What are the characteristics of nickel-metal hydride batteries? Why is Toyota still using nickel-met

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When it comes to hybrid power, in addition to Honda, a latecomer, Toyota, which is the first to bring this technology to our country, is reliable in the domestic market. Obviously Toyota is eating this sweetness. In January 2019, the sales of the eighth-generation Camry reached 19,720, of which hybrid models accounted for 21%. The more affordable compact model Ralink sold 26,681 units in January, of which hybrid models accounted for 20%.

However, many consumers still have a question about hybrid vehicles. Why do most new energy vehicles (such as Tesla, Weilai, BYD, etc.) use lithium-ion batteries? Toyota also blindly focuses on using nickel-metal hydride batteries. ? Today, when our daily necessities have long used lithium-ion batteries, does the use of nickel-hydrogen batteries seem outdated? Is this a way to reduce production costs when leaving the factory? In fact, the use of nickel-metal hydride batteries in hybrid vehicles has considerable advantages. Not only Toyota, but also hybrid vehicles of many brands such as Ford and General Motors choose nickel-metal hydride batteries as the storage medium for electric energy.

The rechargeable battery with a voltage of 1.2V that we use daily is a nickel-metal hydride battery.

1. There are thousands of kinds of batteries, safety first

The reason why Ni-MH batteries can become the first choice for many cars is that they have unparalleled safety and reliability. On the one hand, the nickel-hydrogen battery electrolyte is a non-flammable aqueous solution. On the other hand, the specific heat capacity and heat of evaporation of the nickel-metal hydride battery are relatively high, while the energy density is relatively low, which means that even if extreme abnormalities such as short-circuit and puncture occur, the battery temperature rise is not large and will not cause combustion. . Finally, as a relatively mature battery product, the product quality control of Ni-MH battery is less difficult and the yield rate is higher.

As of the end of 2014, more than 73% of the world's hybrid electric vehicles used nickel-metal hydride batteries, with a total of more than 8 million. These hybrid electric vehicles have experienced serious safety accidents caused by the batteries during their use. As the representative of commercial hybrid vehicles, Toyota Prius, thanks to the excellent charging and discharging procedures, the batteries of these vehicles have not experienced significant life decay after ten years of use. Therefore, mature Ni-MH batteries are currently the most valuable batteries for commercial applications.

Foreign testers conducted artificial uninterrupted charging tests on the battery pack of Prius, and the battery pack did not cause serious safety accidents.

2. Shallow charging makes long life

Secondly, Ni-MH batteries have good fast charge and discharge performance. The battery capacity of hybrid electric vehicles is not large. For example, the latest eighth-generation Camry Dual Engine has a battery capacity of only 6.5 kWh, which is more than half of the capacity of plug-in hybrids, which can easily exceed 10 kWh. However, due to the working mode of the hybrid power system, the battery must be charged and discharged quickly, and under this working condition, the nickel-hydrogen battery has more advantages than the lithium-ion battery.

Although the energy density of Ni-MH batteries is only 60-80% of that of Li-ion batteries (Li-ion batteries are 100J/m), the characteristics of Ni-MH batteries require lower requirements for safety protection, temperature control, etc., so they do not need to be bulky. Easily find your own place in a hybrid car.

The special power system of hybrid electric vehicles, under a reasonable power output strategy, generally only uses 10% of the battery capacity during driving. Even in the most extreme cases, the maximum battery usage is only about 40%. In other words, about 60% of the battery’s power is never used. This battery management strategy called shallow charging is related to the improvement of nickel-chromium batteries and the greatly improved battery memory effect of nickel-metal hydride batteries. Greatly extend its service life, the number of charge and discharge cycles can reach more than 10,000 times.

The US 'Consumer Report' surveyed more than 36,000 Prius car owners and concluded that the vehicle has high reliability and very low cost of use. In response, 'Consumer Reports' used a first-generation Prius that had traveled 330,000 kilometers for 10 years and a new first-generation Prius that had traveled only 3,200 kilometers a decade ago to conduct the same standard fuel consumption test and performance test. The results show that the old car and the new car that have been used for 10 years of driving 330,000 kilometers maintain the same level in terms of fuel consumption and power performance, indicating that the nickel-metal hydride battery pack and hybrid system are still working normally.

Many overseas media have high reliability evaluations of Prius hybrid electric vehicles that use nickel-metal hydride batteries.

Since 2015, new energy vehicles (pure electric and plug-in hybrid) have become popular in the domestic market. Because of this type of lithium-ion battery, the battery life will be reduced after a certain number of years, and the power will be charged in the low temperature environment in winter. The obvious attenuation has caused many car owners to have obvious endurance anxiety during use. And this result is precisely caused by the characteristics of lithium-ion batteries. Therefore, for new energy vehicles with a vehicle age of 3-4 years, the highest value retention rate is only 45%, which is much lower than the lowest 60% of fuel vehicles (the same vehicle age).

3. Environmentally friendly batteries can make environmentally friendly cars

Although lithium-ion batteries have no memory effect, the number of charge and discharge cycles is generally only about 600, and the battery life is greatly reduced in the complex use environment of high current, fast charge, fast discharge, and overcharge and overdischarge. In addition, because lithium-ion batteries use organic electrolyte solutions, the resistance increases rapidly at low temperatures, and their performance at 0°C has been greatly attenuated, which cannot meet the requirements of normal use at -10°C. In contrast, nickel-metal hydride batteries, due to the use of alkaline electrolyte aqueous solutions, their working temperature can be as low as -40 ℃, so in winter, the performance of hybrid vehicles, such as power and economy, does not change significantly.

Ni-MH batteries have fewer components, high recycling value, and low recycling difficulty.

Finally, because Ni-MH batteries do not contain highly toxic substances, they are more environmentally friendly. The important components of nickel-metal hydride batteries are nickel and rare earths, which have high recycling value (with residual value) and low recycling difficulty. They can basically be recycled and reused, achieving sustainable development of materials. They are known as the most environmentally friendly batteries.

In contrast, lithium-ion batteries are much more difficult to recycle. The chemically active nature of lithium-ion batteries is that the technical route for their recycling is quite complicated. The batteries must first be pre-treated, including discharge, disassembly, crushing, and sorting. After disassembly, the plastic and metal casings can be recycled, but at a high cost: because the residual voltage is still as high as hundreds of volts (excluding 18650 batteries), and there is a certain degree of danger; the battery casing is not self-removable for safety reasons. It takes a lot of effort to open the form; in addition, the cathode materials of lithium-ion batteries are also varied, and the acid-base solutions for recycling are highly demanded. With current technology, lithium-ion battery recycling is undoubtedly a loss-making business.

In addition to the above, Ni-MH batteries have the advantages of stable discharge characteristics, smooth discharge curves, and low heat generation. Therefore, before a major breakthrough in battery technology, this relatively low-energy density nickel-hydrogen battery is still the best partner for hybrid vehicles that do not require high battery power. The industry continues to reflect on the safety of electric vehicles, and the blind pursuit of high energy density has become the focus. Many professionals pointed out that, theoretically, battery energy density is inversely proportional to safety. The company pursues high energy density, and safety issues are exposed. Although it is not clear how much the fire incidents that have occurred are related to the pursuit of energy density, as high-nickel ternary batteries enter the market, new energy vehicles are facing higher safety technical requirements.

How to strike a balance between high energy density and improved safety has become a major problem that needs to be solved urgently in the industry. Each company has improved overall safety from multiple levels of single cell, module design, and battery pack structure design.

Security anxiety

In 2018, there were several recalls of new energy vehicles, and the new energy vehicles, which are gradually getting out of mileage anxiety, seem to be in a dilemma of safety anxiety. This is a sentiment from an executive of a listed company in the new energy vehicle equipment industry. Fire incidents frequently enter the public's field of vision, and new energy vehicle safety issues are being mentioned more and more frequently.

At the end of October 2018, the relevant person in charge of the State Administration for Market Regulation said that according to public opinion information, there have been more than 40 new energy vehicle fires in 2018. In view of the fact that vehicle fire accidents may be related to factors such as electrical wiring and fuel power cells, and part of the evidence disappears or changes during the combustion process, defect investigation is very difficult. As of the end of October 2018, the bureau had organized the defective product management center to initiate 10 new energy vehicle defect investigations, conducted 5 on-site fire accident investigations with relevant departments, and urged relevant production companies to implement 5 recalls. Among them, the recall involves 35,600 defective vehicles from 24 models of 5 companies, mostly due to electronic control and mechanical failures.

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