Why Batteries Fail
For the applications engineer designing reliable products dependent on batteries for power an understanding of the potential failure modes of the cells employed is essential. This is to enable him to ensure that potential faults have been designed out of the cells themselves and that unsuitable or uncontrolled operating conditions during manufacture or use of the cells can be prevented or avoided. Batteries with different cell chemistries or constructions may fail in different ways. This report outlines some of the most common cell failures and suggests preventative measures which need to be considered when specifying cells for a new battery application.
Why Cells Fail
Cell design faults
The logical place to start the analysis is in the design of the cell itself. Unfortunately it is the area where the applications design engineer has least knowledge and which he is least able to influence. Cell design faults such as weak mechanical design, inadequate pressure seals and vents, the specification of poor quality materials and improperly specified tolerances can be responsible for many potential failures. Unless he is qualified in physical chemistry, and has experience in components design and access to detailed cell design data and specialist equipment such as mass spectrometers and electron microscopes, there's not much the applications engineer can do to assure himself of the quality of the cell design just from the specifications. What can be done however is accelerated life testing on sample cells to verify that they meet the desired reliability requirements for the proposed application. Before any cells are adopted for your battery application they should undergo thorough qualification testing to identify any potential weaknesses. For more information see Battery Testing. If you don't have the necessary equipment to carry out these tests your friendly pack designer should be able to do it for you.
Manufacturing processes out of control
This is an area where the applications engineer can begin to have some influence. A cell may be well designed, but once it gets out of the design lab and into the factory its fate is determined by the factory manager. In well managed companies this should not be a problem, but a badly run production facility can introduce numerous potential failure sources into the cell. This is less likely to be a problem in a large automated plant with a well known brand name to protect, but if you are looking for the lowest cost cell manufacturer you need to be conscious that corners may be cut to achieve the cost targets. Some symptoms to watch out for:-
- Manual production methods. - It is very difficult to achieve precision and repeatability using manual assembly methods and lack of precision means potential short circuits, leaks, unreliable connections and contamination. This doesn't just apply to back street operations using low cost labour. Even with well managed companies, when new technologies are introduced, the initial customer requirements are usually supplied by hand made products or products made on semi automated machinery until the demand is established and the investment in automated production machinery is justified.
- Handling damage such as scratches or crushing of the electrode sub-assembly.
- Out of tolerance components create similar problems as with imprecise assembly noted above.
- Burrs on the electrode current collector foils give rise to short circuits.
- Voids reduce cell capacity, increase impedance and impede heat dissipation.
- Contamination of the active chemicals gives rise to unwanted chemical effects which could result in various forms of cell failure such as overheating, pressure build up, reduced capacity, increased impedance and self discharge and short circuits.
- Lubricants or debris left in the casing materials.
- Poor control of electrode particle morphology. Particle size needs to be very small and uniform to achieve the cell's specified power handling capability.
- Process out of control. - A typical example is variable coating thicknesses of the active chemicals on the electrodes. Once again the results could affect cell capacity, impedance and self discharge. Process control also applies to the temperature and humidity of the air in the production plant as well as to the dimensional accuracy of the components.
- Use of unapproved alternative materials. This is not necessarily obvious but it certainly happens. Tests on samples may be needed to verify this.
- Weld/sealing quality - This can result in poor, unreliable connections and localised heat build up.
- Mechanical weaknesses. In smaller cells the most likely problem will be leakage of the electrolyte. Larger cells will be more prone to cracking or splitting, which also cause leakage, or distortion which means the cells may not fit into the enclosure designed for them.
- Poor sealing results in leakage and loss of active chemicals or water ingress, corrosion and potential safety problems.
- Quality systems and quality management. - After the design of the cell itself, these are perhaps the most important factors affecting cell failures. The manufacturing facility needs to have in place, at key points in the production process, controls which set limits to, and monitor continuously , all the parameters which can ultimately affect the quality and reliability of the product. Corrective actions should come into play automatically whenever the specified limits are approached to ensure they are never breached. Not only should the system be in place but it should be seen to be fully operational. Records should be kept as evidence that the system is at all times operating correctly.
All of these points can be verified by the battery applications engineer to give confidence in the proposed cell supplier provided the cell suppliers allow access to their manufacturing plants.
See also Battery Manufacturing
Battery performance gradually deteriorates with time due to unwanted chemical reactions and physical changes to the active chemicals. This process is generally not reversible and eventually results in battery failure. The following are some examples:-
- Corrosion consumes some of the active chemicals in the cell leading to increased impedance and capacity loss
- Chemical loss through evaporation. Gaseous products resulting from over charging are lost to the atmosphere causing capacity loss.
- Change in physical characteristics (morphology) of the working chemicals.
- Crystal formation. Over time the crystal structure at the electrode surface changes as larger crystals are formed. This reduces the effective area of the electrodes and hence their current carrying and energy storage capacity.
- Dendritic growth. This is the formation of small crystals or treelike structures around the electrodes in what should be an aqueous solution. Initially these dendrites may cause an increase in self discharge. Ultimately dendrites can pierce the separator causing a short circuit.
- Passivation. This is a resistive layer which builds up on the electrodes impeding the chemical action of the cell.
- Shorted cells. Cells which were marginally acceptable when new may have contained latent defects which only become apparent as the ageing process takes its toll. This would include poor cell construction, contamination, burrs on metal parts which can all cause the electrodes to come into contact with each other causing a short circuit.
- Electrode or electrolyte cracking. Some solid electrolyte cells such as Lithium polymer can fail because of cracking of the electrolyte.
The ageing process outlined above is accelerated by elevated temperatures.
Uncontrolled operating conditions
Good batteries are not immune to failure which can be provoked by the way they are used or abused. High cell temperature is the main killer and this can be brought about in the following situations.
- Bad applications design
- Unsuitable cell for the application
- Unsuitable charging profile
- Environmental conditions. High ambient temperatures. Lack of cooling.
- High storage temperature
- Physical damage is also a contributing factor
Most of these conditions result in overheating of the cell which is what ultimately kills it.
Abuse does not just mean deliberate physical abuse by the end user. It also covers accidental abuse which may be unavoidable. This may include dropping, crushing, penetrating, impacts, immersion in fluids, freezing or contact with fire, any of which could occur to an automotive battery for instance. It is generally accepted that the battery may not survive all these trials, however the battery should not itself cause an increased hazard or safety problem in these circumstances.
Battery failures may not necessarily be due to natural wearout or abuse by the user. They could well be caused by malfunctions in the systems in which they are installed. Batteries used in automotive applications could be subject to a variety of problems, any of which could wreck the battery, such as:
- Sensor failures
- Circuit interruptor failure
- Fan or pump failures
- Loss of cooling fluid
- Incorrect or missing BMS messages
- BMS failure
- Loss of communications or interference
- Charger failure (overcharging)
To identify the route cause of the failure the vehicle On Board Diagnostics (OBD) should be correlated with the data logging in the BMS.
How Cells Fail
The actions or processes outlined above cause the cells to fail in the following ways:
Active chemicals exhausted
In primary cells this is not classed as a failure since this is to be expected but with secondary cell we expect the active chemicals to be restored through recharging. As noted above however ageing will cause the gradual depletion of the active mass.
Change in molecular or physical structure of the electrodes
Even though the chemical composition of the active chemicals may remain unchanged, changes in their morphology which take place as the cell ages can impede the chemical actions from taking place, ultimately rendering the cell unusable.
Breakdown of the electrolyte
Overheating or over-voltage can cause chemical breakdown of the electrolyte.
In Lithium cells, low temperature operation or over-current during charging can cause deposition of Lithium metal on the anode resulting in irreversible capacity loss and eventually a short circuit.
Increased internal impedance
The cell internal impedance tends to increase with age as the larger crystals form, reducing the effective surface area of the electrodes.
This is another consequence of cell ageing and crystal growth. Is is sometimes recoverable through reconditioning the cell by subjecting the cell to one or more deep discharges.
Increased self discharge
The changing crystal structure of the active chemicals as noted above can cause the electrodes to swell increasing the pressure on the separator and, as a consequence, increasing the self discharge of the cell. As with all chemical reactions this increases with temperature.
Unfortunately these changes are not usually reversible.
Gassing is generally due to over charging. This leads to loss of the active chemicals but In many cases this can also be dangerous. In some cells the released gases may be explosive. Lead acid cells for instance give off oxygen and hydrogen when overcharged.
Pressure build up
Gassing and expansion of the chemicals due to high temperatures lead to the build up of pressure in the cell and this can be dangerous as noted above. In sealed cells it could lead to the rupture or explosion of the cell due to the pressure build up unless the cell has a release vent to allow the escape of the gasses. Pressure build up can cause short circuits due to penetration of the separator (see next) and this is more of a problem in cylindrical cells which tend to resist deformation under pressure compared with prismatic cells whose cases have more "give" thus mitigating the pressure effect somewhat.
Penetration of the separator
Short circuits can be caused by pentration of the separator due dendrite growth, contamination, burrs on the electrodes or softening of the separator due to overheating.
Before the pressure in the cell builds up to dangerous limits, some cells are prone to swelling due to overheating. This is particularly true of Lithium polymer pouch cells. This can lead to capacity loss due to deteriorating contact between the conductive particles within the cell as well as external problems in fitting the cell into the battery enclosure.
Overheating is always a problem and is a contributing factor in nearly all cell failures. It has many causes and it can lead to irreversible changes to the chemicals used in the cells, gassing, expansion of the materials, swelling and distortion of the cell casing. Preventing a cell from overheating is the best way of extending its life.
The rate at which a chemical action proceeds doubles for every 10°C increase in temperature. The current flow through a cell causes its temperature to rise. As the temperature rises the electro-chemical action speeds up and at the same time the impedance of the cell is reduced leading to even higher higher currents and higher temperatures which could eventually lead to destruction of the cell unless precautions are taken.
Consequences of Cell Faults
The failure mechanisms noted above to not always lead to immediate and complete failure of the cell. The failure will often be preceded by a deterioration in performance. This may be manifest in reduced capacity, increased internal impedance and self discharge or overheating. If a degraded cell continues in use, higher cell heat dissipation may result in premature voltage cut off by the protection circuits before the cell is fully charged or discharged reducing the effective capacity still further.
Measurement of the State of Health of the cells can provide an advance warning of impending failure of the cell.
There are several possible failure modes associated with the complete breakdown of the cell, but it is not always possible to predict which one will occur. It depends very much on the circumstances.
- Open circuit - This is a fail safe mode for the cell but may be not for the application. Once the current path is cut and the battery is isolated, the possibility of further damage to the battery is limited. This may not suit the user however. If one cell of a multicell battery goes open circuit then the whole battery will be out of commission.
- Short circuit - If one cell of a battery chain fails because of a short circuit, the rest of the cells may be slightly overloaded but the battery will continue to provide power to its load. This may be important in emergency situations.
Short circuits may be external to the cell or internal within the cell. The battery management system (BMS) should be able to protect the cell from external shorts but there's not much the BMS can do to protect the cell from an internal short circuit.
Within the cell there are different degrees of failure.
Solid connection between electrodes causes extremely high current flow and complete discharge resulting in permanent damage to the cell.
Small localised contact between electrodes. Possibly self correcting due to melting of the small regions in contact caused by the high current flow which in turn interrupts the current path as in a fuse.
The existence of a soft short could possibly be indicated by an increase in the self discharge of the cell or by a cell with a higher self discharge than the rest of the population. This indicator is unfortunately less ponounced in larger cells where it is most needed.
- Explosion - This is to be avoided at all costs and the battery must incorporate protection circuits or devices to prevent this situation from occurring.
- Fire - This is also possible and as above the battery should be protected from this possibility.
See also Battery Safety
Occasionally you may find that an apparent fault in the battery is actually a fault external to the cells. It could be in the charger or in the protection circuitry. This may occur when a "perfectly good" charger is unable to charge the battery. It is possible that this could be caused by a mismatch in the protection limit settings between the battery and the charger. The charger voltage regulation may not have the range to cope with a fully discharged battery or the current limits may be set too low to allow the initial current inrush into the battery when the charger is switched on.
It is also possible that a fault in the protection circuit could cause the battery to discharge. The possibility of external faults should therefore be verified before the cells are blamed.
Maximising Battery Life
The first step is to ensure that the most appropriate battery is chosen for the application.
The second step is to select a cell supplier who can be relied upon to provide a safe reliable product.
The next step is to verify that the chosen cells meet the desired specification under every expected condition of use and that inbuilt safety devices such as pressure vents function correctly.
Once the cell has been chosen the ancillary electronic circuits can be specified. The most important of these are the safety circuits which ensure that the cells are maintained within their specified operating temperature, current, and voltage limits. This should also include the specification of the charger.
See Battery Protection Methods
Failure Prevention Design Reviews
The design process for a new cell technology could take up to 10 years or more. Failure prevention sould be an important agenda item during regular design reviews which sould take place during this period. See Failure Modes and Effects Analysis.
When finished battery packs become available, they should be subject to qualification tests as stand alone units and as part of the qualification testing of the product in which they are used and also with the associated charger. These tests should identify whether there are any undesired interactions between these units.
Don't use up the battery's life unnecessarily by storing it at too high temperatures.
After taking such care during the design process, don't let the pack manufacturing introduce potential faults into the battery. Make sure that the factory is implementing the necessary quality systems.
Provide the user with recommended operating and maintenance procedures for the battery (including reconditioning if this is possible) and ways of monitoring the battery State of Health.
See also Lithium Battery Failures
For more information see Battery Life and Battery Reliability and How to Improve it