One of the key elements of any electric vehicle is its battery. In this section, we provide detailed information on battery types and their advantages and disadvantages for consumers who want to understand this element more fully.
A battery is an electrochemical energy storage device that can release an electrical charge when needed. It generally consists of an anode, a cathode and an electrolyte (separator). Different battery types are typically identified by the materials that make up one or more of those components (e.g., lead-acid).
Batteries may be made up of one or more cells, which can be connected (in series) to provide a higher voltage. For example, a typical 12-volt car battery is made up of six cells connected internally, while a battery pack for a battery electric vehicle (BEV) may have hundreds of individual cells. Battery characteristics that are particularly important for automotive use include their energy density and power density.
Energy density is a measure of how much energy a battery can hold. The higher the energy density, the longer it will last before needing to be recharged.
Power is the rate at which energy is used. Power density is a measure of how much power a battery can deliver on demand; that is, how quickly it can release its energy (and conversely, how quickly it can be recharged).
Here are some common types of commercial automotive batteries and some of their characteristics and advantages.
Lead-acid batteries are used in conventional cars and trucks for starting, ignition, lighting and other electrical functions.
They are relatively inexpensive and have a high power density but a relatively low energy density.
Nickel-Metal Hydride (Ni-MH) batteries are commonly used in today’s hybrid vehicles, and in low-cost consumer applications, such as electric razors, toothbrushes, cameras, and camcorders.
Their cost is moderate and they have an energy density about twice that of lead-acid batteries. However, their power density is lower in terms of volume (space required).
They also have a higher self-discharge rate and so tend to discharge when left unused. Although they are capable of delivering rapid power bursts, repeated rapid discharges with high loads reduce the battery’s cycle life.
Consequently, they are better suited to hybrid applications than BEVs, which typically experience deep discharge cycles.
Lithium-ion batteries are commonly used in cell phones and laptop computers and they are becoming the battery of choice for plug-in hybrids and BEVs, as well as some conventional hybrids.
Their energy density and power density are both typically several times those of lead-acid and NiMH batteries, and their charge/discharge efficiency is also higher.
They are, however, more expensive and in their most common form, their temperature must be well controlled, sometimes necessitating an elaborate and costly battery cooling system in the vehicle.
Because of their high energy density, lithium-ion batteries are the preferred choice for many plug-in hybrids and BEVs either currently or soon to be available.
The lithium polymer battery is similar to other lithium-ion batteries except it uses a solid plastic (polymer) electrolyte. This means its cell shape is not restricted to the cylindrical form of most others and can be altered to conform to specific spaces within a vehicle, thus making better use of space.
Its other characteristics are similar to those of other Li‑ion batteries. Li-poly batteries are already being used in some hybrid vehicles.
There are several Lithium-ion battery variants. These variants change according to internal chemistry, specifically the material used in the battery’s cathode. The most common cathode materials are cobalt oxides and manganese oxides.
The Lithium iron phosphate battery uses lithium-ion chemistry but with an iron phosphate cathode. (F is the chemical symbol for iron, thus LFP). Compared to other Lithium-ion batteries, it offers superior heat and chemical stability with no risk of fire in the event of an overcharge or short circuit.
It also typically has a higher peak-power rating, but its energy density is significantly lower than in other Lithium-based batteries.
Lithium iron phosphate batteries are now being used in hybrids and BEVs by some automakers, who consider that their safety and power advantages outweigh their lower energy density.