BU-602: How does a Battery Fuel Gauge Work?

The lifespan of a battery cannot be defined by the number of cycles or age alone but to a large extent by its usage (or misusage). As the capacity fades, the runtime gets shorter. The smart battery captures this capacity fade by reading the previous energy delivered, but these vital health statistics remain mostly hidden from the user. The battery continues to be a “black box” that conceals vital performance records and masks when the battery should be replaced.

One of the main tasks of the smart battery is to establish communication between the battery, charger and user. A fuel gauge indicating state-of-charge fulfills this in part. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears. Figure 1 shows a fuel gauge of a battery that is 75 percent charged with three lights glowing.

While the SoC information displayed on a battery or a display screen is helpful to the user, the readout does not guarantee the runtime. The fuel gauge resets to 100 percent with a full recharge regardless of how much capacity the battery can store.

A serious breach of trust occurs if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a 4-hour runtime, a battery holding half the capacity would run for only 2 hours. Many users are unaware that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.

Other than applying a controlled discharge, there is no reliable method to measure the capacity of “chemical battery” but there is a way to read the “digital battery.” The term chemical battery refers to the actual capacity derived by discharging a fully charged pack, whereas the digital battery is a peripheral monitoring circuit that stores the estimated capacity derived by coulomb counting when charging and discharging a battery as part of field use.

The SMBus battery stores the factory-set design capacity in Ah or 100 percent by default. With each full charge, the battery resets the full-charge flag and during discharge, the coulomb counter measures the energy consumed. The in-and-out-flowing coulombs can be used to estimate battery state-of-health known as full charge capacity (FCC). As the battery fades with usage and time, so also does the delivered energy decrease, and the FCC number will decline. The FCC accuracy of a battery that is being deep cycled is about +/-5 percent compared to capacity readings taken by discharging. Periodic calibration will improve the FCC accuracy.

Capacity can also be estimated by coulomb counting during charging. This works best with an empty battery. A battery with a 100 percent capacity will receive the full coulomb count; one with only 50 percent capacity will only accept half before the battery reaches full charge. Not knowing the residual SoC when the coulomb count begins will affect the accuracy. SoC can be estimated by measuring the battery’s open circuit voltage (OCV), but this only gives a rough approximation as agitation after charge or discharge, as well as temperature, affects the OCV.

Tri-state Fuel Gauge

The SoC and capacity information can be shown on a linear display using colored LEDs. The green lights indicate the usable capacity; the empty part of the battery is marked with un-lit LEDs; and the unusable part is shown with red LEDs. Figure 2 illustrates a tri-state fuel gauge. The results can also be shown on a digital display.

The tri-state fuel gauge reads the “learned” battery information on the SMBus and displays it on a multi colored LED bar. The illustration shows a partially discharged battery of 50% SoC with 20% empty and 30% unusable.

The tri-state fuel gauge provides state-of-function (SoF), the ultimate in battery diagnostics. Some device manufacturers are hesitant to offer this feature to consumers because this could lead to elevated warranty claims. A replacement only becomes mandatory if the battery capacity drops below 80 percent; keeping the evidence hidden is seen as the least disruptive method. SoF can always be accessed by a service code. SoF works best for industrial uses.

Vehicles with electric propulsion do not show the charge but only the remaining driving range, thus hiding the capacity. To accommodate capacity fade that would shorten the driving range, the EV battery is being oversized and does not fully charge and discharge when new. Only as the battery ages and the capacity fades does the charging range gradually increase. Shorter driving ranges only become apparent once this grace buffer has been consumed. (See BU-1003: Electric Vehicle)

References

[1] Courtesy of Cadex