LiFePO4 Battery: Charging, Lifespan, Storage and Recycling

Ⅰ. Charging methods of LiFePO4 battery pack

We recommend using the CCCV charging method to charge the lithium iron phosphate battery pack, that is, constant current first and then constant voltage. The constant current is recommended to be 0.3C, and the constant voltage is recommended to be 3.65. That is, we use 0.3C current to charge during the constant current process. When the battery voltage reaches 3.65V, we use 3.65V voltage constant voltage charging; stop charging when the charging current is lower than 0.1C (or 0.05C), which means the battery is fully charged. When you charge with a constant voltage power supply, you must see the charging current clearly. It is recommended not to charge with too high a voltage. After adjusting the voltage, we need to ensure that the charging current is below 0.5C to ensure the performance of the battery. The following is the charging method of lithium iron phosphate battery pack.

1. Chopper charging method

To use the chopping method to charge. In this method, the current of the constant current source remains unchanged, and the switching tube is controlled so that it is turned on for a period of time and then turned off for a period of time, and the cycle repeats. The advantage of this method is that when the battery is charged by an external circuit, the ion generation inside the battery needs a certain response time. If it is continuously charged, it may reduce the potential of lithium iron phosphate battery capacity. After charging for a period of time, we add a turn-off time to allow the ions generated at the two poles of the battery to have a diffusion process, so that the battery has a "digestion" time. This will greatly increase the utilization rate of the battery and improve the charging effect.

2. Constant current charging method

During the whole charging process, we keep the charging current constant by adjusting the output voltage. We keep the charging current constant, and the charging rate is relatively low. The constant current charging control method is simple, but the acceptable current capacity of the lithium battery pack gradually decreases with the charging process. In the later stage of charging, the power receiving capacity of the power battery decreases, and the utilization rate of charging current is greatly reduced. The advantage of this method is that the operation is simple, convenient, easy to implement, and the charging power is also easy to calculate.

3. Constant voltage charging method

During the charging process, the output voltage of the charging power supply remains constant. Automatically adjust the charging current as the state of charge of the lithium iron phosphate battery pack changes. If the specified voltage constant value is appropriate, it can not only ensure the full charge of the power battery, but also minimize gas evolution and water loss. This charging method only considers the change of the single state of the battery voltage, and cannot effectively reflect the overall charging status of the battery. Its initial charging current is too large, which often causes damage to the power battery. In view of this shortcoming, constant voltage charging is rarely used.

4. Constant current and constant voltage charging method

 This charging method is a simple combination of the above two. The first stage uses a constant current charging method to avoid excessive charging current at the beginning of constant voltage charging. The second stage adopts the constant voltage charging method, which avoids the phenomenon of overcharging caused by constant current charging. Lithium iron phosphate battery packs, like any other sealed rechargeable batteries, must be controlled for charging, otherwise the batteries will be easily damaged. Lithium iron phosphate batteries generally adopt the charging method of constant current first and then voltage limiting.

When charging, we also need to master certain charging skills. We need to use appropriate chargers, control the charging rate, and avoid overcharging.

Ⅱ. The service lifespan of LiFePO4 battery

The service lifespan of a new lithium iron phosphate battery is usually defined by the following key factors:

1. Cycle Life: This is the most common way to measure battery life. Usually defined as the number of charge-discharge cycles a battery can undergo until its capacity is reduced to 80% of its initial capacity. For example, if a battery has an initial capacity of 100Ah. When its capacity is reduced to 80Ah, we can say that the battery has reached the end of its cycle life.

2. Calendar life: This is the length of time that the battery can be stored under specific usage conditions (such as temperature, humidity, etc.). Even when a battery is not being used, its performance will gradually degrade over time. This degradation is usually related to chemical reactions inside the battery.

3. Capacity fading: This refers to the reduction of the energy storage capacity of the battery after a long period of use. When the capacity of the battery is reduced to 80% of the initial capacity, it is usually considered the end of battery life.

4. Increased internal resistance: During the use of the battery, its internal resistance will gradually increase, which will lead to a decrease in the discharge efficiency of the battery. When the internal resistance increases to a certain extent, the performance of the battery will not meet the requirements of use.

5. Safety: If the battery has safety problems such as liquid leakage, thermal runaway, short circuit, etc., it will also be regarded as the end of battery life.

Among them, cycle life and capacity decay are the main basis for judging the service life of lithium iron phosphate batteries. Generally, when the capacity of the battery decays below 80% of the initial capacity, or the cycle life of the battery reaches the life declared by the manufacturer, we can consider the service life of the battery to be over. However, this does not mean that the battery is no longer usable, just that its performance is no longer up to the original standard.

Ⅲ. Recycling of LiFePO4 battery

1. Recycling process

At present, the recycling process of waste lithium iron phosphate batteries can be divided into fire recycling and wet recycling.

(1) Wet recycling 

It is a relatively mature process used in the recycling process of waste lithium iron phosphate batteries. This is mainly based on the enrichment of metal Li by acid leaching, and the recovery of LiFePO4 with different impurity removal processes. The method has high recovery rate and good separation performance, and is widely used.

(2) Fire recycling 

It is to disassemble and crush the battery and then incinerate at high temperature. The metal elements in the electrode material are converted into stable metal oxides, and then separated and recycled. This technological method has a short flow and no waste water is generated, but the operating environment is unfriendly, the energy consumption is high, and the product obtained has low purity.

2. Recycling steps

Lithium iron carbonate battery recycling includes the following steps:

(1) Decomposition: We decompose the waste battery and separate the government, materials and electrolyte. In this step, we need to take strict safety measures to avoid leakage of chemical substances inside the battery.

(2) Crushing: We need to sort the crushed materials and separate the useful materials for subsequent recycling.

(3) Regeneration: We regenerate the separated useful materials to make them usable again. In this step, the battery needs to undergo high-temperature treatment, chemical reaction and other processes in order to remove the impurities in the material.

(4) Preparation: We prepare recycled materials to make them new battery materials. In this step, we need to carry out fine processing and assembly in order to prepare high-quality batteries.

3. Recycling Features

(1) Immature recycling technology

The existing data show that the recycling of waste lithium iron phosphate batteries is divided into two types: one is to recover metals, and the other is to regenerate lithium iron phosphate cathode materials.

(2) Significant harm

Chemical substances such as LiPF6, organic carbonates, and copper contained in lithium iron phosphate batteries are all included in the national hazardous waste list. LiPF6 is highly corrosive and easily decomposes when exposed to water to produce HF. Organic solvents and their decomposition and hydrolysis products will cause serious pollution to the atmosphere, water, and soil, and cause harm to the ecosystem. Heavy metals such as copper accumulate in the environment and eventually endanger humans through the biological chain. Once phosphorus enters lakes and other water bodies, it is very easy to cause eutrophication of water bodies. It can be seen that if the discarded lithium iron phosphate batteries are not recycled, it will be extremely harmful to the environment and human health.

(3) Rapid growth and large amount of scrap

Since the development of the electric vehicle industry, China is the world's largest consumer market for lithium iron phosphate. Especially from 2012 to 2013, it grew at a rate of nearly 200%. In 2013, the sales volume of lithium iron phosphate in China was about 5797t, accounting for more than 50% of the global sales volume.

In 2014, 75% of lithium iron phosphate cathode materials were sold to China. The theoretical life of lithium iron phosphate batteries is 7 to 8 years (calculated in 7 years), and it is expected that about 9400t of lithium iron phosphate will be scrapped by 2021. If such a huge amount of waste is not dealt with, it will not only bring about environmental pollution, but also energy waste and economic loss.

(4) Incomplete recycling system

The national "863" plan, "973" plan and "Eleventh Five-Year" high-tech industry development plan all divide lithium iron phosphate batteries into key support areas. However, the battery production technology requirements are relatively strict, and the battery price is relatively high, so it is only used in electric motorcycles and a small number of cars. Therefore, vehicle power batteries have not yet been scrapped in large quantities. A systematic and professional vehicle power battery recycling system has not yet been established. There are certain problems in the existing recycling system, and the recycling efficiency is low. This problem is mainly caused by the following aspects:

① Weak awareness of recycling

For a long time, there has been little publicity and education on the recycling of waste batteries in my country, resulting in the lack of in-depth understanding of the pollution hazards of waste batteries and the lack of awareness of conscious recycling.

② Incomplete recycling system 

A system dedicated to recycling batteries has not yet been established in China, mainly the extensive collection of small workshops. my country is a big country in the production and consumption of lithium-ion batteries, but due to its large population, the number of batteries per capita is relatively small. For a long time, recycling companies have not recycled individual lithium-ion batteries that have no recycling value.

③ High recycling cost

A large number of lithium iron phosphate materials are used in the positive electrode of power or energy storage batteries, and the demand is far greater than that of ordinary small batteries. Therefore, recycling lithium iron phosphate has high social value. However, its recycling cost is high, and the lithium iron phosphate battery does not contain precious metals, so its economic value is low.

④ High barriers to entry

Enterprises wishing to engage in the recycling and disposal of used batteries must apply for a hazardous waste business license in accordance with the provisions of the "Environmental Protection Law of the People's Republic of China" and the "Administrative Measures for Hazardous Waste Experience Permits". However, there are not many companies that can achieve large-scale recycling qualifications. On the contrary, there are a large number of small-scale and low-tech companies, which makes it difficult for batteries to be collected centrally.

⑤ Less recyclable amount

A large number of used batteries are scattered in the hands of the people, but the people have no place to put them, so they are disposed of together with domestic garbage, so that the waste batteries recovered from individuals are almost zero. Most of the recycling is waste generated in the production process of manufacturers or old materials in stock, and the number of large power batteries recycled is even less.

Ⅳ. Storage of LiFePO4 battery

The following are considerations and recommendations for lithium iron phosphate battery storage:

1. State of charge: It is recommended to charge the lithium iron phosphate battery to about 50% state of charge before storage. This is an ideal storage charge state. A state of charge that is too high or too low can negatively affect the life of the battery.

2. Ambient humidity: Avoid exposing the battery to a high-humidity environment during storage to prevent moisture from invading the inside of the battery.

3.Charging frequency: There is no fixed charging frequency requirement for lithium iron phosphate batteries, we can charge according to actual needs. It is generally recommended that during storage, if the battery charge drops to a low level (such as around 30%), it should be charged to prevent the battery from being over-discharged.

4. Self-consumption (self-discharge): The self-discharge rate of lithium iron phosphate batteries is relatively low, usually about 1-2% per month. This means that during storage, the battery naturally loses some of its charge. In case of prolonged storage, it may be necessary to periodically check the battery state of charge and perform an appropriate top-up charge.

5. Storage temperature: Lithium iron phosphate batteries should be kept within a suitable temperature range during storage, and the recommended storage temperature is usually around 20°C. Excessively high or low temperatures may affect battery performance and lifespan.

Ⅴ. What is the lifespan of a LiFePO4 battery?

Lifepo4 batteries can last 5 – 10 years when properly maintained. Note that, lithium-iron phosphate batteries last longer based on maintenance. Generally speaking, to prevent poor performance, you need to avoid extreme overcharging or your battery will pack up sooner than expected.

Ⅵ. Do LiFePO4 batteries need to be fully charged?

If you let them drain completely, you won't be able to use them until they get some charge. Unlike lead-acid batteries, lithium iron phosphate batteries do not get damaged if they are left in a partial state of charge, so you don't have to stress about getting them charged immediately after use.

Ⅶ. Can LiFePO4 catch fire?

While all lithium-ion batteries have the potential for thermal runaway, LiFePO4 batteries are less prone to this issue due to their stable chemical structure. They are also incombustible, able to withstand harsh conditions, and less likely to release flammable gases or catch fire in the event of a hazardous event.

Ⅷ. How do LiFePO4 batteries fail?

Battery capacity declines when storage temperature increases. In the storage state, the harsh storage conditions (high temperature and high charge state) will increase the self-discharge of LiFePO4 power cells and make the aging of the cells more obvious.

Ⅸ. Is it bad to completely drain a LiFePO4 battery?

Most lead-acid batteries experience significantly reduced cycle life if they are discharged below 50% DOD. LiFePO4 batteries can be continually discharged to 100% DOD and there is no long-term effect. However, we recommend you only discharge down to 80% to maintain battery life.