Energy & Environment

Banking on Batteries

An EV's batteries are made up of many smaller batteries wired together to obtain the desired voltage. Think of it this way: Amps give the motor torque and volts give it speed. But you can't just add as many batteries as you want to increase either-each battery is the size of a toaster and weighs close to 70 pounds.

Deep-cycle, lead-acid batteries, used extensively in industrial vehicles, marine applications, forklifts and golf carts, are generally the choice battery for EVs. They are designed to be repeatedly drawn down to a low state of charge before being recharged and are tough enough to survive fluctuating recharge cycles as might be experienced when solar, wind or hydroelectric energy sources are used. Freeway EVs may have 20 of these batteries, while smaller EVs may use only half as many. Deep-cycle, lead-acid batteries are affordable, widely available in many shapes and sizes, and are backed by an efficient recycling industry.

Sealed lead-acid (SLA) batteries are used in all small EVs and in some larger ones. These batteries require no maintenance, where flooded-cell types need regular replenishment of the water lost to evaporation and electrolysis. SLA batteries cost more than flooded types and have a reputation of a slightly shorter life span.

A nickel-cadmium (nicad) battery yields twice as much power and storage capacity of lead-acid batteries for the same battery space. But because nicad batteries are expensive, easily two to three times the price of lead-acid batteries, and recycling these batteries is a challenging and expensive undertaking, use of nicad batteries is generally restricted to electric motorcycles and electric-assist bicycles.

Quality lead-acid batteries have a 500-cycle life span, similar to sealed-lead and nicad batteries. If an EV's battery pack is driven within 50 percent of its capacity an average of four days per week, it will be three years before the battery pack needs replacement. Deeper discharges and more frequent use will shorten the life span to as little as two years. Some EV users, however, report five or more years of good utility from their EV's battery pack.