Lithium-ion technology has seen use in laptops and cell phones as well as numerous other devices for over a decade. People have grown accustomed to the expectations of these batteries. Cell phones lose their charge-holding capacity after a few years, and laptops with strenuous use can see batteries last less than a year. But car manufacturers claim 10 or more years of dependable service life from their batteries. What makes an automotive lithium-ion battery more durable and longer lasting than its counterparts in use in other devices?
The real difference does not lie in battery technology. The difference lies in how the battery is depleted and charged. In a standard laptop or cell phone system, the battery can be nearly fully depleted before shutting down. Then it is plugged in for charging and charge to near capacity. This process is extremely tough on the li-ion technology. It reduces its useable life, but provides 100 percent of the batteries available power. In short, batteries can be smaller because they can be charged to full capacity and all of the power can be realized.
In the automotive setup, the lithium-ion battery is treated differently to increase longevity at the expense of output. Many hybrid and electric vehicles utilize what is known as a 50-80 charge. In short, this means a li-ion battery would never be discharged below 50 percent and would never be charged over 80 percent of its real capacity. This area between 50 and 80 percent is where the battery can safely operate for 10 or more years. It's a gentle discharging and charging process that never allows the battery to drop much below half of its capacity or reach its full capacity.
However, there is a serious drawback to extending the battery life through this method. In order to provide a decent electric range, EVs are equipped with a battery that is close to twice the size that is actually needed for the application. This means a vehicle must carry additional weight, weight that is actually useless in increasing the range of the vehicle, but vital for the longevity of the battery.
An example is provided below. The Chevy Volt carries a 16 kilowatt hour lithium-ion battery pack that weighs in around 400 pounds. The vehicle is only capable of using 8 kilowatt hours of energy from the battery. It must carry the additional 200 pounds which in turn decreases its electric only range by a measurable amount. When the vehicle reaches its end of service life 10 years from now, the battery is expected to still be able to output 12 kilowatt hours of juice. Still more than is needed to power the vehicle, but not enough to keep the battery in its safe zone for an extended period of time.
Another downside is cost. Batteries cost roughly $1,000 per kWh. That puts the battery pack in the Volt at an estimated price of $16,000. It only needs a battery pack priced at $8,000 outputting 8 kWh. The additional $8,000 is money spent for energy not needed and weight not desired.
The advancement of battery technology will likely find a way around the dilemma. But for now, EVs buyers will be reluctantly carrying a lot of dead weight onboard. Maximizing energy output and maximizing longevity is a battle that battery makers face and the answer is not clear cut right now. Lithium-ion technology is relatively new and advancements come almost daily, a breakthrough is sure to come and somebody will discover a solution to the problem that currently plagues this technology.
Source: Car and Driver August 2009