What is the bipolar nickel-metal hydride battery used in the new Aqua?

Battery performance has been said to be "lithium ion > nickel metal hydride", but is it too late?

The "bipolar nickel-metal hydride battery" used in the new Aqua has become a hot topic. According to the article, this battery is described as "battery output doubled" and "powerful and smooth acceleration", and fuel efficiency has improved significantly. Bipolar nickel-metal hydride batteries, are they really that great? "I want to properly introduce myself as a hybrid master."

First of all, the conclusion is that the performance (specs) of the new Aqua as a power unit is exactly the same as its siblings Yaris and Yaris Cross. Both models are equipped with a lithium-ion battery. It's true that the battery output and acceleration are significantly improved compared to the previous generation Aqua, but it's not the bipolar Ni-MH battery that made it possible to achieve high performance.

In the first place, Toyota's hybrid system called THS-II, engine output, motor output, and battery output cannot be changed easily. That's why Mazda vehicles using THS-II have exactly the same output. If we were to introduce the performance of the bipolar nickel-metal hydride battery, it would be "same as the lithium-ion battery used in Yaris and other vehicles."

This is where it gets interesting. THS-II has been using nickel-metal hydride batteries and lithium-ion batteries together in the same model. In that case, the base is the former with low battery performance. Therefore, even if a high-performance lithium-ion battery was used, it could not be made smaller, and the performance could not be extracted. Until now, hybrid vehicles have been held back by low-performance batteries.

For that reason, I once asked Toyota's battery development staff, "Why don't you stop using nickel-metal hydride batteries?" Then, "There is still room for development (laughs)." It was a secret weapon, a bipolar nickel metal hydride battery. In terms of specific performance, the new battery seems to have the same performance as the lithium-ion battery in the same space!

No, to be precise, the capacity of the lithium-ion battery is 4.3Ah for the same size. A bipolar nickel-metal hydride battery is 5Ah, so it is rather small. In this case, the advantages of nickel-metal hydride batteries that are inexpensive, have stable performance and durability (both the previous generation Prius and the previous generation Aqua have a battery life of 300,000 km or more) will come out! "No, no, the evolution of technology is interesting."

By the way, what is a bipolar type? Ordinary batteries have independent cells, just like dry batteries. As shown in the illustration, the inside of the cell consists of a positive electrode, a negative electrode, and a separator together with current collectors for conducting electricity. Since the bipolar type has + and - poles attached to one current collector, it will be a simple structure if you can understand that it cannot be separated.

If you are a railway fan, you can imagine an "articulated truck" like the Odakyu Romancecar. The simple structure eliminates wasted materials and space. As a result, if the size is the same, the size can increase the capacity. It is said to be easy to cool. Above all, when compared with the "battery package", it is wonderful that it has performance equal to or greater than that of a lithium-ion battery!

This will make you wonder, "Why didn't you hire me before?" In the case of conventional independent cells, even if there is a defective cell, it is enough to replace it. However, if the bipolar type has one defect, the battery package itself cannot be used. Thanks to long experience and a lot of know-how, it was possible to adopt it because it reached a level where there were no defects during manufacturing.

In any case, Toyota has a new "weapon". It will be possible to pursue the cost reduction of THS-II. In the middle of the 2030s, which is before carbon-free in 2050, engine-equipped vehicles, including hybrid vehicles, will no longer be sold in Europe and Japan. It will do its best as an eco-friendly vehicle until December 31, 2049, when sales of gasoline will continue in Europe and Japan.

(Mitsuhiro Kunisawa)