Battery Management System Testing

with the development of society, more people are pursuing a "green" lifestyle, and the use of electric energy transportation equipment in the automobile industry has increased significantly. They have very low emissions or zero emissions, and they save the cost of oil in power, but their functions will not be reduced.

their design is a basic change in the automobile industry-a brand-new drive system, technology, etc., as well as new test plans and schemes. With the increase in the use of electric energy in vehicles, the life and use of these tools have brought new testing and inspection challenges to the industry.

in the automotive industry, lithium batteries are often used in many mixing devices and plug-in mixing devices. The design requirement of the battery is to have a very sophisticated system to ensure its long service life and safety, which means that there is a big challenge-designing an efficient battery management system (BMS)-the battery needs to store high levels of energy to use to drive the car.

Figure 1 Battery Management System

In order to assist in the testing of BMS systems, DMC engineers and software engineers have cooperated with Pickering to provide major manufacturers with a BMS testing scheme based on PXI modules that can be used to simulate battery systems. In this article, we will discuss some necessary tests and the reasons for their existence. We will show how to use PXI and why it is the perfect solution to this complex problem.

1. to Establish Battery Management System

there will be an inherent change in the production of lithium batteries, which requires a BMS system with higher performance and good fixation. The BMS system must be able to simulate the state of any battery performance in a module, either through the monitoring of some event or the battery state stack for balancing. A battery stack can combine good and bad batteries, and can also adapt to harsh environmental conditions. During the development and quality inspection of BMS system, the change and use of these battery packs are realized by simulation technology. BMS is a solution that can be used smoothly in the production environment of products.

BMS systems can be used in hybrid motor (HEV), motor (EV) and plug-in motor (PHEV) systems. A typical BMS controls all the functions of a battery storage system (ESS), including voltage and current control of the battery pack, and can be used for battery voltage testing, battery balancing, charge calculation package status, battery temperature and health detection, thus ensuring the safety and optimal operation of the battery pack.

BMS modules and related replacement modules must read voltage parameters from the battery stack and related temperature and current and voltage sensors. From there, BMS processes the input signal, uses logic judgment to control the operation and safety of the battery pack, records the input state and controls the communication between the state and the output through a series of analog and digital components. If you want to test a BMS system efficiently, you should include two basic functions: the first is to accurately simulate the input signals of the sensor and the battery stack in the BMS system; the second is to calculate, collect and process the output of the digital and analog signals obtained by the BMS system processing these inputs.

2. System design

2. Scheme Design

two main reasons for calibrating the BMS of the battery pack separately, including safety and life.

many people know that lithium batteries can be used on notebook computers and mobile phones. The good news is that the power of lithium batteries is 6 times that of ordinary lead batteries and 3 times that of ordinary nickel batteries. In addition, under proper design, the battery pack has a greater possibility of being recycled. However, when you put more capabilities in a small space, this increases usability, but you need to consider the safety of the battery pack even more.

battery is released in the form of current and voltage to provide electrical energy. The release of uncontrollable part of the energy may cause the emission of some toxic substances, such as smoke, sparks, explosions or a mixture of other things. All lithium battery systems use flammable electrolyte, and there is a possibility of withstanding thermal breakdown. When you heat these things up, it will reach a critical temperature. At this time, it will automatically heat up and burn or explode.

out-of-control energy may occur in harsh ways of use, such as impact, breakdown or combustion, which can be slowed down in a mechanical safety system and through correct physical design. However, they may also short-circuit the battery, abnormal release speed, too fast energy accumulation, excessive demand, or continuous charging, which will weaken the battery. These factors can be avoided through the correct design and inspection of battery safety and monitoring systems. These aspects have a name called BMS (Battery Management System).

BMS can also be used to track the exact state of the battery's power supply, which is used to maintain its service life. The usable life of the battery will be reduced during simple overcharging or overdischarging. According to this, BMS must include a very accurate discharge evaluation part. Once you cannot directly calculate the discharge of the battery, the parameters of the discharge state must be calculated by voltage, temperature, current and other parameters based on the manufacturer. A BMS is a system that can perform these tests and calculations. The accuracy of detecting the discharge state of the BMS system can be used to evaluate the performance and life of the battery.

3. Battery Pack

lithium battery has a phosphate negative electrode and a carbon rod as the anode, and has an open circuit voltage of 3.2V and a power supply voltage of 3.6V. The lithium-nickel-manganese-cobalt oxide cathode and the carbon rod anode have a nominal voltage of 3.7V and a maximum discharge voltage of 4.2V. A typical battery stack is 96 batteries connected in series, which provides a voltage of more than 350V. High voltage allows a finer linear drive system to transmit energy, which is more energy-saving than a low-voltage system, but high voltage requires a very careful operation, and any damage to the electronic system must be avoided. Testing and verifying a new BMS system on a real battery stack is not a feasible solution, because the occurrence of an error may cause damage to other parts and also cause injury to the inspector. The BMS can only be connected to the real battery pack if it is guaranteed to be correct. The second problem is that the characteristics of faults and real battery packs cannot be transformed to simulate the controllable state of BMS design. During fault injection, especially during the development process, NPI can be successfully designed by testing software and hardware.

DMS puts forward the requirement for Pickering to make a battery pack that can be used to simulate low power for the purpose of checking the design of BMS. This battery pack is programmed, each battery has an output voltage, and this battery pack can also be used for charging and discharging.

4. Design Challenges

Pickering already has experience in designing a single PXI battery emulator for mobile phones. So, with the passage of time, a feasibility survey has been completed. In order to maintain the compactness and low cost of the system, the final decision is that we should concentrate as many channels as possible in a simple module. In addition, the price of this product is still acceptable in the automotive industry.

But PXI is not an ideal platform designed for high-density, multiple battery emulators, but it is not impossible. That module platform will not limit specifications. However, since all other modules are available and PXI is widely accepted, this shows that we must focus on the PXI scheme.

obviously, the final product must be reliable, compact and safe to use. A PXI chassis can support 18 sub-slots, so according to the design requirements, each slot must simulate 6 OFF of the battery pack-this requires 16 modules in the slot to simulate 96 battery packs. This density limits the size of each battery. During the time of the customer's project, most of the functions can be realized.

each battery needs to provide 300mA current and generate an extra voltage of 4.2V for each battery, it is a great challenge for the chassis backplane to provide enough power for each slot-and we must have a fast enough response speed to simulate a battery.

PXI backplanes can provide up to 6A of current to each module in a 5V power supply-this is currently available in most PXI backplanes. However, 5V DC transmission is not very good at present, so the loss of important energy has to be considered in the design, which will bring thermal load to the chassis. The solution to this problem is to power the battery from a positive 5V power supply, but this also requires the use of a 12V power supply.

each battery emulator uses an integrated isolated DC to DC converter, which is used to provide an isolated power supply and then modulated by a fast linear calibrator. This fast calibrator requirement allows remote simulation at one point without adding additional voltage to the output, and cannot use an output external galvanic capacitor.

this linear calibrator needs to distribute the power under the maximum load, it is obvious that each battery on the module is limited in one space, which limits the heat dissipation effect of the calibrator. The solution to this problem is to use a calibrator specially designed for automotive use. It can withstand high temperatures. Protective measures have been established inside, and PCB copper plates are used for cooling. This efficient cooling system in the PXI chassis makes these copper plates very ventilated-working very well, especially when the equipment is placed in the PXI module.

safety and insulation are also design challenges. In a battery pack of 96 cells connected in series, if each cell has an output voltage of 4.2V, it may cause a fatal output voltage. This balanced battery has to be designed so that the common voltage terminal of each battery exceeds two disconnects, so that the battery can still be controlled. This improved design uses a digital insulator to provide a control interface for the PXI backplane, and a secure dual threaded pin system with default configuration, which allows the user to connect the modules in such a way that it will be closed if the user disconnects the cable of the PXI front panel or the modules or all the modules in the system.

BMS system also simulates the charging state of the battery. Simple power supplies are unidirectional-they are only source current or reverse current, and do not exist at the same time. The BMS system has to be competent for two states, but the reverse current requires less than the source current. This problem can be solved by integrating a controllable current load. This design preloads the power supply. When the BMS is charged, the current load will ensure that the power supply is still the source current.

all the problems and solutions have been discussed, and the final suggestion is to put the new 6-channel battery emulator together, which is what Pickering provides for DMC. After exchanging views on the limitations of the design, the problem has become clear that this is the predetermined scheme for this complete battery pack simulator.图片

Figure 3

5. System

Pickering continuously improved the design scheme and finally built the first module, which is called 41-752. The problem here is not simply defined as hardware design. In order to test and use 41-752 to achieve the goal, the software team and the hardware team work together to test the software compatibility of the module and the design of the new version of software, including the manual software front panel. With this, the user can control the battery on any module.

When the module shows good performance, other modules are produced and sent to DMC. DMC integrated this scheme into their test system and sent it to the end user. After successful testing and some very minor adjustments, the user accepted the product, and then 41-752 also began to sell on the market.

However, due to the use of PXI as a platform, this has caused some problems, such as flexibility in application and the speed of using PXI design. Therefore, some design improvements have been made now. Backplane power supply, PCI control bus, chassis ventilation system and PC-based software, all of which allow improvement at a high speed. In the first 41-752 system, it took only 8 weeks from design to output, which is impossible on other platforms.

41-752 is now available for purchase. It provides 6-channel simulation with a voltage of up to 7V and a current of up to 300mA. This high isolation barrier is graded at 750V, each battery is connected in series, and supports the user's Class D connector. A flexible safety interlock system allows the system to be safely connected and used. The Pickering 40-923 chassis is used to support high-power modules, which can provide energy and the current required by 108 batteries in the battery pack.

Figure 4

Figure 5 The module is integrated into the chassis.

6. other tests

In addition to the battery simulator, the PXI module supports analog temperature sensors, analog and digital I/O interfaces, high-voltage switches at the output of the battery pack, and communication between the CAN port and BMS. This system is composed of two PXI chassis and some external circuits. This complete system is a compact unit, which is embedded in a 1.5-meter frame and meets the requirements of all customers.

Figure 6 Use Site of Battery Management System (BMS)

7. summary

in the coming days, we will see more and more electric vehicles on the road, which may be hybrid or fully electric. In order to ensure the long life of the battery and protect the system from damage, a BMS system will be a very good component. The test will meet the customer's requirements. PXI test system can solve the problems of small size, low cost and flexibility.

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