I always get excited to hear about a cool new portable design that uses TI chips inside – but then sometimes get equally flustered when I find out there’s no battery fuel gauge in the product.
You see, I'm part of the large team at TI that is dedicated to battery management, and more specifically on the ICs that tell you how much battery capacity you have left in your handy device. We call these "fuel gauges," or sometimes "gas gauges." and it baffles me sometimes how a system designer could create a battery-powered product, but not give the user a way to reliably tell how close to empty that battery is. Think about some of the products you use. and how the "battery" has made you angry. Has your phone ever said you had X% of battery left, and then suddenly shut down, requiring a recharge? (Note: it probably wasn't using a TI gauge.) Has your kid ever happily rolled his or her new remote-controlled toy from the land into the water, only to have it die in the middle of the water? Have you ever been documenting your awesome stunts (or your kid's birthday), but the battery dies in the middle of the slopes or party? Maybe the camera or device said "100%" when you started, but what does that mean? How does it translate to run-time in minutes or hours?
Back in the day when your cellular phone could only make phone calls, you might have been able to go a week between re-charges, and a battery indicator with a few bars might have been sufficient. You weren't worried about whether your charge would last until the next time you got home. Now that your phone is a little computer, and you rely on it for so many functions, daily battery anxiety is a common reality. You need fair warning if it's about to run out so you're not surprised. The same goes for just about any battery powered device. If you design mission-critical equipment, do you want your users to be out at a remote site when the battery gives out?
All of these unfortunate scenarios directly impact users' experiences, and their impression of your product (and brand). It's not only the reliability of your hardware that shades their opinions, but also the reliability of the information that your product gives them, including reliability of the battery predictions. Many of their complaints may be about the "battery," but really are directly attributable to the poor performance of the fuel gauge. If it reports 10%, but then shuts down because the battery is actually empty, they are not happy. If the fuel gauge is wrong in the other direction, it might report 0% too early, while there actually is energy still available in the battery. In this case, the user complains about short run-time. Either way, a lack of a gauge or choosing the wrong gauge can lead to an unhappy customer.
To be sure, not every battery-powered application needs a fuel gauge - and I would never try to sell sand in the desert, but most really could improve their user experience immensely if they had that feature. There are several reasons I have heard for NOT using a fuel gauge.
Some customers try to roll their own gauging solution, perhaps using the ADC in their microcontroller to measure the battery voltage. While it's true that battery voltages typically go down as they become empty, but relying on voltage alone is only appropriate if your load is extremely light relative to the battery capacity. Even then, the flatness of the Li-ion/Li-poly voltage curve means the voltage may not change by more than a few mV for a significant portion of the discharge. If your load is dynamic, then you can throw voltage look-up tables out the window. The internal impedance of the battery means that the voltage you see at the terminals will bounce up and down as your load current and operating temperature changes. Since battery impedance increases with age and use, the bounces will become more severe over time. Did you ever hang up a call on your old phone and see the battery bar bounce back up?
Fig. 1, Measuring open circuit voltage in a Li-Ion battery.
In order to accurately predict the remaining capacity of a battery, a more sophisticated algorithm is usually required that takes into account voltage, temperature, current, history, and other factors. You could try to come up with your own solution to handle these variables, or you could save your time and rely on TI's years of experience refining algorithms and specialized hardware.