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September 28, 2014


Electronics & Systems
Mobile Devices
The Basics
Nadim Maluf

No, it is not “Batteries Made Simple,” nor “Better Make Sense,” though BMS do indeed try to accomplish both in a very indirect and implicit way.

BMS stands for Battery Management Systems. These are electronic systems, both hardware and software, whose primary function is to control the operation of the battery. In order for batteries, and more specifically lithium-ion batteries, to deliver the requisite safe performance, they must operate within some very well defined, and in many cases, strict limits. For example, a lithium-ion battery cannot be charged above a certain voltage specified typically by the manufacturer in the range of 4.2V and 4.35V. Maximum current values and temperature limits are other examples. Failure to observe these limits will result at the very least in performance degradation, and quite likely in a seriously unsafe outcome such as fire or even death. A Chinese flight attendant died in 2013 while using her iPhone 5 during charging; her electrocution was attributed to a counterfeit charger she purchased in China.

BMS cover several functions including charging the battery, measuring the battery’s amount of stored charge, and making many decisions to ensure the battery remains within a safe operating mode.

The fuel gauge, the device responsible for giving you the percentage of “battery full” in your mobile device, is an integral part of the BMS. Fuel gauges were practically inexistent until a startup company called Benchmarq introduced them in the early 1990s, initially for notebook PCs. Fuel gauge functionality is integrated today in the power management integrated circuits (known as PMIC) manufactured by companies such as Qualcomm and Texas Insruments, yet sadly, there has been very little if any meaningful innovation added since Benchmarq — I will resist the temptation of openly promoting Qnovo here. For example, the accuracy of the fuel gauge in your smartphone is quite poor, and can often be as high as 5 to 10 percentage points. Next time you look at your mobile device and it reads 20% battery remaining, keep in mind that may be as little as 10% or as high as 30%. Worse yet, device manufacturers routinely fail at translating this reading into a meaningful usage number like  hours of remaining use.

Battery charging is another function of the BMS. Yet charging remains extremely primitive. Most mobile devices today charge using a method called constant-current constant-voltage (abbr. CCCV) that was invented in the 19th century to charge lead-acid batteries. Its simplicity certainly made it irresistible; but there is no free lunch. CCCV charging has now been clearly established as a primary cause of battery damage. Next time you look at your mobile device and wonder why it is not lasting you a full day as it did when it was new, you can start by pointing the finger to CCCV charging. Yet, most mobile devices still stick with this archaic charging approach.

If you are a battery user, you also might want to see additional information such as the health of your battery. Nope! You can’t get it from present-day BMS in your mobile device. You may want to charge your mobile device faster. Nope! You can’t do it. You may want to know whether your battery may have been defective from the onset. Nope again! Both you and the device manufacturer are in the dark. Yes, you can walk today into the store of your favorite wireless carrier (or operator) and tell them that your battery was defective, and there is virtually little they can do to prove or disprove your concern. Insist a little and you will walk away with a replacement smartphone or mobile device. And while you are at it, let them know that you want more features such as faster charging!

This is the sad state of battery management today. It’s not because innovation is lacking or the technology is behind. Solutions do exist. Device manufacturers are slow to implement innovation. So let them know what you want!

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