BU-801a: How to Rate Battery Runtime

The consumer market follows set standards in specifying battery runtime, but these are loose and some associations develop their own rules by using the lightest load patterns possible to achieve good numbers. This results in specifications that carry little resemblance to reality. Under pressure from consumer associations, manufacturers have finally agreed to standardize testing procedures on key consumer products.

The Camera and Imaging Products Association (CIPA) set up in Japan in 2002 succeeded in developing a standardized battery-life test for digital cameras. Under the test scheme, the camera takes a photo every 30 seconds, half of them with flash and the other without. The test zooms the lens in and out all the way before a shot is taken 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 and most new cameras adopt 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 energy consumption. To take these conditions into account, the computer industry developed a standard called MobileMark 2014, an application-based benchmark that reflects usage patterns for business users. MobileMark is built on the 20-year history of BAPCo (Business Applications Performance Corporation) in building benchmarks to evaluate platform technologies.

Not everyone agrees with the established norm and opponents say that the convention trims the applications down and ignores real-world practices. The setting of brightness is one example. The display is one of the most power-hungry components of a modern laptop or tablet, and at full brightness the screen delivers 250 to 300 nits. (A nit is a unit of brightness equal to one Candela per square meter. Candela is a unit of luminous intensity.) MobileMark is said to use 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.

Mobile 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 touch screen displays, video presentations, web surfing, as well as the use of GPS, camera, voice dialing and Bluetooth. Web-browsing and video-watching typically cut the runtime in half compared to mere talking on the phone.

Although battery capacity has increased and the circuits have become more efficient, these improvements do not compensate for the added features of a smartphone that also sees longer and more frequent usages between charges than mere talking. A new energy crisis is emerging.

Figure 1 illustrates battery improvement against the lack of energy with the early analog cell phones during the 1990s, the sudden excess energy with the digital phones, and the lack of energy in modern smartphones and tablets. Improvements in battery capacity cannot keep pace with added consumption.

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.

Smartphones and tablets operate on a single Li-ion cell, and this has a decisive advantage over a laptop battery. Cell-matching is not required as with the laptop packs running at 14.40V in a typical 4S2P configuration. This simplifies cell manufacturing and increases the yields by opening the flood-gates for broader performance acceptance.

Most users would not notice if a battery came with the specified 100 percent capacity or only 90 percent. Manufacturers know that consumer batteries vary in capacity and that the user will not test them, especially those that are non-replaceable. No rules exist as to what constitutes an acceptable battery other than meeting the obligatory 80 percent within the warranty period.

References

[1] Courtesy of Cadex