Safety analysis of lithium battery in UPS application and the requirement of BMS for the system (1)

In the UPS power supply industry, with the increasing backup time of user demand, the ratio of battery and UPS reaches 1:1 on average, and even more than 2:1 in some industries. Therefore, as an energy storage component, battery plays an important role in UPS system. Lithium battery features smaller volume, lighter weight and long cycle life, which has become the focus of energy storage component reform, also represents the reform pace of UPS power industry, and is closely related to the UPS. But in recent years, the explosion of lithium batteries in mobile phones, laptops and electric vehicles has once again pushed the safety of lithium batteries to the forefront. So the following we will talk about the safety analysis of lithium battery in UPS application and the requirements of the system for BMS.


1. Safety analysis of lithium iron phosphate battery

1.1 Factors affecting the safety of lithium battery

The thermal runaway of lithium battery may occur when it is affected by some factors, such as high ambient temperature (with external fire source), mechanical damage, serious manufacturing defects inside the battery, etc., but the damage degree of thermal runaway is quite different due to different materials. In addition, the larger the capacity of the cell, the more intense the thermal runaway.

LFP has the highest decomposition temperature and the lowest heat release in lithium battery materials, and does not release oxygen (O2), so if the decomposition is restrained, the occurrence of thermal runaway will be restrained, and the system will be safer and more reliable.


1.2 High environmental temperature resistance of lithium iron phosphate battery

The service environment of lithium iron phosphate battery below 50 ℃ will not affect the battery life, and will not be as severe as the requirements of other battery life for ambient temperature.


1.3 High cycle life of lithium iron phosphate battery

Under the condition of 1C charging and discharging, lithium iron phosphate battery can maintain 80% capacity after 2500-3000 cycles.


1.4 Safety test

The lithium iron phosphate battery core shall pass GB / T 31485-2015 and other relevant certification, and must pass the following severe safety verification tests, such as nailing, short circuit, overcharge, over-discharge, high temperature and extrusion, so as to ensure no fire and explosion in the process.


1.5 Comparison and analysis of lithium batteries of different systems

At present, the supporting battery of UPS is lithium iron phosphate battery, which has the characteristics of high safety, environmental protection, multiple cycles and strong temperature resistance.

The comparison is as follows:


Battery Category

Energy Density

Wh/kg

Power Density

W/kg

Safety

Remarks

Lithium iron phosphate

~125

higher

high

The calculated energy density of mass-produced aluminum shell core is up to 100 at pack level; the voltage platform is very flat and the energy is well developed; the energy storage is high and cheap. Almost no thermal runaway (the temperature of thermal runaway is above 800 ℃), safe.

Ternary lithium

~170

high

medium

The calculated energy density of mass-produced aluminum shell core can exceed 200 ℃ in the future; the runaway temperature is more than 200 ℃, which needs to be combined in many aspects to meet the safety requirements

Lithium manganate

~110

higher

medium

The biggest problem is that it is easy to dissolve at high temperature, doping and surface treatment are improved, but its stability and safety are not as good as that of iron lithium.

Lithium manganese iron phosphate

~150

low

higher

The material maturity is low, the electronic resistance is high and the service life is poor.

Lithium titanate

~80

high

high

Application technical problems: 1) low platform voltage, many system connections; 2) high temperature gas expansion; 3) high cost.


2. Requirement of UPS system for BMS

Although BMS can not solve the fundamental problem of lithium battery safety, when UPS chooses the appropriate single cell as lithium iron phosphate battery, the stability and adaptability of the system depend on BMS. Therefore, the research and development of BMS are mostly industry users rather than core manufacturers. For UPS power plants, we have to develop our own, so as to be close to users and enhance stability and adaptability.


2.1 In application, the requirements of UPS system for BMS are as follows:

1) Monomer equalizing voltage: when the monomer voltage is greater than this value, turn on equalizing;

2) Recovery charging voltage: after charging protection, when the maximum single voltage is greater than this value, recovery charging;

3) Recovery discharge voltage: after the discharge protection, when the minimum single voltage is greater than this value, the recovery discharge;

4) System shutdown monomer value: when the lowest monomer is less than this value, the system will automatically power off and shutdown;

5) The highest single level 1 alarm value: the highest single level 1 alarm value;

6) Second level alarm value of the highest monomer: when the highest monomer is greater than this value, the second level alarm will be generated, and then the charging contactor will be disconnected;

7) The lowest single level 1 alarm value: the lowest single level 1 alarm value;

8) Second level alarm value of the lowest unit: when the lowest unit is less than this value, the second level alarm will be generated, and then the discharge contactor will be disconnected;

9) First level alarm value of maximum temperature: when the maximum temperature is greater than this value, charging is prohibited;

10) Second level alarm value of maximum temperature: when the maximum temperature is higher than this value, the charging and discharging contactors are disconnected, and the yellow light and red light are on;

11) Minimum monomer charging off value: during charging, when the minimum monomer is greater than this value, the charging contactor is disconnected and the charging is closed;

12) Discharge current charging closing value: when the discharge current is greater than this value, the charging contactor is disconnected;

13) Discharge overcurrent level I alarm value: discharge current level I alarm value;

14) Discharge overcurrent secondary alarm value: when the discharge current is greater than this value, the discharge contactor is disconnected;

15) Charging overcurrent level I alarm value: charging current level I alarm value;

16) Charging overcurrent level II alarm value: when the charging current is greater than this value, the charging contactor is disconnected and it is not allowed to resume charging within 3 minutes;

17) Communication time overflow value: when the communication time between the single sample and the main board is longer than this value, the charging and discharging contactors are disconnected, and the yellow light and red light are on;

18) Current battery capacity value: configure battery capacity.


2.2 BMS functional analysis

1) System composition of BMS

BMU: responsible for voltage collection, temperature collection and battery equalization.

BCU: it is responsible for the collection of total voltage and current, the monitoring and control of the status of the whole battery system, data display, storage, external part communication and other functions.


2) BMS design

BMU design:

Each module is equipped with a BMU, and the main control chip adopts 16 bit DSP;

It can monitor 12 lithium batteries, and the voltage measurement accuracy of single battery is ± 5mv. Each battery scanning cycle is 200ms; each LECU is designed with 6 temperature measurement points to accurately obtain the internal temperature distribution of the module and provide accurate temperature information for BMS. Temperature measurement range - 30 ~ + 90 ℃; temperature measurement accuracy ± 2 ℃;

The single battery is equipped with equalizing circuit, and the maximum equalizing current can be designed as 0.1A; the equalizing current is selected according to the battery size.


BCU design:

It adopts 32-bit DSP main control chip; the hardware has various interfaces, and has LCD interface, providing convenient and intuitive interface for users; providing RS485 interface, convenient and computer interface, convenient for debugging, BCU has 1mbit power-off protection memory, recording the status data during the operation of BMS, and 4G memory card.


2.3 Functional characteristics of BMS


1) System self test

The system will conduct self-test when the power is turned on. If everything is normal, send out a signal that can work normally. If there is a problem, send out a fault signal and cut off the strong current switch. The system self-test information includes: no primary and secondary faults. There is no fault in each actuator (signal self diagnosis of each output port of controller) and sensor of BMS.


2) Charging protection, discharge protection, thermal protection, overcurrent protection, safety wire protection

In case of over-current, over-voltage, under voltage, unbalanced single battery voltage, high temperature and large temperature difference of battery (including the whole system and each module), inform UPS host to request to turn off the charging and discharging circuit, and cut off the charging and discharging circuit of battery pack when it is not allowed after a certain time. When the protection factor of the protected battery disappears, the protection function will be cancelled.


3) Judgment and treatment of battery failure

During the use of the battery pack, the real-time parameters of the single battery and battery pack are recorded at any time, and the validity of the single battery and battery pack is judged by a certain mathematical model. If it is found that there is a battery failure in the system, which is about to fail, and the inconsistency with other batteries increases, the ups host is informed through the CAN bus communication mode to maintain the battery pack.


4) Fault warning and handling

During the use of battery pack, monitor the relevant parameters of BMS system, determine the current status of battery pack system through fault judgment conditions, and report to UPS for processing.


5) Insulation resistance measurement and high voltage electricity early warning management

It can measure the insulation resistance of the battery to the shell in real time, and judge whether the insulation strength of the system meets the requirements according to the insulation resistance.


6) Power bus precharge function

When the UPS is powered on, the battery system is required to charge the capacitance of the high voltage bus. In a certain period of time, the precharge is confirmed to be successful according to the difference between the battery terminal voltage and the power bus voltage.


7) Can bus communication

Can bus is used to communicate with subsystem module, UPS host and charger.


8) Battery voltage and current measurement

It can measure the current working voltage and current of the battery pack in real time. At the same time, according to the collected voltage and current data of the battery pack and the SOC of the battery pack, it can calculate the maximum discharge power that can be output and the maximum charging power that can be accepted. The accuracy of battery voltage measurement is ± 0.5V, and the accuracy of current measurement is 0.5%.


9) SOC estimate

In the process of real-time charging and discharging, the capacity of battery pack can be monitored online, and the remaining capacity of the whole system of battery pack can be given at any time.


10) SOC equalization of BMS internal battery

Real time monitoring of the voltage status of the single battery in the battery pack, through the built-in equalization circuit, to ensure the consistency of SOC of all single batteries.


11) Low power consumption of the system

According to the actual situation of BMS system, it can turn on or off the power of subsystem; on the basis of judging the completion of equalization, it can enter the ultra-low power consumption mode of self closing.


12) Single cell voltage measurement

Through the simulation measurement circuit, the voltage of each cell is measured in real time for BMS system analysis. The measurement accuracy of cell voltage is ± 5mv.


13) Multi point temperature measurement in BMS system

The internal temperature of BMS can be measured in real time through the simulation measuring circuit for analysis and heat balance control of BMS system. The temperature measurement range is - 20 ~ + 90 ℃; the temperature measurement accuracy is ± 2 ℃.


14) Heat balance function

Through the analysis of the internal temperature, the air inlet and exhaust control the cooling fan to ensure the temperature balance in the BMS system, and each module ensures two temperature probe points.





3.Conclusion

It can be seen from the above that, compared with other types of batteries, lithium iron phosphate battery has low cost, the best safety and meets the environmental protection requirements. Therefore, although it has the disadvantages of low energy density, it is very suitable for electric vehicle power battery, large-scale energy storage system and UPS equipped with batteries and other occasions.


It is feasible and safe to apply the lithium iron phosphate battery to the UPS system:

1) Lithium iron phosphate battery technology has been fully mature

In recent years, with the rapid development of electric vehicle industry, countries all over the world have formulated the exit schedule of fuel vehicles. Xin Guobin, Vice Minister of the Ministry of industry and information technology of China: "some countries have formulated timetables to stop production and sales of traditional energy vehicles. At present, the Ministry of industry and information technology has also started relevant research and will work out timetables with relevant departments in China." this further promotes the technical development of lithium iron phosphate battery, which has been fully mature.


2) The safety of lithium iron phosphate battery is guaranteed

Due to different internal principles, compared with the ternary lithium battery, the lithium iron phosphate battery does not have any potential safety hazards, which is higher than the lead-acid battery.


3) The cost of lithium iron phosphate battery has been greatly reduced

Taking 30 minutes of UPS discharge with 100kVA as an example, the one-time input cost of lithium iron phosphate battery is only 1.2 times of that of lead-acid battery, which is equivalent to the whole system, and the total cost is only increased by less than 10%.


4) Lithium iron phosphate battery can last for 10 years

The lithium iron phosphate battery can be fully charged and discharged for more than 3000 times, and the lead-acid battery only has about 300 times. Therefore, when used as a UPS, the lithium iron phosphate battery can be used for more than 10 years. In terms of the cost of the whole life cycle, it has been significantly lower than the UPS system equipped with lead-acid battery.


5) The volume of lithium iron phosphate battery is greatly reduced

The same capacity of lithium iron phosphate battery occupies only 1 / 3 to 1 / 2 of the area and volume of lead-acid battery, which can greatly reduce the installation and maintenance costs.


To sum up, in UPS power supply industry, with the technical progress of lithium iron phosphate battery cell factory, the maturity of UPS factory BMS, and the improvement of safety, stability and adaptability of lithium battery in UPS application, it will bring a revolution of UPS energy storage components in a certain period of time, which is the inevitable result of technological development and social progress.



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