0 Introduction
Nickel-hydrogen (MH-Ni) batteries are currently the mainstream batteries for hybrid electric vehicles. With the development of economy and technology, my country has made great progress in the research and development of hybrid electric vehicles. At present, hybrid electric buses are gradually entering the market. Regarding the selection and application of MH-Ni power sources for hybrid electric vehicles, different companies have different opinions, mainly focusing on several issues, such as the judgment accuracy of SOC and the consistency of batteries. Based on the actual supporting applications of our products in hybrid electric vehicles in recent years, we have accumulated some experience and put forward some of our own views for discussion with you.
1 Selection of power source for hybrid electric vehicles
The main discussion here is about the selection of battery capacity and power for hybrid electric vehicles, taking MH-Ni battery as an example, but it also has certain reference value for batteries for other hybrid electric vehicles.
1.1 Selection of battery capacity
Different vehicle designs have different requirements for batteries. Currently, hybrid electric vehicles mainly include micro-hybrid (42V power system) and full hybrid, and full hybrid includes series type, parallel type, hybrid type, and external charging type (PHEV). Relatively speaking, the parallel type has lower requirements for battery capacity, while the series type and PHEV require higher capacity. Regardless of the form of electric vehicle, the calculation method for battery capacity requirements is roughly the same, that is, the required battery capacity is calculated based on the actual operating conditions of the battery.
For example, for Beijing working conditions, the specific system is:
a: 160A discharge for 30s, rest for 100s; b: 40A charge for 30s, rest for 100s (17 cycles); c: 50A charge for 30s, 190A discharge for 40s (4 cycles); d: rest for 80s; e: 50A charge for 30s, rest for 103s (6 cycles).
From the above working cycle, we can see that the highest output capacity in continuous charging or discharging is in step c (reaching 6.77Ah). In HEV, the normal application SOC range of MH-Ni battery is 30%~80% (the Japanese Toyota Prius car uses 40% capacity). Assuming that after step b, the battery SOC reaches the intermediate state of 55% SOC, at this time the battery discharges from 55% SOC to the lowest point of 30% SOC, and at least 6.77Ah capacity must be released. Then the minimum capacity of the battery required is:
6.77Ah/(55%-30%)=27.08Ah In the actual selection process, considering the battery life and application reliability, a certain redundancy is generally required. According to the US Freedom-CAR hybrid electric vehicle battery test manual, the redundancy can be set at 30%. The battery capacity should be:
27.08Ahxl30%=35.2Ah, which means a battery of about 35Ah should be selected.
When there are other auxiliary electrical appliances in the car (such as air conditioning), their corresponding power consumption should be considered before calculation.
1.2 Battery power selection
Hybrid electric vehicles have relatively high requirements for battery power. For batteries, increasing battery power will increase the cost, weight, or volume of the battery accordingly. For the entire vehicle, as long as the battery power can meet the application requirements, it is not appropriate to excessively pursue indicators. Also based on the above Beijing operating conditions, the battery capacity is selected as 35Ah. Calculate the required battery power based on the worst case. Step c requires the highest power, the discharge current is 190A, and the duration is 40s. Assuming that this requirement needs to be met at 30% SOC, the minimum discharge voltage of the battery is not less than 0.9V. The discharge rate of the battery is:
190A/35Ah=5.4C Assuming that the average discharge voltage of the battery is 1.15V at this discharge rate and 30% SOC, the required battery power is at least:
1.15V×190A=218.5W also requires 30% redundancy for the battery power. The power requirement for continuous discharge of the battery for 40s at 30% should be:
218.5Wxl30%=284W Under low temperature conditions, the discharge power of MH-Ni batteries will be greatly affected, and the power requirements for batteries at low temperatures should also be relatively low, mainly for starting power. For example, the peak power requirement for power-assisted hybrid electric cars in the United States at room temperature is 20kW, while the starting power requirement at low temperatures of ~30qC is 5kW. The specific detection method is shown in Table 1.
2. Battery consistency
Battery consistency is a problem that everyone often mentions, but there is no clear concept of what battery consistency is, what is the basis and method for judging it, and what consistency level should be achieved. Consistency should refer to the differences between various parameters such as voltage, internal resistance, SOC, capacity, battery decay rate, self-discharge rate, and the rate of change of each parameter at any time after batteries of the same specifications and models form a battery pack, as well as whether the performance differences caused by external conditions such as temperature are consistent. The main purposes of requiring battery consistency are as follows: first, to protect the battery from overcharging and over-discharging; second, to improve system reliability and life; third, to avoid excessive differences in battery performance that affect the normal use of the system.
There is no specific assessment standard for consistency. From the actual operation, considering the entire battery system, some parameters are not easy to detect, such as the battery decay rate, the self-discharge rate of each battery, the change rate of each parameter, the DC internal resistance of the battery, etc., so these parameters are not easy to list as assessment standards. Battery capacity, voltage, ohmic internal resistance and system temperature uniformity are relatively easy to detect and can be used as assessment standards.
To what extent should each parameter be considered to meet the consistency requirements? We believe that as long as the purpose of consistency can be achieved, these parameters will be discussed in detail below.
2.1 Battery Capacity
During the application process, the consistency of battery capacity cannot be tested, and the consistency of the battery capacity in the initial state (that is, before the system battery pack is assembled) should be mainly assessed.
In the previous discussion of the SOC situation, we can know that the maximum allowable deviation of SOC is 13%, which is also the maximum allowable requirement for battery capacity difference. That is, if the deviation is lower than this, it will not affect the normal application of the system.
In actual production, most manufacturers currently control it at 5%, which can fully meet the requirements of electric vehicles.
2.2 Battery voltage
The consistency of battery voltage is closely related to SOC. First, we need to determine the method for judging voltage consistency. Below 20% SOC, the voltage difference of the battery is quite obvious. For example, 0~1.2V can be regarded as SOC=0%. At 20% SOC, the voltage is around 1.25V, and the voltage difference with 0% SOC is at least 50mV (at this time, a 10-combination battery module may reach more than 500mV). At 20%~80% SOC, the voltage difference is very small, and the voltage is in the range of 1.25-1.35V. This voltage refers to the open circuit voltage of the battery, and the open circuit time is generally longer (more than 4h). As the open circuit time increases, the voltage difference in the range of 20%~80% SOC will be smaller. For example, if the self-discharge is left for 28d, the voltage difference may be only about 30mV. Above 80% SOC, the voltage will also be quite different. Therefore, it is impossible to formulate a unified standard based on the open circuit voltage. From the perspective of consistency, the main thing is not to affect the normal application of the system, so the consistency of the battery during use should be considered.
From the battery charge and discharge curve, we can know that when the SOC is lower than 20% and higher than 80%, the battery voltage changes dramatically, so it is not very meaningful to examine the consistency beyond this range.
The actual SOC range is between 20% and 80%, and the voltage is relatively stable. The consistency of the voltage of each battery in this range should be mainly examined. Based on actual production experience, when charging or discharging in this range, the voltage difference of each battery should not exceed 5mV (regardless of the SOC).
2.3 Consistency of internal resistance
Internal resistance is divided into ohmic internal resistance (AC internal resistance at standard 1kHz) and DC internal resistance (measured by short-time charging or discharging with large current). The internal resistance of MH-Ni batteries used in HEV is relatively small. For example, the internal resistance of a 40Ah battery is normally 1~1.2mQ, but its detection accuracy is closely related to the measuring equipment. As long as there is a slight deviation in the operation of some equipment and instruments, the measurement error will be relatively large, and the normal error is about 20%. Generally speaking, as long as the battery is qualified and the performance is normal, the battery performance will not be much different within this deviation range. However, beyond this range, the battery performance may be different, such as cold soldering during the battery manufacturing process. Although it has no significant impact on the battery capacity, it will affect the power performance, which can be distinguished by the detection of ohmic internal resistance. Therefore, the internal resistance deviation is generally controlled within the range of ±20%.
DC internal resistance can better reflect the consistency of internal resistance during battery application. However, it is impossible to test the DC internal resistance of each battery during production and application, so it is not suitable as a basis for assessment.
Specifically involving the entire power system, another issue to consider is the connection resistance between batteries, which must be controlled for consistency. Because this part has a slightly larger resistance, it will heat up during use, which will cause a series of problems. However, the internal resistance of this part is very small, only a few milliohms, and it is not easy to detect directly. It can be controlled by detecting the voltage drop on it during the charging and discharging process. Its consistency can be controlled according to the battery connection method, process, etc.
2.4 Temperature uniformity
Temperature is one of the factors that has the greatest impact on the performance of MH-Ni batteries. Uneven temperature not only affects the consistency of battery capacity and the determination of SOC during use, but more importantly, it will accelerate the decay of some batteries with high temperatures, thus affecting the service life of the entire system. Temperature consistency is mainly for the design of system cooling structure, which refers to the degree of difference in ambient temperature around each battery in the power system during use.
Studies on the discharge power, capacity and charging efficiency of MH-Ni batteries at different temperatures show that at 0~30℃, the battery power changes by 4%~5% for every 5℃ change in temperature (increases with increasing temperature); below 0℃ and above 30℃, the power changes by 2%~3% for every 5℃ change in temperature; above 0℃, the ambient temperature has little effect on the discharge capacity, but below this temperature, the discharge capacity differs by 30%~50% for every 10℃ difference; as for the charging efficiency, at 30-50℃ (the maximum temperature limit for electric vehicles is generally 50℃), the charging efficiency (Coulomb efficiency) will decrease by about 5% for every 5℃ increase in temperature.
As the temperature rises, the corrosion rate of the alloy increases. Panasonic's research shows that when the ambient temperature rises from 60℃ to 70℃ and 80℃, the life coefficient of the hydrogen storage alloy decreases from 1.59.79_0.40 respectively. That is, starting from 60℃, the alloy life is shortened by half for every 10℃ increase in temperature. In the application process of hybrid electric vehicles, the maximum temperature is generally controlled not to exceed 55℃.
The main problem of MH-Ni battery during use is high temperature. On the one hand, the maximum application temperature should be controlled to avoid problems such as thermal runaway. On the other hand, according to the above analysis, the difference in battery capacity should not exceed 5% in production. During use, the difference in ambient temperature of each battery in the battery pack should not exceed 5°C. The temperature difference of the battery pack of Toyota Prius in Japan is controlled to be no more than 5°C (it can reach 10°C at a lower ambient temperature), while that of Honda Insi Qut is relatively low, no more than 3°C.
3 Other problems in the current research process of automotive power systems
3.1 Dispersion of research
At present, each component and the whole vehicle are studied in different units, and there are great problems in combining the two. The whole vehicle product should be the leader, and the design of each component and the whole vehicle should be comprehensively considered. From the perspective of ensuring application and protecting batteries, comprehensive design can be carried out to improve the reliability and service life of the whole vehicle system. In this regard, the combination of Toyota and Panasonic in Japan is relatively good. The design of the power system in its Prius car is closely integrated with the design of the whole vehicle, and the space and integrity of the car are fully considered, such as the structure of the power system, the electromagnetic compatibility of the management system, and other issues; with the deepening of research, its power system has been reduced from the first generation of 288V to the current 202V, which requires a series of changes to the motor, the whole vehicle system, etc. This cannot be done without full penetration of both parties.
In China, this aspect is far from enough. Many units also conduct research on battery and power management systems in different units, which brings another level of integration problem. Management system and battery research are two completely different industries, and it is difficult for both parties to penetrate each other's products. Not to mention vehicle units and battery production units. In China, most vehicle units make requirements to battery manufacturers according to their own needs.
3.2 System Reliability
Reliability is the most important indicator when entering the market. At present, most of our power systems have only been tested by the whole vehicle for a short time. Basically, it started from the "15th Five-Year Plan", during which the system and the whole vehicle have been continuously modified. The longest continuous driving time on the whole vehicle may not exceed 3 years, and the existing problems are still being dealt with.
4 Conclusion
In fact, the whole vehicle system in my country may not have been running continuously without fault for more than 2 years. Strengthening the reliability research of vehicle power system and whole vehicle is the key issue currently faced.
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