To put it simply, we selected the battery in terms of its capacity and characteristics.
There are lead-battery, nickel-cadmium battery, etc. besides lithium-ion battery.
I'd like to explain them in comparison, because it will help understanding.
First is the lead-battery. The major difference form the lithium-ion battery is that the electric energy charged is not so high, about the half capacity of the lithium-ion battery.
So, if we use the lead-battery and need the same electric energy as that of the lithium-ion battery, we have to load twice of the battery in weight.
In 1994, the Libero EVs, which were sold to governmental agencies and electric power companies, were equipped with the lead-battery, and so the weight of the battery became about half of the vehicle weight.
Now I think back about the electric vehicles at that time, most of them ran on a lot of batteries, as it seemed like battery transporter. On the other hand, the nickel-cadmium battery features that the battery capacity has reduced if it is charged before it has run out, which is called “memory effect”.
So, the way of charging the nickel-cadmium battery before it has run out, which is as seen in the electric vehicles, is not favorable. The capacity is also less than the lithium-ion battery.
Therefore, the i-MiEV uses the lithium-ion battery which has the capacity of achieving the sufficient driving distance and the characteristics of withstanding the long-term charge and discharge cycle.
What the i-MiEV differs from the other electric vehicles in the past lies in a thorough pursuit of the same extent of safety as that of normal vehicles. The tests were very severe. The battery has been developed to have an endurable performance against the assumed long-term use of 10 years, so the tests are very severe and demanding to meet the required performances and assurance level. For example, we verified the safety by driving on the long rugged road, or tested in the cold environment as cold as the vehicle to be frozen, we verified with our thoroughly accumulated experiences. As for the suspension system, we have been verifying while continuing giving a very strong vibration for a long time. Actually, the i-MiEV is heavier by about 200kg than the normal gasoline-powered model of “i”, but of course we have taken the corresponding secure measures. In addition, the system of “MiEV OS” equipped in the i-MiEV is always monitoring to secure the occupant's safety, that's why the safety is always secured even in the case of unexpected accidents.
There are two aspects of the safety regarding the battery. One is an approach from the material.
Some years ago, you know, there occurred accidents of a battery burn in personal computers or mobile phones, but the major cause for this problem was an internal short-circuit due to the entry of metallic impurities into the inside of the battery. Actually, the general-purpose lithium-ion battery used to have a tendency to easily produce heat in case of abnormal conditions such as an internal short-circuit, etc. resulting from the positive-electrode material property. However, we further enhanced the safety by developing our own thermally-stable positive-electrode material and using the thick and high-strength insulating sheet, etc. The other is a battery charging method. The charging voltage used in mobile phones or personal computers is normally 4.2 volt in relation to single battery cell. In this case, the electric energy taken out from the battery becomes larger, while the battery condition becomes more severe. In case of the i-MiEV, the battery charge is controlled by the lower voltage. So, this mechanism allows for less fluctuation of the battery capacity depending on the using frequency or the storage period and improve the long-term durability and the safety. As you are already aware, the battery, which is a power source of the i-MiEV, has been developed from the stage of selecting its material through to doing various kinds of tests related to the vehicle safety or durability.
