What kind of batteries are in a Prius? Why are they different than the ones in the Tesla Roadster? Why is Honda changing the kind of batteries in its hybrid Civic? Thanks to a burgeoning market for hybrids and electric vehicles, batteries are the latest automotive technology to leave consumers scratching their heads.
Electric and hybrid vehicles do not all use the same type of batteries, and some automakers even use different types of batteries in different models.

So here are some battery basics that should help explain today's in-car battery technology and what it means to you.

Lead Acid

This is the conventional automotive battery, the oldest type of rechargeable battery that has been around since the 1800s. Basically every automobile uses a lead-acid battery to run its electrical system and accessories like the radio and headlights. Even hybrids like the Toyota Prius use a lead acid battery to run these secondary systems. Early electric cars were powered by lead acid batteries, and even General Motor's EV1 was initially powered by lead acid.

But lead-acid batteries have serious limitations. They don't have great energy storage abilities, they're heavy, and the chemicals inside are hazardous. Also, lead-acid batteries have a relatively short lifespan.

Nickel Metal Hydride (NiMh)

NiMh has a higher storage capacity than many types, including lead acid. This is good because the more energy that can be stored in a small space, the easier it is for designers to pack enough batteries onboard to power the car. NiMh batteries have fewer toxic chemicals than lead acid batteries as well. NiMh technology has been around since the 1970s so it's proven and relatively inexpensive. That's part of the reason some automakers are sticking with it.

For example, Honda pointed out that the Honda CR-Z met all the company's performance, fuel economy and price-point goals using NiMh batteries, which is why it didn't opt for fancier, more expensive batteries. The same goes for Toyota, which has said that the Prius will continue to use NiMh batteries for the foreseeable future.

The disadvantages are that the NiMh batteries need to be fully discharged regularly to avoid "memory" which shortens the battery's life. They also generate more heat than NiCad or lead-acid batteries while charging. Heat and heavy loads can also reduce battery life.

Lithium-ion (Li-ion)

Lithium-ion batteries are safer and less toxic than the others. Compared to NiMh and lead acid batteries, Li-ion allows for the most energy storage in the smallest space, which makes it ideal for automotive uses. Li-ion batteries aren't affected by "memory" so they don't need to be fully discharged to maintain a long life, making them basically maintenance free. Li-ion batteries can also be stored for a long period of time without losing their charge.

Li-ion is the newest of the battery types and is being used in cars like the Chevrolet Volt and Nissan Leaf. Tesla also uses Li-ion batteries in its Roadster. But Li-ion batteries are more expensive than NiMh batteries – one reason why Honda chose to stick with NiMh in the $20,000 CR-Z.

Both the Nissan Leaf and Chevy Volt are notably more expensive. This is, in part, because both cars have much larger battery packs, but it's also because those packs are made with more expensive Li-ion batteries. The 2012 plug-in Prius will use all Li-ion batteries, and Toyota says the plug-in version will be at least $3,000 more expensive than the standard Prius, no doubt in part because of the more expensive Li-ion batteries onboard. However, General Motors thinks the extra cost is worth it saying "...the fact that lithium-ion batteries have a higher energy-to-weight ratio and as a result weigh less than nickel metal hydride systems in similar applications means that they are cost competitive."

Honda has said publicly that the next generation Civic Hybrid will use Li-ion batteries which should allow for a boost in fuel economy. It will be interesting to see how Honda pulls this off, however, because the new Civic Hybrid certainly won't be a $30,000 car, not when the current model costs about $24,500 and the Prius sells for about the same amount.

Li-ion batteries do have drawbacks. They work best if they're never fully charged or discharged. That means an electronic monitor or protection switch must be installed, adding complexity and cost. Finally, Li-ion batteries do not work well in extreme temperatures. So the Chevy Volt has a system in place that heats and cools the batteries to prolong their life.

The Future

While there are other battery technologies being developed, progress on this front is not moving as rapidly as automakers would like. So for the foreseeable future, hybrids and EVs are going to be powered by either NIMh or Li-ion batteries. Like every new technology, battery prices will eventually come down, but it's going to take time, as right now global battery manufacturing capacity is still fairly constrained.

Lithium-ion is obviously a better and more efficient way to power modern hybrids and EVs, but it's also more expensive. Li-ion batteries are sort of like a post-modern overhead cam, multi-valve, high-output V8 -- expensive but worth it. NiMh technology is like a reliable and affordable four-cylinder engine -- adequate for sure, but not the top dog.