The components of NiMH batteries include a cathode of Nickel-hydroxide, an anode of Hydrogen absorbing alloys and a Potassium-hydroxide (KOH) electrolyte. The energy density of NiMH is more than double that of a lead acid battery but less than lithium ion batteries.
In 1997, Toyota developed a cylindrical NiMH battery that powered the Rav4EV as well as the e-com electric vehicle. Since 1997, Toyota has continually improved its NiMH batteries by reducing size, improving power density, lowering weight, improving the battery pack/case and lowering costs. In fact, the current NiMH battery, which powers the third generation Prius, costs ¼ that of the 1st generation.
Nickel-metal batteries are ideal for mass producing affordable conventional hybrid vehicles due to their low cost, high reliability and high durability. There are first-generation Prius batteries still on the road with over 200,000 miles and counting. That is why NiMH remains the battery of choice for Toyota’s conventional hybrid line up.