In the UPS power supply industry, as the backup time required by users is longer and longer, the matching amount of battery and UPS has reached 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 the reform of energy storage components, and also represents the reform pace of UPS power supply industry, which is closely related to UPS host. However, in recent years, the incidents of lithium battery combustion and explosion in mobile phones, notebooks and electric vehicles have once again pushed the safety of lithium batteries to the forefront. Next, let's 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 batteries

Lithium batteries may be thermally out of control when affected by some factors, such as excessive ambient temperature (with external fire source), mechanical damage, serious manufacturing defects inside the battery, etc., but the degree of harm after thermal out of control varies greatly with different materials. In addition, the larger the capacity of a single battery, the more intense the thermal runaway.

Remarks in the figure: LCO lithium cobalt oxide, NCM nickel cobalt manganese, NCA nickel cobalt aluminum, LMO lithium manganese oxide, LFP lithium iron phosphate

Figure 1: decomposition temperature range of different cathode materials

Figure 2: comparison of heat release of different cathode materials

It can be seen that LFP has the highest decomposition temperature and the lowest heat release in lithium battery materials, and will not release oxygen (O2), so if the decomposition is prevented, the occurrence of thermal runaway is prevented, and the system is more safe and reliable.

1.2 lithium iron phosphate battery is resistant to high ambient temperature

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

Figure 3: temperature life comparison diagram of lead-acid battery and lithium iron phosphate battery

1.3 high cycle life of lithium iron phosphate battery

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

Figure 4: relationship between cycle times and discharge depth of lithium iron phosphate battery

1.4 safety test

Lithium iron phosphate battery cells need to pass gb/t 31485-2015 and other relevant certifications, and must pass the following harsh safety verification tests such as nailing, short circuit, overcharge, over discharge, high temperature, extrusion, etc., so as to ensure no fire and explosion in this process.

Figure 5: safety test to be passed by cell and pack

1.5 comparative analysis of lithium batteries in different systems

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

The comparison is as follows:

Figure 6: comparative analysis of lithium batteries in different systems

2、 Requirements of UPS system for BMS

Although BMS cannot solve the fundamental problem of the safety of lithium battery, the stability and adaptability of the system depend on BMS when UPS selects the appropriate single cell lithium iron phosphate battery. Therefore, now the research and development of BMS are mostly industry users, rather than cell manufacturers. For UPS power plants, we have to research and develop ourselves to get close to users and enhance stability and adaptability.

Figure 7: system block diagram of BMS

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

1) Unit equalization voltage: when the unit voltage is greater than this value, the equalization is turned on;

2) Resume charging voltage: after charging protection, when the maximum monomer voltage is greater than this value, resume charging;

3) Restore discharge voltage: after discharge protection, when the minimum monomer voltage is greater than this value, restore discharge;

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

5) 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, a second level alarm is generated, and then the charging contactor is disconnected;

7) Lowest single level-1 alarm value: lowest single level-1 alarm value;

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

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

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

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

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

13) Level 1 alarm value of discharge overcurrent: Level 1 alarm value of discharge current;

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

15) Level 1 alarm value of charging overcurrent: Level 1 alarm value of charging current;

16) Level II alarm value of charging overcurrent: 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 sampling plate and the main board exceeds 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 single voltage acquisition, temperature acquisition and battery equalization.

BCU: collect the total voltage and current; Monitoring and control of the state of the whole battery system; Data display, storage and external communication.

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 ℃;



A single battery is equipped with an equalizing circuit, which can design a maximum equalizing current of 0.1A; Select the balanced current according to the size of the battery.

BCU design:

Adopt 32-bit DSP main control chip; The hardware has various interfaces and LCD interface, providing users with convenient and intuitive interfaces; RS485 interface is provided, which is convenient for computer interface and debugging. There is a 1mbit power down protection memory on the BCU to record the status data during BMS operation, and there is a 4G memory card.

2.3 functional characteristics of BMS

1) System self check

Self check the system when the power is turned on. If everything is normal, send a signal that it can work normally. If there is a problem, send a fault signal and cut off the strong current switch. The system self-test information includes: there is no level 1 and level 2 fault. BMS actuators (self diagnosis of signals at each output port of the controller) and sensors have no faults.

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

When the battery (including the whole system and each module) has overcurrent, overvoltage, undervoltage, unbalanced voltage of single battery, high temperature and large temperature difference, notify the ups host to request to turn off the charging and discharging circuit. When it is not allowed after a certain time, cut off the charging and discharging circuit of the battery pack by itself. When the protective factor of the protected battery disappears, the protection function should be cancelled.

3) Judgment and treatment of battery failure

During the use of the battery pack, record the real-time parameters of the single battery and the battery pack at any time, and judge the effectiveness of the single battery and the battery pack through a certain mathematical model. If it is found that the battery in the system has failed, will fail, and the inconsistency with other batteries increases, notify the ups host through CAN bus communication for battery pack maintenance.

4) Fault warning and handling

During the use of the battery pack, monitor the relevant parameters of the BMS system, determine the current state of the battery pack system through the fault determination conditions, and report it to UPS for treatment.

5) Insulation resistance measurement and high voltage early warning management

It can measure the insulation resistance of the battery pack 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 UPS is powered on, the storage battery system is required to charge the capacitance of the high-voltage bus. Within a certain period of time, determine whether the precharge is successful according to the difference between the terminal voltage of the storage battery and the voltage of the power bus.

7) Can bus communication

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

8) Battery voltage and current measurement

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

9) SOC estimation

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

10) BMS internal battery SOC balance

Monitor the voltage status of the single battery inside the battery pack in real time, and ensure the consistency of SOC of all single batteries through the built-in equalization circuit.

11) Low power consumption of the system

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

12) Voltage measurement of single battery

By simulating the measuring circuit, the voltage of each single battery is measured in real time for BMS system analysis. The voltage measurement accuracy of single battery is ± 5mv.

13) Multi point temperature measurement inside BMS system

By simulating the measuring circuit, the internal temperature of BMS is measured in real time for BMS system analysis and thermal balance control. Temperature measurement range -20 ~ +90 ℃; Temperature measurement accuracy ± 2 ℃.

14) Heat balance function

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

3、 Application case pictures

Figure 8: AVIC Tec UPS lithium iron phosphate battery system test

Figure 9: Field Application of AVIC Tec UPS lithium iron phosphate battery system

  4、 Conclusion

It can be seen from the above that compared with other types of batteries, lithium iron phosphate batteries have the advantages of low cost, best safety and meet environmental protection requirements. Therefore, although they have the disadvantages of low energy density, they are still very suitable for electric vehicle power batteries, large-scale energy storage systems and UPS equipped batteries.

The application of lithium iron phosphate battery in uninterruptible power supply system has full feasibility and safety:

1) Lithium iron phosphate battery technology has been fully mature

In recent years, the electric vehicle industry has developed rapidly, and countries around the world have formulated a timetable for the withdrawal of fuel vehicles. Xin Guobin, Vice Minister of the Ministry of industry and information technology of the people's Republic of China, "some countries have formulated a timetable to stop the production and sales of traditional energy vehicles. At present, the Ministry of industry and information technology has also launched relevant research and will work with relevant departments to formulate China's timetable", which further drives the technological development of lithium iron phosphate batteries and has been fully mature.

2) The safety of lithium iron phosphate battery is guaranteed

Due to different internal principles, compared with ternary lithium battery, lithium iron phosphate battery has no potential safety hazards, and its safety is higher than that of lead-acid battery.

3) The cost of lithium iron phosphate battery has decreased significantly

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

4) Lithium iron phosphate battery can be guaranteed for 10 years

The lithium iron phosphate battery can be fully charged for more than 3000 times, and the lead-acid battery is only 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 is much lower than the UPS system equipped with lead-acid battery.

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

Lithium iron phosphate batteries with the same capacity occupy only 1/3 to 1/2 of the area and volume of lead-acid batteries, which can significantly reduce the installation and maintenance costs.

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