As the author of BatteryUniversity.com, I get many interesting enquiries from battery users. A man writes, “Hi, I am looking for an answer to a perplexing question. A co-worker and I have identical cell phones from the same provider. Moving into a new house, she complained of short battery runtime. I told her she was out of her mind, but then I noticed my battery behaving differently when I travel. Is there some mysterious force that’s draining the battery?”
Yes, there is a hidden force that drains the battery but it’s not mystical. When turned on, a cell phone is in constant communication with the tower, transmitting small bursts of power once every second or so to check for incoming calls. To save energy, the signal strength adjusts the transmission power to only what is needed. If the cell phone is close to a repeater tower, the energy required to communicate is very low. Move farther away or enter an area with high electrical noise, such as a shopping mall, hospital or factory, and the required energy increases. An analogy can be made to sitting in a restaurant. When the surroundings are quiet, the voices can be kept low, but as the crowd grows everyone needs to talk a bit louder.
Living in sight of a tower has advantages and your cellular battery will last longer between charges. Where you park your cell phone in the house also affects runtime. A manager of a large cellular provider in the UK said his son experienced shorter standby times after moving from the upstairs bedroom to the basement. If possible, leave your cell phone in an upstairs room facing a tower. When traveling by car place it near a window rather than on the floor but avoid direct exposure to the sun.
Similar rules apply to TETRA and P25 radio systems, cordless telephones, Wi-Fi and Bluetooth devices. A wireless headset that communicates with the cell phone from belt to ear provides longer runtimes than when placing the handset on the dining-room table while cooking in the kitchen. Although the quality of communication stays the same, the Bluetooth headset needs to work harder when placed farther away from the user.
The energy savings only apply when the wireless device is in the “on” position. When “off,” the load on the battery is very low and only provides power for housekeeping functions such as maintaining the clock and monitoring key commands. Housekeeping and self-discharge consume 5 to 10 percent of the available battery energy per month.
During the last few years, standby and talk-times on cell phones have improved. Besides increases in the specific energy of lithium-ion, improvements in receiver and demodulator circuits have achieved notable energy savings. Figure 1 illustrates the reduction of power consumption in these circuits since 2002. We must keep in mind that the savings apply mainly to standby and receiving circuits. Transmitting still requires about five times the power of the receiving and demodulation.
Figure 1: Reduction in power consumption
Cell phones have achieved notable power savings in the receiver and demodulator circuits. Transmitting needs the most power.
Souece: Sieber et al. (2004).
Laptop batteries fare badly in terms of life span. Laptops are demanding bosses that request a steady stream of power under poor working conditions, toiling in an unbearable heat of 40–45°C (104–113°F). In addition, the battery is exposed to a high voltage by being kept at full charge. High heat and dwelling at full state-of-charge, not cycling, cause short battery life in laptops.
Laptop batteries have further demands — they must be small and lightweight. The laptop battery should be invisible to the user and deliver enough power to endure a five-hour flight. In reality, the battery runs for only about 90 minutes. Batteries are getting better; however, the request for higher performance counteracts the capacity gain, resulting in roughly the same runtime with more powerful features.
Although users want longer runtimes, computer manufacturers are hesitant to add larger batteries because of increased size, weight and cost. A survey indicates that given the option of a larger size with added weight to gain longer runtimes, most users would settle for what is offered today. For better or worse, we have learned to live with what we have.
In the search for low-cost batteries, consumers may inadvertently purchase counterfeit batteries that are unsafe. The label appears bona fide and the buyer cannot distinguish between an original and a forged product. Cell phone manufacturers are concerned about these products flooding the market and advise customers to use approved brands; defiance could void the warranty. Manufacturers do not object to third-party suppliers as long as the aftermarket batteries are well built, safe and approved by a safety agency.
Caution also applies to purchasing counterfeit chargers. Some unsafe aftermarket chargers do not terminate the battery correctly and rely on the battery’s internal protection circuit to cut off when full. The need for redundancy is important because a bona fide battery could have a malfunctioning protection circuit that was damaged by a static charge. If, for example, the maker of the counterfeit battery relies on the charger to terminate the charge, and the charger builder has full confidence that the battery will turn off when ready, the combination of these two products can have a lethal effect.
Some laptop manufacturers disallow aftermarket batteries by digitally locking the pack with a tamperproof security code. This is done in part for safety reasons, because the potential damage resulting from a faulty laptop battery is many times greater than that of a cell phone.