damaged both by over-voltage which can occur during charging and by under-voltage due to excessive discharging. This scheme allows voltage limits to be set for both charging and discharging. Batteries can be particularly vulnerable to overcharging. (See the section on Charging ). By providing the charger with inputs from voltage and temperature sensors in the battery, the charger can be cut off when the battery reaches predetermined control limits. The diagram above only shows a single voltage cut off from the charger, however multiple protection circuits can be implemented to provide a comprehensive protection scheme involving the charger as well as the protection built into the battery.
It should be noted that each protection device added into the main current path will increase the effective internal impedance of the battery, as much as doubling it in the case of single cell batteries. This adversely affects the battery's capability of delivering peak power.
When the charging system involves communications between the battery and the charger it is called an Intelligent Charging System. An example of an Intelligent Battery is provided in the section on Battery Management Systems. An industry standard for specifying the communications link has been defined. This is the SMBus and this is supported by chip sets which have been developed to facilitate this protocol. Although the SMBus is convenient, many manufacturers still prefer to use proprietary solutions.
As well as sending signals to the charger the intelligent battery can turn on warning lights or send signals about the battery condition to the user. Monitoring is an essential component of Battery Management Systems.
With many cell chemistries the electrochemical process can give rise to the generation of gases, particularly during conditions of over charge. This is called gassing. If the gases are allowed to escape the active mass of chemicals in the cell will be diminished, permanently reducing its capacity and its cycle life. Furthermore the release of chemicals into the atmosphere could be dangerous. Manufacturers have therefore developed sealed cells to prevent this happening. Sealing the cells however gives rise to a different problem. If gassing does occur, pressure within the cell will build up, this will usually be accompanied by a rise in temperature which will make matters worse, until the cell ruptures or explodes. To overcome this second problem sealed cells will normally incorporate some form of vent to release the pressure in a controlled way if it becomes excessive. This is the last line of defence for an abused cell if all the other protection measures fail. Cells are not meant to vent under normal operating conditions.
Circuit Interrupt Device (CID)
For smaller cells an alternative method of dealing with excess pressure is available. This is a small mechanical switch which interrupts the current path through the cell if the internal pressure exceeds a predetermined level. This method is not siutable for high power cells because of the difficulty of incorporating switches which can break the high currents typically causing over-pressure in the cell.
Unfortunately there is no easy way of monitoring the internal pressure of standard cells to facilitate the implementation of simple pressure control mechanisms particularly for high current applications and the product designer is dependent on the efficacy of the safety vent and the use of systems based on temperature monitoring to provide protection from excessive pressure build up within the cells.
See also Pressure Effects.
There is the possibility of explosion if a sealed cell is encased in such a way that it cannot vent. The vents are often tiny and usually go unnoticed. Standard battery holders won't block the vents, but encapsulating the battery in epoxy resin to make a solid power module certainly will.
PTC (Pressure, Temperature, Current) Switch
A slightly more complex safety device which inhibits current surges and interrupts the circuit in case of excess pressure or over temperature. It resets and does not permanently disable the battery when triggered, however tripping may irreversibly increase their electrical resistance by up to a factor of two increasing the possibility of a later catastrophic failure. Typically used in small cylindrical cells such as the 18650.
In multi-cell applications each cell should have its own over-voltage detection device. Several temperature sensors will also be required since the pack may not have a uniform temperature across all the cells. Series connected cell chains would normally require only a single current monitoring and protection device unless provision is made for charging or bypassing individual cells. In such cases each cell will also require its own current monitor. Such complication is unfortunately necessary in high voltage packs containing long series cell strings. This is because individual cells may become overstressed and cause the premature failure of the whole battery. Why this arises, and how to avoid it, is discussed in the section on Cell Balancing.
While the battery can detect and initiate protective actions for events within the battery system, there are some applications which require the battery to respond to external events. This could be an out of tolerance condition such as a high temperature in some other part of the application which requires the power to be shut off. In the case of an automobile accident for instance, an inertia switch should isolate the battery. In these situations the battery needs to incorporate a switch in the main current path which can be triggered by an external signal. This does not necessarily need to be a separate switch since it could be possible to design the battery's over current protection circuit to accept a trigger from an external source.
Capacitive and Inductive Loads
Capacitive and inductive loads may be subject to large current surges as the load charges up. These surges can be sufficient to trip the current protection circuits but may not be of long enough duration to damage the battery. If the application does not allow the current surge to be designed out, then the protection circuit should incorporate a timer or some other device to delay or disable the current cut-off during expected short duration current pulses.
The object of protection is to maximise the life of the battery. Electronic protection circuits themselves draw current from the battery, reducing the effective capacity of the battery to supply the desired load. Low quiescent current is therefore an essential requirement for protection circuits.
Procedures and Discipline
No amount of electronics will protect a cell from bad management practices.
- We know that elevated temperatures are bad for batteries. We should therefore ensure that cells are stored in a cool environment.
- We know that shorting the terminals can be dangerous. We should ensure that handling and packing methods prevent this from happening.
- We know batteries have a finite life. We should make sure the stores works on a FIFO basis.
- Cell manufacturers set operating limits and conditions for their cells. We should ensure that these recommendations are respected during all stages of the procurement, manufacturing and shipping processes.
Protection During Manufacturing
Safe handling procedures for batteries in general are given in the section on User Safety Instructions.
In addition, any electronic circuitry included within the battery pack may be susceptible to damage from electrostatic discharges (ESD) caused by mishandling during the production process. Static electricity may build up on the human body due to contact or friction with insulators and other synthetic materials such as plastics and styrofoam cups, plastic bags and clothing. Its effect is particularly strong in a dry atmosphere. If the charged person then touches an object at a lower potential or ground/earth potential such as circuit boards or components, the charge will be dissipated through that path. This charge is sufficient to damage transistors and integrated circuits. Even if the static sensitive devices are not handled directly they can be damaged by touching the pins or connectors on the printed circuit board.
Standard precautions to avoid electrostatic damage include, the prohibition of casual handling of items on the production line (by visitors or managers), the wearing of grounding straps by anyone touching components or printed circuit boards, conductive flooring, conductive packaging, the labeling of static sensitive components and the avoidance of static prone materials near the production line.
See also Battery Safety