Chemical changes of a battery. The batteries are electrochemical devices that convert chemical energy into electrical energy or vice versa by means of controlled chemical reactions between a set of active chemical products.
Unfortunately the desired chemical reactions in which the battery depends are usually accompanied by unwanted chemical reactions that consume some of the active chemicals or impede their reactions.
Even if the Cell’s active chemicals are not affected over time, the cells may fail due to unwanted chemical or physical changes in the electrolyte maintenance joints.
Under different conditions of pressure, temperature, electric field and reaction duration, active chemicals in a hot can break or combine in many different ways.
According to Guoxian Liang, of the Phostech lithium materials company, The following combinations of the elements used in the cathodes of the iron phosphate lithium cells have been found in some impure products in addition to the active compound. Desired LiFePo 4:
Fe 3 (po 4) 2, Li 3 fe 2 (po 4) 3, fe 2 po 5, fe 2 P 2 o 7, FePO 4, fe (po 3) 3, fe 7 (po 4) 6, FE 2 P 4 o 12, fe 3 (po 4) 2, fe 3 (P 2 o 7) 2 , FePLi 2 o, LIPO 3, li 2 o, li 3 PO 4, Li 4 P 2 o 7, fe 2 o 3, fe 3 o 4, feo, fe, FEP, Lifeo 2, li 5 FeO 4, LiFeP 2 o 7, li 2 FEP 2 o 7, li 9 fe 3 (P 2 o 7) 3 (PO 4) 2, P 2 or 5, and Others.
These compounds are present only from the material of the cathode, but there are many other elements present in the anodes, electrolytes, binders and other additives that are used in the cell, having many possible combinations.
The result is a reduction in the amount of chemicals active in the cell and the consequent reduction of their capacity. Subjecting the battery to excessive currents also provides problems such as a reduction in the battery life cycle. See charging times in another item and loading speed BELOW.
It is the task of cell designers and battery application engineers to create the electrochemical recipe and stable operating conditions to ensure that the desired reactions are optimized and undesirable side effects are Suppressed.
Effects of temperature
The internal chemical reactions of the battery are driven either by voltage or temperature.
The hotter the battery, the chemical reactions will occur faster. High temperatures therefore can provide higher performance, but at the same time the speed of unwanted chemical reactions will increase resulting in a decrease in battery life.
The lifespan and load retention depends on the rate of self-discharge and self-discharge is the result of an unwanted chemical reaction in the Cell.
similarly, adverse chemical reactions, such as electrode passivation, corrosion and gas chambers, are common causes of reduced life-cycle.
therefore, the temperature affects both the life of that and the life cycle, as well as load retention, because they are due to chemical reactions.
Even batteries that are specifically designed around chemical reactions, temperature (such as Zebra batteries) are not immune to heat-induced failures, which are the result of parasitic reactions within cells.
the Arrhenius equation defines the relationship between the temperature and the velocity at which the chemical action proceeds. This shows that the rate increases exponentially as the temperature increases.
As a general rule, for every 10 °c increase in temperature the reaction rate is doubled. thus, an hour at 35 º C is equivalent in the life of the battery to two hours at 25 º C. The heat is the enemy of the battery and as a sample of Arrhenius, even a small increase in temperature will have a great influence on the performance of the battery that affects both the desired and unwanted chemical reactions.
The following chart shows how the life of the high tubular capacity of Ironclad lead-acid batteries used in emergency applications over the years varies with the operating Temperature.
Keep in mind that the work at 35 º c, the batteries offer more than their nominal capacity, but their life is relatively short, while a long life is possible if the batteries are kept at 15 º C.
As an example of the importance of storage temperature conditions: nickel-metal hydride (NiMH) chemistry, in particular, is very sensitive to high temperatures. The tests have shown that continuous exposure to 45 º C will reduce the life cycle of an I-MH battery 60 percent and as with all batteries, the self-discharge rate is doubled with every 10 °c increase in Temperature.
Apart from the gradual deterioration of the cell over time, under conditions of abuse, the effects of temperature can lead to premature cell failure. This can occur even under normal operating conditions if the heat rate generated by the battery exceeds the heat loss rate to the Environment.
In this situation, the battery temperature will continue to increase to a condition known as thermal exhausts which ultimately result in disastrous Consequences.
the bottom line is that high temperatures during storage or use seriously affect the life of the battery.
Effects of pressure
These problems are related only to sealed cells.
The increase of the internal pressure within a cell is usually the consequence of increasing temperature. Several factors can play a role in increasing pressure temperature I.
Excessive currents or a high ambient temperature will raise the cell temperature and the consequent expansion of the active chemicals, in turn causing the internal pressure to rise in the Cell.
The overload also causes an increase in temperature, but it is more serious, the overload can also cause the release of the resulting gases in a greater accumulation of the internal pressure.
unfortunately, increased pressure tends to magnify the effects of high temperatures by increasing the rate of chemical reactions in the cell, not only the desired galvanic reaction, but also other factors such as self-discharge rate or Extreme cases, which contribute to the thermal leakage.
Excessive pressures can also cause mechanical failures within the cells, such as short-circuits between the parts, interruptions in the current route or swelling of the cell casing, or in the worst case the actual rupture of the cell cover. All of these factors tend to reduce the life of the Battery’s Potential.
Usually this type of problem occurs only in situations of Abuse. However the manufacturers have no control over how the user treats the cells once they have left the factory, and for safety reasons, the pressure release outputs are incorporated into the cells to provide a controlled release of the Pressure if there is the possibility that it could reach dangerous levels.
Discharge depth (DOD)
At a certain speed and discharge temperature, the quantity of active chemical products transformed with each load-discharge cycle will be proportional to the depth of the Discharge.
The relationship between the life cycle and the discharge depth appears to be logarithmic as shown in Figure 1. In other words, the number of cycles produced by a battery increases exponentially if the discharge is low.. This is true for most cell chemical reactions.
(the curve only looks like a logarithmic curve but, in reality, is a reciprocal curve set in logarithmic paper).
There are important lessons here, both for designers and Users. By limiting the possible DOD in the application, the designer can dramatically improve the life cycle of the Product. In the same way the user can obtain a much longer life of the battery by using cells with a capacity of little more than required or by the head of the reserve stack before it is completely discharged.
It is common for cells to use for “microcycle” (small load and unload current APPLICATIONS) a life cycle of between 300,000 and 500,000 Cycles.
Mobile phone users often recharge their batteries when recommended is only 25 to 30 Percent. In this low DOD a lithium-ion battery can be expected to reach between 5 and 6 times the battery life cycle is specified to assume full discharge each cycle. thus, the life cycle improves considerably if the recommended is Reduced.
Nickel cadmium batteries are kind of an exception to this Rule. Subjecting the battery to partial discharges only gives rise to the memory effect called that can only be reversed by deep discharge.
Some applications, such as electric vehicles or marine use, may require the maximum capacity to be extracted from the battery which means very high battery discharge. Dod. Special “deep cycle battery” can damage general purpose batteries and should be used for this type of applications since the total Discharge.
In particular, the typical SLI automotive batteries are only designed to work up to 50% of the recommended, while the traction batteries can work at 80% to 100% of the Recommended.
The life cycle of the lithium batteries can be increased by reducing the load when the cutting voltage. This essentially gives the battery a partial load rather than fully load, similar to working on a minor DOD as in the example Above.
The battery life is also influenced by the rate Reduction. The load capacity in discharge rates is due to the transformation of active chemicals that cannot keep pace with the current Consumed.
The result is an incomplete or unwanted chemical reaction and an associated reduction in capacity as noted in the paragraph on chemical changes Above.
This can be accompanied by changes in the morphology of the electrode crystals, such as cracks or growth of the crystals that negatively affect the internal impedance of the Cell. Similar problems occur during CHARGING.
There is a limitation on how quickly lithium ions can enter the Anode’s collation Layers. Try to force too much of the current through the battery during the results of the charging process on the surplus ions to have deposited on the anode in the form of lithium metal.
Known as lithium plating, This results in a loss of irreversible Capacity.
At the same time, maintaining the highest necessary tensions for fast loading can lead to the breakdown of the electrolyte which results in a loss of Capacity. From the above it can be deduced that with each charge/discharge cycle the irreversible and accumulated loss of capacity will Increase.
Although this may be imperceptible, ultimately, reducing the capacity in the cell will result in the inability to store the energy required by the Specification. In other words, the end of its useful life is reached and since the loss of capacity is caused by the high current operation, it is possible to expect that the life cycle of the battery is shorter, the greater the current it Carries.
The rechargeable batteries each have a characteristic voltage range of work associated with the particular chemistry used in each Cell.
The practical tension limits are a consequence of the emergence of unwanted chemical reactions that take place beyond the safe working range.
Once all active chemicals have been transformed into the composition associated with a fully charged cell, forcing more electrical energy into the cell will cause them to heat up and to initiate the most desired reactions among the chemical components Descomponiéndolos in ways that cannot be recombined.
Thus trying to load a cell above its upper voltage limit can produce irreversible chemical reactions that can damage the Cell. Failure to control the increase in temperature and the pressure accompanying these events could lead to the rupture or explosion of the cell and the release of hazardous chemicals or Fires.
similarly, discharging a cell below its lower limit of recommended voltage can also result in permanent, albeit less dangerous, damage due to adverse chemical reactions among active chemicals.
The protection circuits are designed to keep the cell within its recommended range and to work with the limits set to include a safety margin. This is discussed in more detail in the protection Section.
Estimates of the normal life cycle assumes that the cells will only be used within their specified operating limits, however this is not always done in practice and at the same time we move further away from the limits set for a short period or by a Minor margin usually will not cause immediate destruction of the cell but its life cycle will be affected.
For example, continuously along the discharges of 0.2 v NiMH cells can result in a loss of 40 percent of cycle life, and 0.3 v Over-discharge of lithium-ion chemistry can result in a loss of 66 percent of the Capacity. The tests have shown that the overload of 0.1 V or 0.25 volt lithium batteries will not give rise to safety problems, but can reduce the life cycle by up to 80 percent.
Therefore the load and discharge control are essential to preserve the life of the battery