Batteries for Electric Cars

Battery manufacturers are tooling up for the electric vehicle, but what would happen if it failed? Could there be a déjà vu of the fuel cell in the 1990s, or the bio fuels in the last decade that cannot survive without heavy government subsidies? The US Department of Energy (DOE) has admitted that some critical parameters of Li-ion are not met. Newer NiMH batteries, which are cheaper and safer than Li-ion, are also suitable for the electric powertrain but these mature systems are often excluded from government grants for research.

There are no ideal contenders for the electric powertrain, and lithium-ion remains a good choice. Out of the five candidates illustrated in Figure 1, Li-nickel-manganese-cobalt (NMC), Li-phosphate and Li-manganese stand out as being superior. The popular Li-cobalt (not listed) used in consumer products was thought to be not robust enough; nevertheless, this high energy dense “computer battery” powers the Tesla Roadster and Smart Fortwo ED.

Figure 1: Batteries for Electric Cars. Challenges, opportunities and outlook for 2020. The compromises are in safety, specific energy, cost and temperature performance.
Note: The further the shapes extend outwards on the axis, the better the battery will be. With special permission from the Boston Consulting Group (BCG), ©2010

The illustration compares batteries in terms of safety; specific energy, also known as capacity; specific power, or the ability to deliver high current on demand; performance, the ability to function at hot and cold temperatures; life span, which includes the number of cycles delivered as well as calendar life; and finally cost. The figure does not mention charge times. All batteries offered for EV powertrains can be charged reasonably fast if a suitable electrical power outlet is available. A charge time of a few hours is acceptable for most users, and super-fast charging is the exception. Nor does the table reveal self-discharge, another battery characteristic that needs scrutiny. In general, Li-ion batteries have low self-discharge, and this parameter can mostly be ignored when the battery is new. However, aging when exposed to heat pockets can increase the self-discharge of the affected cells and cause management problems. Among the EV battery candidates, Li-phosphate exhibits a higher self-discharge than the other systems.

None of the five battery candidates in the figure above show a significant advantage over others, and the size of the spider fields are similar in volume, although different in shape. Focusing on one strong attribute inevitably discounts another. NCA, for example, has a high capacity but presents a safety challenge, whereas Li-phosphate is a safer system but has lower capacity. In the absence of a clear winner, car manufacturers include peripherals to compensate for the deficiencies. Battery manufacturers in turn assist by custom-designing the cell to strengthen the important characteristics needed for the application. Here is a brief summary of the most important characteristics of a battery for the electric powertrain.

Safety is one of the most important aspects when choosing a battery for the EV. A single incident blown out of proportion by the media could turn the public against such a vehicle. Similar concerns occurred 100 years ago when steam engines exploded and gasoline tanks caught fire. The main concern is a thermal runaway of the battery. Carefully designed safety circuits with robust enclosures should virtually eliminate this, but the possibility of a serious accident exists. A battery must also be safe when exposed to misuse and advancing age.

Life Span reflects cycle count and longevity. Most EV batteries are guaranteed for 8–10 years or 160km (100 miles). Capacity loss through aging is a challenge, especially in hot climates. Auto manufacturers lack information as to how batteries age under different user conditions and climates. To compensate for capacity loss, EV manufacturers increase the size of the batteries to allow for some degradation within the guaranteed service life.

Performance reflects the condition of the battery when driving the EV in blistering summer heat and freezing temperatures. Unlike an IC engine that works over a large temperature range, batteries are sensitive to cold and heat and require some climate control. In vehicles powered solely by a battery, the energy to moderate the battery temperature, as well as heat and cool the cabin, comes from the battery.

Specific energy demonstrates how much energy a battery can hold in weight, which reflects the driving range. It is sobering to realize that in terms of output per weight, a battery generates only one percent the energy of fossil fuel. One kilogram (1.4 liter, 0.37 gallons) of gasoline produces roughly 12kWh, whereas a 1kg battery delivers about 120 Wh. We must keep in mind that the electric motor is better than 90 percent efficient while the IC engine comes in at only about 30 percent. In spite of this difference, the energy storage capability of a battery will need to double and quadruple before it can compete head-to-head with the IC engine.

Specific power demonstrates acceleration, and most EV batteries respond well. An electric motor with the same horsepower has a better torque ratio than an IC engine.

Cost presents a major drawback. There is no assurance that the battery’s target price of $250–400 per kWh, which BCG predicts, can be met. The mandated protection circuits for safety, battery managements for status, climate control for longevity and the 8–10-year warranty add to this challenge. The price of the battery alone amounts to the value of a vehicle with IC engine, essentially doubling the price of the EV.