The energy storage system in electric cars comes in the form of a battery. Battery type can vary depending on if the vehicle is all-electric (AEV) or plug-in hybrid electric (PHEV). Current battery technology is designed for extended life (typically about 8 years or 100,000 miles). Some batteries and can last for 12 to 15 years in moderate climates, or eight to 12 years in extreme climates. There are four main kinds of batteries used in electric cars: lithium-ion, nickel-metal hydride, lead-acid, and ultracapacitors.

Types of electric car batteries
Lithium-ion batteries
The most common type of battery used in electric cars is the lithium-ion battery. This kind of battery may sound familiar – these batteries are also used in most portable electronics, including cell phones and computers. Lithium-ion batteries have a high power-to-weight ratio, high energy efficiency and good high-temperature performance. In practice, this means that the batteries hold a lot of energy for their weight, which is vital for electric cars – less weight means the car can travel further on a single charge. Lithium-ion batteries also have a low “self-discharge” rate, which means that they are better than other batteries at maintaining the ability to hold a full charge over time.

Additionally, most lithium-ion battery parts are recyclable making these batteries a good choice for the environmentally conscious. This battery is used in both AEVs and PHEVs, though the exact chemistry of these batteries varies from those found in consumer electronics.

Nickel-metal hydride batteries
Nickel-metal hydride batteries are more widely used in hybrid-electric vehicles, but are also used successfully in some all-electric vehicles. Hybrid-electric vehicles do not derive power from an external plug-in source and instead rely on fuel to recharge the battery which excludes them from the definition of an electric car.

Nickel-metal hydride batteries have a longer life-cycle than lithium-ion or lead-acid batteries. They are also safe and tolerant to abuse. The biggest issues with nickel-metal hydride batteries is their high cost, high self-discharge rate, and the fact that they generate significant heat at high temperatures. These issues make these batteries less effective for rechargeable electric vehicles, which is why they are primarily used in hybrid electric vehicles.

Lead-acid batteries
Lead-acid batteries are only currently being used in electric vehicles to supplement other battery loads. These batteries are high-powered, inexpensive, safe, and reliable, but their short calendar life and poor cold-temperature performance make them difficult to use in electric vehicles. There are high-power lead-acid batteries in development, but the batteries now are only used in commercial vehicles as secondary storage.

Ultracapacitors
Ultracapacitors are not batteries in the traditional sense. Instead, they store polarized liquid between an electrode and an electrolyte. As the liquid’s surface area increases, the capacity for energy storage also increases. Ultracapacitors, like lead-acid batteries, are primarily useful as secondary storage devices in electric vehicles because ultracapacitors help electrochemical batteries level their load. In addition, ultracapacitors can provide electric vehicles with extra power during acceleration and regenerative braking.

How do electric car batteries work?
All-electric vehicles have an electric traction motor in place of the internal combustion engine used in gasoline-powered cars. AEVs use a traction battery pack (usually a lithium-ion battery) to store the electricity used by the motor to drive the vehicle's wheels. The traction battery pack is the part of the car that must be plugged in and recharged, and its efficiency helps determine the overall range of the vehicle.

In plug-in hybrid electric vehicles, the electric traction motor is powered by a traction battery pack much like an AEV. The primary difference is that the battery also has a combustion engine. PHEVs run on electric power until the battery is depleted and then switch over to fuel which powers an internal combustion engine.  The battery, usually lithium-ion, can be recharged by being plugged in, through regenerative braking, or by using the internal combustion engine. The combination of battery and fuel gives PHEVs a longer range than their all-electric counterparts.