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Want to know the facts about your battery?
The battery is a bag of chemicals and their combined interaction is not so precisely predictable as a simple rotating mechanism.
LiPo research. Extracts from https://mpoweruk.com/lithium_failures.htm#lifetime
Cells deteriorate over time, even if properly used or not used at all.
A stored battery under ideal conditions self-discharges at about 5% of residual charge. It is quite possible for a long storage (3-6 months) to take the voltage low enough that the control circuitry in the battery shuts of attempts to recharge.
Every charge / discharge cycle shortens the battery life. If under lab conditions a number of cycles is indicated, there is nothing that can be done by a user to increase it. The battery is considered at end of life if it can only produce 80% of its 'as new' capacity, when fully charged.
All balanced, multi-cell batteries develop weaker cells during their life. Sudden battery failure, often temporary, occurs when the stronger cell reverse charges the weaker cell.
Temporary failure can switch off or glitch digital electronics.
The manufacturers charger can develop a fault, where it over or undercharges, leading to premature battery failure.
Chemical changes within the cell when charged or discharged outside of its design parameters, including temperature range causes rapid deterioration and shortened life.
The Arrhenius equation defines the relationship between temperature and the rate at which a chemical action proceeds. It shows that the rate increases exponentially as temperature rises. As a more convenient rule of thumb, an approximation which is true for temperatures around room temperature - for every 10 °C increase in temperature the reaction rate doubles. Thus, an hour at 35 °C is equivalent in battery life to two hours at 25 °C. Heat is the enemy of the battery and as Arrhenius shows, even small increases in temperature will have a major influence on battery performance affecting both the desired and undesired chemical reactions.
Temperature Effects
Heat is a major battery killer, either excess of it or lack of it, and Lithium secondary cells need careful temperature control.
Low temperature operation
Chemical reaction rates decrease in line with temperature. (Arrhenius Law) The effect of reducing the operating temperature is to reduce rate at which the active chemicals in the cell are transformed. This translates to a reduction in the current carrying capacity of the cell both for charging and discharging. In other words its power handling capacity is reduced. Details of this process are given in the section on Charging Rates
Furthermore, at low temperatures, the reduced reaction rate (and perhaps contraction of the electrode materials) slows down, and makes more difficult, the insertion of the Lithium ions into the intercalation spaces. As with over-voltage operation, when the electrodes can not accommodate the current flow, the result is reduced power and Lithium plating of the anode with irreversible capacity loss.
High temperature operation
Operating at high temperatures brings on a different set of problems which can result in the destruction of the cell. In this case, the Arrhenius effect helps to get higher power out of the cell by increasing the reaction rate, but higher currents give rise to higher I2R heat dissipation and thus even higher temperatures. This can be the start of positive temperature feedback and unless heat is removed faster than it is generated the result will be thermal runaway.
Thermal runaway
Several stages are involved in the build up to thermal runaway and each one results in progressively more permanent damage to the cell.
The first stage is the breakdown of the thin passivating SEI layer on the anode, due to overheating or physical penetration. The initial overheating may be caused by excessive currents, overcharging or high external ambient temperature. The breakdown of the SEI layer starts at the relatively low temperature of 80ºC and once this layer is breached the electrolyte reacts with the carbon anode just as it did during the formation process but at a higher, uncontrolled, temperature. This is an exothermal reaction which drives the temperature up still further.
(Lithium Titanate anodes do not depend on an SEI layer and hence can be used at higher rates.)
As the temperature builds up, heat from anode reaction causes the breakdown of the organic solvents used in the electrolyte releasing flammable hydrocarbon gases (Ethane, Methane and others) but no Oxygen. This typically starts at 110 ºC but with some electrolytes it can be as as low as 70ºC. The gas generation due to the breakdown of the electrolyte causes pressure to build up inside the cell. Although the temperature increases to beyond the flashpoint of the gases released by the electrolyte, the gases do not burn because there is no free Oxygen in the cell to sustain a fire.
The cells are normally fitted with a safety vent which allows the controlled release of the gases to relieve the internal pressure in the cell avoiding the possibility of an uncontrolled rupture of the cell - otherwise known as an explosion, or more euphemistically, "rapid disassembly" of the cell. Once the hot gases are released to the atmosphere they can of course burn in the air.
At around 135 ºC the polymer separator melts, allowing the short circuits between the electrodes.
Eventually heat from the electrolyte breakdown causes breakdown of the metal oxide cathode material releasing Oxygen which enables burning of both the electrolyte and the gases inside the cell.
The breakdown of the cathode is also highly exothermic sending the temperature and pressure even higher. The cathode breakdown starts at around 200 ºC for Lithium Cobalt Oxide cells but at higher temperatures for other cathode chemistries.
By this time the pressure is also extremely high and it's time to run for the door.
Mechanical Fatigue. The electrodes of Lithium cells expand and contract during charging and discharging due to the effect of the intercalation of the Lithium ions into and out of the crystal structure of the electrodes. The cyclic stresses on the electrodes can eventually lead to cracking of the particles making up the electrode resulting in increased internal impedance as the cell ages, or in the worst case, a breakdown of the anode SEI layer which could lead to overheating and immediate cell failure.
Non-Uniformities
Non-uniform current flow due to localised defects in the region of the interface between the separator and the anode surface can also give rise to Lithium plating. Examples of such defects are:
Manufacturing Defects
Mechanical deformation of the components
Blockage or deformation of the separator pores
Uneven anode coating
High current tortuosity due to uneven pressure across the current path
Non-uniform contact between the separator and the anode
Delamination of the current collector
Contamination of the active chemicals
Local electrolyte drying
Abuse
Physical damage
Copper deposition as a result of prolonged over discharge
While the bulk current through the cell may nor be excessive, these defects can cause an uneven current density in the flow of Lithium ions into the surface of the anode creating local “hot spots” of high current which can not easily be accommodated into the anode intercalation layers. The corresponding high concentrations of Lithium ions give rise to Lithium plating.
Because of their flexible casing, pouch cells are more vulnerable to several of these defects than rigid cased cylindrical and prismatic cells.
Pressure effects. When gasses cause the cell to swell, the active elements no longer remain in electrical contact. The elements that do remain connected electrically, heat up more under load, because of their smaller contact area and generate more gas. Continued cell use means sudden failure is certain. Fire is a possibility.
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