As with other aspects of vehicle design, battery selection is a compromise
Just as the engine is the defining element in a conventional vehicle’s powertrain, the battery is at the core of the electric vehicle’s propulsion system. Which is why, as the auto industry makes its inexorable transition from mechanical to electrical powertrains, it’s important to understand the differences in battery types and their characteristics.
While the electric motor fulfils the role of the engine, actually driving the wheels in an electric vehicle (EV), it is the battery that constrains the motor’s performance, for it not only stores energy, its characteristics determine how quickly and in what quantity that energy can be released to the motor.
That’s why the battery has been the limiting factor in electric vehicle development for the past century or so, and why it continues to be the most important factor in EV design.
Battery basics
In principle, a battery is an electrochemical energy storage device that can release an electrical charge when needed. A single battery cell consists primarily 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 comprise one or more cells, which can be connected together (in series) to provide a higher voltage. For example, a typical 12-volt car battery is made up of six cells and a battery pack for an electric vehicle (EV) may have hundreds of individual cells.
Battery characteristics that are particularly important for automotive use include energy density and power density:
Energy density (Wh/kg) 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 density (W/kg) 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).
The primary challenge for EV batteries is, and always has been, storing enough electrical energy to provide competitive performance with vehicles powered by internal combustion engines, while also achieving adequate driving distance (range) before requiring recharging.
The range issue is complicated by the fact that recharge times for EV batteries are typically much longer than those required for gasoline or diesel fill-ups.
Adding more battery capacity would potentially increase the driving range, but doing so also increases weight, which reduces range, so the two factors must be balanced against each other.
Battery types
Some common types of commercial automotive batteries and their characteristics are:
Lead-Acid – 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. They are too heavy and have insufficient range for use in EVs today.
Nickel-Metal-Hydride – Nickel-Metal-Hydride (NiMH) batteries are commonly used in today’s hybrid vehicles, as well as in low-cost consumer applications such as electric razors and toothbrushes. Their cost is moderate and they have an energy density about twice that of lead-acid batteries, although their power density is lower in terms of volume (space required).
They are capable of delivering rapid power bursts, but repeated rapid discharges with high loads reduce the battery’s cycle life, which makes them better suited to hybrid applications than to pure electric vehicles.
Lithium-ion (Li-ion) – 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 battery electric vehicles, 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.
Lithium Polymer (Li-poly) – The lithium polymer battery is similar to other lithium-ion batteries except it uses a solid plastic (polymer) electrolyte, which means its cell shape is not restricted to the cylindrical form of most others.
That means its shape 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.
Battery development is currently among the hottest areas of research in the automotive industry, with multiple new types being announced every year. Which one will become dominant, if any one does, is far from resolved at this point. It may not even be invented yet.
