Our sensitivity to the environment has led society to enact “green” initiatives that not only reduce our personal carbon footprints, but also increase recycling and reduce the amount of waste that enters landfills. Although, when green issues are raised, our first thoughts may be of larger electronic products such as computers or TVs, even small, portable equipment such as cell phones and personal media players need to be considered. In particular, we need to consider the impact of batteries on the environment.
Portable electronic devices require power to run. Power can be harvested from sources such as the sun (solar cells), mechanical vibration (walking) or thermal differentials to provide the necessary energy. When those are not enough, then batteries are required to store the energy and provide a vehicle for recharging. Most batteries today are either lithium or nickel based.
These battery technologies all have finite life spans that are affected by application and manufacturing. Lithium-ion (Li-ion) batteries have several issues that cause degradation in their capacity, most notably, Li-ion's shelf life. These batteries start “aging” as soon as they are manufactured, which causes their internal resistance to increase. This effect is due to oxidation and limits the life span to about two to three years. Eventually, the resistance gets too high to deliver a full charge. Also, charging will degrade their performance to roughly 300 to 500 cycles before they reach 80% of their original capacity.
Nickel-metal-hydride (NiMH) batteries also suffer from charge-cycle wearing. Discharging under heavy load or high temperature (or worse, both conditions) can severely shorten the service life. NiMH batteries average between 250 and 500 cycles (longer under light loads) before they significantly lose their capacity. Additionally, a maintenance charge is actually detrimental to the cells. A better charging method is to stop charging at full charge and periodically top off the cells at the full-charge rating (i.e., c/1).
To extend battery life, several tactics can be employed, starting with operating temperature. A good portable design will provide ample cooling for the system and keep excess heat away from the battery pack. Lithium technologies are sensitive to heat, which can greatly degrade their life spans.
Also, excessively heating Li-ion batteries may result in catastrophic failure (explosion) and should always be avoided. This can occur by overstressing the discharge of the cells. Most lithium cells have internal mechanical protection to prevent shorting and, thus, excessive heating, but it may still be possible to create these conditions.
Another method for increasing cell life is to be gentler with charge cycling. This applies to both chemistries. Use batteries that are larger than required and charge them to only 90% of capacity and discharge to only 10% to 20%. Damage occurs at the extremes of the cycles, so by setting limits well inside the operating parameters of the batteries, their life will be extended.
To reduce the aging effect, the less current a portable device requires, the less the internal impedance of the battery will affect its operating life. That is, if you draw a low current, the internal resistance can increase and battery can still deliver the required energy to the load.
So, what is the impact of actually extending the life span of various portable equipment and their batteries to environment? There are more than two billion cellular phones and 1.5 billion other portable devices (e.g., media players) in use worldwide, and the number is rapidly growing. If we assume the average life span of these devices is three years, we can estimate that an average of 1.2 billion devices are discarded each year.
Simply extending the battery's life span by only 15% would keep roughly an additional 180,000 portable devices (and their batteries) in operation per year and possibly out of landfills. Also, mines that produce nickel or lithium would not need to increase production quite as rapidly due to the longer life spans of the cells themselves. All of this benefits our environment. So, using less power even in portable devices is good for the environment.
Rick Zarr designed computer and communication equipment before entering applications engineering in 1984. In his current role with National Semiconductor, he focuses on developing partnerships with other technology suppliers. Zarr holds a BSEE degree from the University of South Florida.