How to Rate Battery Runtime

In the past, the battery industry got away with soft standards specifying battery runtimes. Each manufacturer developed their own method, using the lightest load patterns possible to achieve good figures. This resulted in specifications that bore little resemblance to reality. Under pressure from consumer associations, manufacturers finally agreed to standardized testing procedures. 

The Camera and Imaging Products Association (CIPA) succeeded in developing a standardized battery-life test for digital cameras. Under the test scheme, the camera takes a photo every 30 seconds, half with flash and the other without. The test zooms the lens in and out all the way before every shot and leaves the screen on. After every 10 shots, the camera is turned off for a while and the cycle is repeated. CIPA ratings replicate a realistic way a consumer would use a camera. Most new cameras adapt the CIPA protocol to rate the runtime.

The runtime on laptops is more complex to estimate than a digital camera as programs, type of activity, wireless features and screen brightness affect the load. To take these conditions into account, the computer industry developed a standard called MobileMark 2007. Not everyone agrees with this norm, and opponents say that the convention trims the applications down and ignores real-world habits. The setting of brightness is one example. The monitor is one of the most power-hungry components of a modern laptop and at full brightness the screen delivers 250 to 300 nits. MobileMark uses a setting that is less than half of this. Nor does MobileMark include Wi-Fi and Bluetooth; it leaves these peripherals up to the manufacturers to investigate. BAPCO (Business Applications Performance Corporation), the inventor of MobileMark 2007, is led by Intel and includes laptop and chip manufacturers, such as Advanced Micro Devices.

Cell phone manufacturers face similar challenges when estimating runtimes. Standby and talk time are field-strength dependent and the closer you are to a repeater tower, the lower the transmit power will get and the longer the battery will last. CDMA (Code Division Multiple Access) takes slightly more power than GSM (Global System for Mobile Communications); however, the more critical power guzzlers are large color displays, touch screens, video, web surfing, GPS, camera, voice dialing and Bluetooth. These peripherals drastically shorten the advertised runtime specifications if used frequently.

The insatiable appetite for information and entertainment on the go is devouring the excess energy enjoyed during the past 10 years when we used our cell phones for voice only. Although modern handsets draw considerably less power than older models and the battery capacity has doubled in 12 years, these improvements do not compensate for the modern peripherals, and a new energy crisis is in the making. Figure 1 illustrates the lack of energy with analog cell phones during the 1990s, the sudden excess with the digital phones, and the looming energy shortage when making full use of modern features. These power needs are superimposed on a continuously improving battery.

 Power needs of the past, present and future

The capacity of Li-ion has doubled in 12 years and the circuits draw less power; however, these improvements do not compensate for the power demand of the new features, and a new energy crisis is
in the making.

Manufacturers of analog two-way radios test the runtime with a scheme called 5-5-90 and 10-10-80. The first number represents the transmit time at high current; the second denotes the receiving mode at a more moderate current; and the third refers to the long standby times between calls at low current. While 5-5-90 simulates the equivalent of a 5-second talk, 5-second receive and 90-seconds standby, the 10-10-80 schedule puts the intervals at a 10-second talk, 10-second receive and 80-second standby. The runtimes of digital two-way radios are measured in a similar way, with the added complexity of tower distance and digital loading requirements that are reminiscent of a cellular phone.