Among the most modern batteries, lithium ones occupy a special place. In chemistry, lithium is the most active metal.

It has a huge energy storage resource. 1 kg of lithium can store 3860 ampere hours. The well-known zinc lags far behind. His figure is 820 ampere-hours.

Lithium-based cells can produce voltages up to 3.7V. But laboratory samples are capable of producing a voltage of about 4.5V.

Modern lithium batteries do not use pure lithium.

There are currently 3 common types of lithium batteries:

Lithium-ion ( Li-ion). Rated voltage (U nom.) - 3.6V;

Lithium polymer ( Li-Po, Li-polymer or "lipo"). U nom. - 3.7V;

Lithium iron phosphate ( Li-Fe or LFP ). U nom. - 3.3V.

All these types of lithium batteries differ in the cathode or electrolyte material. Li-ion uses a lithium cobaltate cathode LiCoO2, Li-Po uses a gel polymer electrolyte, and Li-Fe uses a lithium ferrophosphate cathode LiFePO 4.

Any lithium battery (or the device in which it operates) is equipped with a small electronic circuit - a charge/discharge controller. Since lithium-based batteries are very sensitive to overcharging and deep discharge, this is necessary. If you “pick apart” any lithium battery from a cell phone, you can find a small electronic circuit in it - this is the protective controller ( Protection IC ).

If there is no built-in controller (or charge supervisor) in a lithium battery, then such a battery is called unprotected. In this case, the controller is built into the device, which is powered by such a battery, and charging is possible only from the device or from a special charger.

The photo shows an unprotected Li-Po battery Turnigy 2200 mAh 3C 25C Lipo Pack. This battery consists of 3 cells connected in series (3C - 3 cell) of 3.7V each and therefore has a balancing connector. The continuous discharge current can reach 25C, i.e. 25 * 2200 mA = 55000 mA = 55 A! And the short-term discharge current (10 sec.) is 35C!

Lithium batteries, which consist of several cells connected in series, require a complex charger equipped with a balancer. This functionality is implemented, for example, in such universal chargers as Turnigy Accucell 6 and IMAX B6.

A balancer is needed to equalize the voltage across individual cells during charging of a composite lithium battery. Due to differences between cells, some may charge faster and others slower. Therefore, a special circuit for shunting the charging current is used.

This is the wiring for the balancing and power cables of an 11.1V LiPo battery.

As is known, overcharging a lithium battery cell (especially Li-Polymer) above 4.2V can lead to an explosion or spontaneous combustion. Therefore, during charging it is necessary to control the voltage on each cell compound battery battery!

Correct charging of lithium batteries.

Lithium batteries (Li-ion, Li-Po, Li-Fe) are charged by CC/CV method (“constant current/constant voltage”). The method is that first, when the voltage on the element is low, it is charged with a constant current of a certain value. When the voltage on the cell reaches (for example, up to 4.2V - depends on the type of battery), the charge controller maintains a constant voltage across it.

First stage lithium battery charge - CC- implemented through feedback. The controller selects the voltage on the element so that the charge current is strictly constant.

During the first charging stage, the lithium battery accumulates most of the power (60 - 80%).

Second stage charge - CV- begins when the voltage on the element reaches a certain threshold level (for example, 4.2V). After this, the controller simply maintains a constant voltage on the element and gives it the current it needs. Towards the end of the charge, the current decreases to 30 - 10 mA. At this current, the element is considered charged.

During the second stage, the battery accumulates the remaining 40 - 20% of the power.

It is worth noting that exceeding the threshold voltage on a lithium battery can cause it to overheat and even explode!

When charging lithium batteries, it is recommended to place them in a fireproof bag. This is especially true for batteries that do not have a special box. For example, those that are used in radio-controlled models (car, aircraft modeling).

Disadvantages of lithium-ion batteries.

The main and most frightening disadvantage of lithium-based batteries is their fire hazard if the operating voltage is exceeded, overheating, improper charging and illiterate operation. There are especially many complaints regarding lithium-polymer (Li-Polymer) batteries. However, lithium iron phosphate (Li-Fe) batteries do not have such a negative feature - they are fireproof.

Also, lithium batteries are very afraid of the cold - they quickly lose their capacity and stop charging. This applies to Li-ion and Li-Po batteries. Lithium iron phosphate (Li-Fe) batteries are more resistant to frost. Actually, this is one of the positive qualities of Li-Fe batteries.

The disadvantage of lithium batteries is that they require a special charge controller - an electronic circuit. And in the case of a composite battery and balancer.

When deeply discharged, lithium batteries lose their original properties. Li-ion and Li-Po batteries are especially susceptible to deep discharge. Even after restoration, such a battery will have a lower capacity.

If a lithium battery does not “work” for a long time, then first the voltage on it will drop to a threshold level (usually 3.2-3.3V). The electronic circuit will completely turn off the battery cell, and then a deep discharge will begin. If the voltage on the cell drops to 2.5V, this may lead to its failure.

Therefore, it is worthwhile to recharge the batteries of laptops, cell phones, and mp3 players from time to time during long periods of inactivity.

Typically, the service life of an ordinary lithium battery is 3 - 5 years. After 3 years, the battery capacity begins to decrease quite noticeably.

What are the types of lithium batteries and their design features?

Lithium batteries have firmly occupied several different niches in the modern market. They are mainly used in all kinds of consumer electronics, portable tools and mobile devices, home appliances, etc. There are even 12 volt lithium batteries for cars. Although they have not yet received widespread use in the automotive industry. The use of lithium batteries in various sectors of the national economy has led to the appearance of many varieties of these batteries on the market. We will look at the main types of lithium batteries in today's article.

We will not write here about the operating principle of Li batteries and the history of their origin. You can read more about it in the article at the given link. You can also read the materials separately about and. And in this material I would like to consider exactly the different types of Li batteries depending on their characteristics and purpose.

So, as for the power and capacity of lithium batteries. The division here is quite arbitrary. In order to produce batteries of different capacities and with different discharge currents, manufacturers change a number of parameters. For example, they regulate the thickness of the layer of electrode mass on the foil (in the case of a roll design). In most cases, this electrode layer is coated with copper (negative electrode) and aluminum (positive) foil. Due to this increase in the electrode layer, the specific parameters of the battery increase.

However, when increasing the active mass, it is necessary to reduce the thickness of the conductive base (foil). As a result, the battery can pass less current without overheating. In addition, an increase in the layer of electrode mass leads to an increase in the resistance of the element. To reduce resistance, more active and dispersed substances are often used for the active mass. Manufacturers “play” with these parameters when producing batteries with certain parameters. A battery cell with thin foil and thick active mass shows high stored energy values. And its power will be low, and vice versa. And this can be adjusted without changing the size of the product.

Rechargeable batteries with different capacity values ​​and discharge current are obtained by changing the following parameters:

• Foil thickness;
• Separator thickness;
• Material of positive and negative electrode;
• Active mass particle size;
• Electrode thickness.

At the same time, battery models designed for higher power are equipped with current leads of larger sizes and weight. This is done to prevent overheating. Also, to increase the discharge current, various substances are used that are added to the electrolyte or to the electrode mass. Batteries with a large capacity usually have small current leads. They are calculated for a discharge current of up to 2C (usually the charge-discharge current of a battery is indicated by its capacity) and a charging current of up to 0.5C. For high-capacity lithium batteries, these values ​​are up to 20C and 40C, respectively.

High-power lithium battery models are designed to power starters, and high-capacity models are designed to power various portable equipment. As for the development of lithium batteries, manufacturers of all kinds of electronics order them from special companies. They develop them taking into account the proposed conditions, and then place them into mass production. When developing modern lithium batteries, the following parameters are taken into account:

• Capacity;
• Standard and maximum discharge current;
• Dimensions;
• Conditions for location inside the device;
• Working temperature;
• Resource (number of charge-discharge cycles) and others.

Various Lithium Battery Designs

Based on their design features, lithium batteries can be divided into two categories:

• Housing design;
• Electrode design.

Electrode design

Roll type

In the image below you can see a Li-Ion battery with a roll-type design.

Roll structure elements are manufactured in two types:

• A roll of electrodes is twisted around a virtual plate. One housing can accommodate several rolls connected in parallel;
• Cylindrical. Various heights and diameters.

The roll design is used where a small capacity battery and power are required. This technology has little labor intensity, since the twisting of the electrode strips and the separator is fully automated. The disadvantage of this design is poor heat removal from the electrodes. In fact, heat is removed only through the end of the element.

From a set of electrodes

Lithium batteries assembled from individual electrodes are used in the production of prismatic batteries.

Heat here is also removed from the end of the electrode. Manufacturers are trying to improve heat dissipation by adjusting the composition and dispersion of the active mass.

Housing design

Cylindrical

It is worth paying attention to cylindrical lithium batteries. They are widely used in various household appliances and electronics. Battery cells are especially popular.

Experts cite the absence of volume changes during long-term use as an advantage of the cylindrical body. This occurs due to the fact that the battery slightly changes its volume during the charging and discharging process. The design of electrodes in such a housing is always roll type. Disadvantages include poor heat dissipation.

Cylindrical lithium batteries may have the following current terminals:

• Screw bournes;
• Regular contact pads.

Where there are higher requirements for current collection, screw borns are used. This is a battery with a high discharge current and large capacity (more than 20 Ah). Numerous tests show that cylindrical lithium batteries with screw-type batteries can withstand currents of no more than 10-15C. And these are the values ​​of short-term load, at which the element quickly overheats. During long-term operation, they can withstand discharge currents of 2-3C. Mainly used in portable power tools.

Battery cells with contact pads are commonly used to form batteries. To do this, they are welded with tape using resistance welding. Sometimes manufacturers already produce elements with petals for independent soldering. Moreover, the type of petals can be different depending on the type of soldering.

The size designation for cylindrical lithium batteries usually includes their dimensions. For example, 18650 lithium-ion cells have a height of 65 mm and a diameter of 18 mm.

Which is widely used in modern consumer electronics and finds its application as an energy source in electric vehicles and energy storage devices in energy systems. This is the most popular type of battery in devices such as cell phones, laptops, electric vehicles, digital cameras and camcorders. The first lithium-ion battery was released by Sony in 1991.

Characteristics

Depending on the electro-chemical circuit, lithium-ion batteries show the following characteristics:

• The voltage of a single element is 3.6 V.
• Maximum voltage 4.2 V, minimum 2.5–3.0 V. Charge devices support voltage in the range 4.05–4.2 V
• Energy density: 110 … 230 W*h/kg
• Internal resistance: 5 ... 15 mOhm/1Ah
• Number of charge/discharge cycles until 20% capacity is lost: 1000-5000
• Fast charge time: 15 min - 1 hour
• Self-discharge at room temperature: 3% per month
• Load current relative to capacity (C):
  • constant - up to 65C, pulsed - up to 500C
  • most acceptable: up to 1C
• Operating temperature range: −0 ... +60 °C (at subzero temperatures, charging batteries is not possible)

Device

A lithium-ion battery consists of electrodes (cathode material on aluminum foil and anode material on copper foil) separated by porous separators impregnated with electrolyte. The electrode package is placed in a sealed housing, the cathodes and anodes are connected to current collector terminals. The housing has a safety valve that relieves internal pressure in emergency situations and violation of operating conditions. Lithium-ion batteries vary in the type of cathode material used. The current carrier in a lithium-ion battery is a positively charged lithium ion, which has the ability to penetrate (intercalate) into the crystal lattice of other materials (for example, into graphite, metal oxides and salts) to form a chemical bond, for example: into graphite with the formation of LiC6, oxides (LiMO 2) and salts (LiM R O N) of metals. Initially, lithium metal was used as negative plates, then coal coke. Later, graphite began to be used. Until recently, lithium oxides with cobalt or manganese were used as positive plates, but they are increasingly being replaced by lithium ferrophosphate, which have proven to be safe, cheap and non-toxic and can be recycled in an environmentally friendly manner. Lithium-ion batteries are used in conjunction with a monitoring and control system - SKU or BMS (battery management system) and a special charge/discharge device. Currently, in the mass production of lithium-ion batteries, three classes of cathode materials are used: - lithium cobaltate LiCoO 2 and solid solutions based on its isostructural lithium nickelate - lithium manganese spinel LiMn 2 O 4 - lithium ferrophosphate LiFePO 4. Electrochemical circuits of lithium-ion batteries: lithium-cobalt LiCoO2 + 6xC → Li1-xCoO2 + xLi+C6 lithium-ferrophosphate LiFePO4 + 6xC → Li1-xFePO4 + xLi+C6

Due to low self-discharge and a large number of charge-discharge cycles, Li-ion batteries are most preferable for use in alternative energy. Moreover, in addition to the BMS system (SKU), they are equipped with inverters (voltage converters).

Advantages

• High energy density.
• Low self-discharge.
• No memory effect.
• No maintenance required.

Flaws

First generation Li-ion batteries were subject to explosive effects. This was explained by the fact that they used a lithium metal anode, on which, during multiple charge/discharge cycles, spatial formations (dendrites) arose, leading to the short circuit of the electrodes and, as a result, fire or explosion. This problem was finally solved by replacing the anode material with graphite. Similar processes occurred on the cathodes of lithium-ion batteries based on cobalt oxide when operating conditions were violated (overcharging). Lithium ferrophosphate batteries are completely free of these disadvantages. In addition, all modern lithium-ion batteries have built-in electronic circuitry that prevents overcharging and overheating due to overcharging.

Li-ion batteries may have a shorter life cycle when uncontrolledly discharged compared to other types of batteries. When fully discharged, lithium-ion batteries lose the ability to charge when the charging voltage is connected. This problem can be solved by applying a higher voltage pulse, but this negatively affects the further performance of lithium-ion batteries. The maximum “life” of a Li-ion battery is achieved when the charge is limited from above to 95% and discharge to 15–20%. This operating mode is supported by the BMS monitoring and control system (SKU), which is included with any lithium-ion battery.

Optimal storage conditions for Li-ion batteries are achieved when charged at a level of 40–70% of the battery capacity and at a temperature of about 5 °C. At the same time, low temperature is a more important factor for small losses of capacity during long-term storage. The average shelf life (service) of a lithium-ion battery is on average 36 months, although it can range from 24 to 60 months.

Loss of capacity during storage:

temperature	with 40% charge	with 100% charge
0⁰C	2% per year	6% per year
25 ⁰C	4% per year	20% per year
40⁰C	15% per year	35% per year
60⁰C	25% per year	40% for three months

According to all current regulations for the storage and operation of lithium-ion batteries, to ensure long-term storage it is necessary to recharge them to 70% capacity once every 6–9 months.

see also

Notes

Literature

• Khrustalev D. A. Batteries. M: Izumrud, 2003.
• Yuri Filippovsky Mobile food. Part 2. (RU). ComputerLab (May 26, 2009). - Detailed article about Li-ion batteries. Retrieved May 26, 2009.

Links

• GOST 15596-82 Terms and definitions.
• GOST 61960-2007 Rechargeable batteries and lithium batteries
• Lithium-ion and lithium-polymer batteries. iXBT (2001)
• Domestic lithium-ion batteries

Galvanic cell	Galvanic cell Daniel | Alkaline element | | Dry element | Concentration element | Zinc air element | Weston normal element
Electric batteries	Lead Acid | Silver-zinc | Nickel-cadmium | Nickel metal hydride | Nickel-zinc battery | Lithium-ion | Lithium polymer | Lithium Iron Sulfide | Lithium Iron Phosphate | Lithium titanate | Vanadium | Iron-nickel
Fuel cells	Direct methanol | Solid oxide | Alkaline
Models	Battery | Electric battery | Fuel cell
Device

Lithium-ion and lithium-polymer batteries

Engineering thought is constantly evolving: it is stimulated by constantly emerging problems that require the development of new technologies to be solved. At one time, nickel-cadmium (NiCd) batteries were replaced by nickel-metal hydride (NiMH), and now lithium-ion (Li-ion) batteries are trying to take the place of lithium-ion (Li-ion) batteries. NiMH batteries have to some extent supplanted NiCd, but due to such undeniable advantages of the latter as the ability to deliver high current, low cost and long service life, they could not provide their full replacement. But what about lithium batteries? What are their features and how do Li-pol batteries differ from Li-ion? Let's try to understand this issue.

As a rule, when buying a mobile phone or laptop computer, we all don’t think about what kind of battery is inside and how these devices differ in general. And only then, having encountered in practice the consumer qualities of certain batteries, do we begin to analyze and choose. For those who are in a hurry and want to immediately get an answer to the question of which battery is optimal for a cell phone, I will answer briefly - Li-ion. The following information is intended for the curious.

First, a short excursion into history.

The first experiments on creating lithium batteries began in 1912, but it was only six decades later, in the early 70s, that they were first introduced into household devices. Moreover, let me emphasize, these were just batteries. Subsequent attempts to develop lithium batteries (rechargeable batteries) failed due to safety concerns. Lithium, the lightest of all metals, has the greatest electrochemical potential and provides the greatest energy density. Batteries using lithium metal electrodes offer both high voltage and excellent capacity. But as a result of numerous studies in the 80s, it was found that cyclic operation (charge - discharge) of lithium batteries leads to changes in the lithium electrode, as a result of which thermal stability decreases and there is a threat of the thermal state getting out of control. When this happens, the temperature of the element quickly approaches the melting point of lithium - and a violent reaction begins, igniting the gases released. For example, a large number of lithium mobile phone batteries shipped to Japan in 1991 were recalled after several fire incidents.

Because of lithium's inherent instability, researchers have turned their attention to non-metallic lithium batteries based on lithium ions. Having lost a little in energy density and taking some precautions when charging and discharging, they received safer so-called Li-ion batteries.

The energy density of Li-ion batteries is usually twice that of standard NiCd, and in the future, thanks to the use of new active materials, it is expected to increase it even further and achieve three times superiority over NiCd. In addition to the large capacity, Li-ion batteries behave similarly to NiCds when discharged (their discharge characteristics are similar in shape and differ only in voltage).

Today there are many varieties of Li-ion batteries, and you can talk for a long time about the advantages and disadvantages of one type or another, but it is impossible to distinguish them by appearance. Therefore, we will note only those advantages and disadvantages that are characteristic of all types of these devices, and consider the reasons that led to the birth of lithium-polymer batteries.

Main advantages.

• High energy density and, as a result, large capacity with the same dimensions compared to nickel-based batteries.
• Low self-discharge.
• High voltage of a single cell (3.6 V versus 1.2 V for NiCd and NiMH), which simplifies the design - often the battery consists of only one cell. Many manufacturers today use just such a single-cell battery in cell phones (remember Nokia). However, to provide the same power, a higher current must be supplied. And this requires ensuring low internal resistance of the element.
• Low maintenance (operating) costs result from the absence of memory effect, requiring periodic discharge cycles to restore capacity.

Flaws.

Li-ion battery manufacturing technology is constantly improving. It is updated approximately every six months, and it is difficult to understand how new batteries “behave” after long-term storage.

In a word, a Li-ion battery would be good for everyone if it were not for the problems with ensuring the safety of its operation and the high cost. Attempts to solve these problems led to the emergence of lithium-polymer (Li-pol or Li-polymer) batteries.

Their main difference from Li-ion is reflected in the name and lies in the type of electrolyte used. Initially, in the 70s, a dry solid polymer electrolyte was used, similar to plastic film and not conducting electricity, but allowing the exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte effectively replaces the traditional porous separator impregnated with electrolyte.

This design simplifies the production process, is safer, and allows the production of thin, free-form batteries. In addition, the absence of liquid or gel electrolyte eliminates the possibility of ignition. The thickness of the element is about one millimeter, so equipment developers are free to choose the shape, shape and size, even including its implementation in fragments of clothing.

But so far, unfortunately, dry Li-polymer batteries have insufficient electrical conductivity at room temperature. Their internal resistance is too high and cannot provide the amount of current required for modern communications and power supply to the hard drives of laptop computers. At the same time, when heated to 60 °C or more, the electrical conductivity of Li-polymer increases to an acceptable level, but this is not suitable for mass use.

Researchers are continuing to develop Li-polymer batteries with a dry solid electrolyte that operates at room temperature. Such batteries are expected to become commercially available by 2005. They will be stable, allow 1000 full charge-discharge cycles and have a higher energy density than today's Li-ion batteries

Meanwhile, some types of Li-polymer batteries are now used as backup power supplies in hot climates. For example, some manufacturers specifically install heating elements that maintain a favorable temperature for the battery.

You may ask: how can this be? Li-polymer batteries are widely sold on the market, manufacturers equip phones and computers with them, but here we are saying that they are not yet ready for commercial use. Everything is very simple. In this case, we are talking about batteries not with dry solid electrolyte. In order to increase the electrical conductivity of small Li-polymer batteries, a certain amount of gel-like electrolyte is added to them. And most Li-polymer batteries used for cell phones today are actually hybrids because they contain a gel-like electrolyte. It would be more correct to call them lithium-ion polymer. But most manufacturers simply label them as Li-polymer for advertising purposes. Let us dwell in more detail on this type of lithium-polymer batteries, since at the moment they are of the greatest interest.

So, what is the difference between a Li-ion and a Li-polymer battery with gel electrolyte added? Although the characteristics and efficiency of both systems are largely similar, the uniqueness of the Li-ion polymer (you can call it that) battery is that it still uses a solid electrolyte, replacing a porous separator. Gel electrolyte is added only to increase ionic conductivity.

Technical difficulties and delays in ramping up production have delayed the introduction of Li-ion polymer batteries. This is caused, according to some experts, by the desire of investors who have invested a lot of money in the development and mass production of Li-ion batteries to get their investments back. Therefore, they are in no hurry to switch to new technologies, although with mass production of Li-ion polymer batteries will be cheaper than lithium-ion ones.

And now about the features of operating Li-ion and Li-polymer batteries.

Their main characteristics are very similar. The charging of Li-ion batteries is described in sufficient detail in the article. In addition, I will only give a graph (Fig. 1) from, illustrating the stages of charge, and small explanations to it.

The charging time for all Li-ion batteries with an initial charging current of 1C (numerically equal to the nominal value of the battery capacity) averages 3 hours. Full charge is achieved when the battery voltage is equal to the upper threshold and when the charging current is reduced to a level approximately equal to 3% of the initial value. The battery remains cold during charging. As can be seen from the graph, the charging process consists of two stages. In the first (a little over an hour), the voltage increases at an almost constant initial charge current of 1C until the upper voltage threshold is first reached. At this point, the battery is charged to approximately 70% of its capacity. At the beginning of the second stage, the voltage remains almost constant and the current decreases until it reaches the above 3%. After this, the charge stops completely.

If you need to keep the battery charged all the time, it is recommended to recharge after 500 hours, or 20 days. Usually it is carried out when the voltage at the battery terminals decreases to 4.05 V and stops when it reaches 4.2 V

A few words about the temperature range during charging. Most types of Li-ion batteries can be charged with a current of 1C at temperatures from 5 to 45 °C. At temperatures from 0 to 5 °C, it is recommended to charge with a current of 0.1 C. Charging at sub-zero temperatures is prohibited. The optimal temperature for charging is 15 to 25 °C.

The charging processes in Li-polymer batteries are almost identical to those described above, so the consumer has absolutely no need to know which of the two types of batteries he has in his hands. And all those chargers that he used for Li-ion batteries are suitable for Li-polymer.

And now about the discharge conditions. Typically, Li-ion batteries discharge to a value of 3.0 V per cell, although for some varieties the lower threshold is 2.5 V. Manufacturers of battery-powered equipment typically design devices with a shutdown threshold of 3.0 V (for all occasions). What does this mean? The voltage on the battery gradually decreases when the phone is turned on, and as soon as it reaches 3.0 V, the device will warn you and turn off. However, this does not mean that it has stopped consuming energy from the battery. Energy, albeit small, is required to detect when the phone's power key is pressed and some other functions. In addition, energy is consumed by its own internal control and protection circuit, and self-discharge, although small, is still typical even for lithium-based batteries. As a result, if lithium batteries are left for a long period of time without recharging, the voltage on them will drop below 2.5 V, which is very bad. In this case, the internal control and protection circuit may be disabled, and not all chargers will be able to charge such batteries. In addition, deep discharge negatively affects the internal structure of the battery itself. A completely discharged battery must be charged at the first stage with a current of only 0.1C. In short, batteries like to be in a charged state rather than in a discharged state.

A few words about temperature conditions during discharge (read during operation).

In general, Li-ion batteries perform best at room temperature. Operating in warmer conditions will seriously reduce their lifespan. Although, for example, a lead-acid battery has the highest capacity at temperatures above 30 °C, long-term operation in such conditions shortens the life of the battery. Likewise, Li-ion performs better at high temperatures, which initially counteracts the increase in battery internal resistance that results from aging. But the increased energy output is short-lived, since increasing temperature, in turn, promotes accelerated aging, accompanied by a further increase in internal resistance.

The only exceptions at the moment are lithium-polymer batteries with dry solid polymer electrolyte. They require a vital temperature of 60 °C to 100 °C. And such batteries have found their niche in the market for backup sources in hot climates. They are placed in a thermally insulated housing with built-in heating elements powered from an external network. Li-ion polymer batteries as a backup are considered to be superior in capacity and durability to VRLA batteries, especially in field conditions where temperature control is not possible. But their high price remains a limiting factor.

At low temperatures, the efficiency of batteries of all electrochemical systems drops sharply. While NiMH, SLA and Li-ion batteries stop functioning at -20°C, NiCd batteries continue to function down to -40°C. Let me just note that again we are talking only about batteries of wide use.

It is important to remember that although a battery can operate in low temperatures, this does not mean that it can also be charged in these conditions. The charge responsiveness of most batteries at very low temperatures is extremely limited, and the charge current in these cases should be reduced to 0.1C.

In conclusion, I would like to note that you can ask questions and discuss problems related to Li-ion, Li-polymer, as well as other types of batteries, on the forum in the accessories subforum.

When writing this article, materials were used [—Batteries for mobile devices and laptop computers. Battery analyzers.

Permissible temperature ranges for charging and discharging lithium-ion batteries

Testing Features

Tests for the number of cycles were carried out with a discharge current of 1C; for each battery, discharge/charge cycles were carried out until 80% of the capacity was reached. This number was chosen based on the timing of the test and for possible comparison of results later. The number of full equivalent cycles is up to 7500 in some tests.
Life tests were carried out at various charge levels and temperatures, voltage measurements were taken every 40-50 days to monitor discharge, the test duration was 400-500 days.

The main difficulty in the experiments is the discrepancy between the declared capacity and the real one. All batteries have a capacity higher than stated, ranging from 0.1% to 5%, which introduces an additional element of unpredictability.

NCA and NMC batteries were most commonly used, but lithium cobalt and lithium phosphate batteries were also tested.

A few terms:
DoD - Depth of Discharge - depth of discharge.
SoC - State of Charge - charge level.

Using Batteries

The number of cycles

At the moment, there is a theory that the dependence of the number of cycles that a battery can withstand on the degree of discharge of the battery in the cycle has the following form (discharge cycles are indicated in blue, equivalent full cycles are indicated in black):

This curve is called the Wöhler curve. The main idea came from mechanics about the dependence of the number of stretches of a spring on the degree of stretching. The initial value of 3000 cycles at 100% battery discharge is a weighted average at 0.1C discharge. Some batteries show better results, some worse. At a current of 1C, the number of full cycles at 100% discharge drops from 3000 to 1000-1500, depending on the manufacturer.

In general, this relationship, presented in the graphs, was confirmed by the results of experiments, because It is advisable to charge the battery whenever possible.

Calculation of superposition of cycles

When using batteries, it is possible to operate with two cycles simultaneously (for example, regenerative braking in a car):


This results in the following combined cycle:


The question arises, how does this affect the operation of the battery, is the battery life significantly reduced?

According to the results of the experiments, the combined cycle showed results similar to the addition of complete equivalent cycles of two independent cycles. Those. The relative capacity of the battery in the combined cycle fell according to the sum of the discharges in the small and large cycles (the linearized graph is presented below).


The effect of large discharge cycles is more significant, which means that it is better to charge the battery at every opportunity.

Memory effect

The memory effect of lithium-ion batteries was not noted according to the experimental results. Under various modes, its total capacity still did not subsequently change. At the same time, there are a number of studies that confirm the presence of this effect in lithium phosphate and lithium titanium batteries.

Battery storage

Storage temperatures

No unusual discoveries were made here. Temperatures 20-25°C are optimal (in normal life) for battery storage, if not used. When storing a battery at a temperature of 50°C, capacity degradation occurs almost 6 times faster.
Naturally, lower temperatures are better for storage, but in everyday life this means special cooling. Since the air temperature in the apartment is usually 20-25°C, storage will most likely be at this temperature.

Charge level

As tests have shown, the lower the charge, the slower the self-discharge of the battery. The capacity of the battery was measured, what it would be during its further use after long-term storage. The best results were shown by batteries that were stored with a charge close to zero.
In general, good results were shown by batteries that were stored with no more than 60% charge level at the start of storage. The numbers differ from those below for a 100% charge for the worse (i.e. the battery will become unusable earlier than indicated in the figure):

Figure taken from article 5 practical tips for using lithium-ion batteries
At the same time, the figures for small charge are more optimistic (94% after a year at 40°C for storage at 40% SOC).
Since a 10% charge is impractical, since the operating time at this level is very short, It is optimal to store batteries at SOC 60%, which will allow you to use it at any time and will not critically affect its service life.

Main problems of the experimental results

No one has conducted tests that can be considered 100% reliable. The sample, as a rule, does not exceed a couple of thousand batteries out of millions produced. Most researchers are unable to provide reliable comparative analyzes due to insufficient sampling. Also, the results of these experiments are often confidential information. So these recommendations do not necessarily apply to your battery, but can be considered optimal.

Results of the experiments

Optimal charging frequency - at every opportunity.
Optimal storage conditions are 20-25°C with a 60% battery charge.

Sources

1. Course “Battery Storage Systems”, RWTH Aachen, Prof. Dr. rer. nat. Dirk Uwe Sauer