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Managing rechargeable batteries with BIF

Posted: 07 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:battery? BIF? Battery Interface Standard? charging? multi-drop interface?

For the foreseeable future, much of modern mobile and embedded consumer electronics will be dictated by the amount of battery power available in wireless untethered designs. Although much work is being done to create a variety of energy harvesting schemes to make such devices truly independent, batteries will always be there in some form to provide a backup and as sources of constant and regular power to drive the electronics.

Billions of lithium-ion batteries are in use; an increasing number are in rechargeable formats. There are many different, mostly proprietary battery interfaces. This lack of a commonly accepted battery interface standard has caused extra work and logistical effort throughout the industry.

The one point of agreement amongst all these different schemes is the importance of using some sort of smart battery technology by which information is exchanged between the mobile terminal host platform and the battery pack. Smart battery technology significantly improves the safety of the end users by providing access to reliable battery authentication, versatile sensing of operating conditions (e.g. multiple temperature sensors, stress sensors, etc.) and comprehensive battery related data sets (e.g. manufacturing parameters, charging recipes). In particular, battery authentication with cryptographically strong algorithms improves end user safety by eliminating the use of potentially dangerous counterfeit batteries not complying with the required safety standards [3] or incompatible with the charging parameters of the mobile device.

Unfortunately, without common agreement as to what 'smart' means in the context of rechargeable batteries, mobile device manufacturers must coordinate, specify, and maintain proprietary solutions from different parties in the ecosystemthemselves, mobile chipset suppliers, battery IC suppliers, and battery pack manufacturers.

Under the umbrella of the MIPI Alliance, the solution to this and other troubling battery problems is working its way through the consumer and mobile industry. Several stakeholders in the mobile device industry started to develop a new industry standard for battery interfaces in the form of MIPI's Battery Interface Specification (BIF). (For a complete description of the specification and its impact on how mobile device developers can incorporate it into their SoC designs read BIF C Battery Interface Standard for Mobile Devices [1], a paper presented at the 2013 Custom Integrated Circuits Conference.)

This article will first look at some techniques and rules that have emerged to help both vendors and users in managing the use of rechargeable batteries, then outline the basic parameters of the BIF specification, and conclude with a description of how it deals with what several characters in the 1967 movie Cool Hand Luke would call "a failure to communicate."

The many rules of battery charging
Battery charging requires particular attention to ensure safe operation for some battery types. This is specifically true for the Lithium-based batteries widely used in portable devices today (see also [5]). While charging these batteries, appropriate constraints must be followed to control charging current and voltage precisely. The charging segment, defined as the charging voltage and current combination, depends generally on battery temperature and battery voltage. The charging segment is selected by a charging algorithm and applied on a precisely controllable CC/CV (constant current/constant voltage) charger.

Figure 1: Lithium battery charging profile.

For a given battery, a charging profile, composed by a set of charging segments, can be established so that charging is safe over temperature and battery voltage, an example of which is shown in figure 1. Disrespecting this charging profile may result in various defects such as accelerated ageing of the battery, over-heating, or even battery physical damage that can cause end user injuries.

Limitations of conventional charging. A portable device charging sub-system is usually designed for a specific battery or family of batteries. It usually cannot guarantee safe operation or identical performance when used with batteries other than the design prototype. This inflexibility limits the choice of battery throughout the product life.

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