Why aren't Ni-mh batteries used for electric vehicles?

It seems to me that Lead-acid and Lithium batteries are the only type of batteries used for electric vehicles. Also every one complains that Lead-acid is too heavy and Lithium is too expensive. So why not split the difference and use Ni-mh batteries? They aren't really heavy or expensive. Why aren't they used?

aarone7 years ago
Chevron owns the patent on the large-format NiMH, and prevents the batteries from being produced or imported into the states. When California was pushing it's zero emissions program, Toyota and GM had placed NiMH batteris in their vehicles. Some of the Rav4 EVs had battery packs that lasted the decade. NiMH batteries are available, but not cheap. Unfortunately, it's a political move, not a technical one. It's hard to tell what the future holds though. The patent on the NiMH battery runs out in 2014. Lithium is still a relatively new battery technology. While NiMH has proven itself, it may be side-stepped simply because it doesn't offer the same potential benefits. Li-Ion is still super expensive. If economies of scale push NiMH into greater popularity, it may see a resurgence. I still look forward to the possibility of the NiMH battery being somewhere in the practical expense range.
While Chevron does own the Cobasys large-format patent for now, Nilar holds a non-competing patent on a large format NiMH battery and they do sell smaller quantities to DIY EV converters. Lithium technologies do have a higher power density per unit of mass, but there aren't any Li-Ion or Li-Polymer battery packs that last more than 7 years. The problem is in the chemistry--Lithium technologies involve "stealing" electrons from the Lithium compounds which causes instability and eventually breakdown of the compounds, while NiMH "traps" extra electrons which jump from molecule to molecule when the battery isn't in use. These "freer" electrons are the reason NiMH batteries are so durable. There are several RAV4 EV's still running around, and engineer's from Panasonic and the personal experience of thousands of small-cell NiMH users confirms these packs can easily last 12-15 years. There are many research packs over 25 years old that are still operating at greater than 95% capacity. The newer low-resistance (so-called pre-charged) NiMH approach the efficiency of Lithium technologies but still lack the power density. Even still, they are a good option. There are several companies (i.e. MAHA who makes the Imedion Powerex batteries) that will sell a quantity appropriate for an EV (say, 10K+ cells) to anyone. I'm in talks with someone there currently for such a deal. If you choose to contact them, please don't abuse it. If they have a large quantity of people asking questions and getting quotes, but not buying they may change their disposition.
CameronSS7 years ago
They have been, and occasionally still are. The ill-fated GM EV1 used Ni-MH batteries, and many modern hybrids use Ni-MHs. They aren't used very often in straight EVs because most people don't want a mid-range battery. Homebuilts and conversions typically use lead-acid batteries, because they're cheap and rugged. In production EVs, lithium-based batteries are usually used because the reduced performance and lifespan of Ni-MH isn't worth the lower cost, which is usually a minor difference in the final price of the car.
orksecurity7 years ago
The tradeoffs are energy density (and charge/discharge rates) versus cost versus lifespan versus bulk. NiMH just isn't at a particularly good trade-off point.