Battery Fuel Gauge

One of the main tasks of the smart battery is to establish communication between the battery and user. A fuel gauge that indicates state-of-charge is part of this effort. 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.

Figure 1: State-of-charge readout of a “smart” battery Signal lights indicate the battery SoC when pressing the TEST button. Courtesy of Cadex

While the SoC information displayed on a  high quality battery or computer screen is helpful to the user, it does not assure sufficient runtime, because the fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious miscount 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 four-hour runtime, the same indication with a battery holding half the capacity would run for only two hours. The user should know that the fuel gauge only shows SoC and capacity is missing.

Other than using coulomb counting and measuring internal resistance, there is no reliable method to calculate the state-of-health (SoH) of a battery; current technologies provide only rough estimations. However, there is a way to display the capacity of a smart battery digitally.

At time of manufacture, the SMBus battery is programmed with a specified capacity, which is 100 percent by default, and the toshiba satellite a350 battery keeps this information as permanent data. With each full charge, the battery resets to the charge flag; and during discharge the coulomb counter measures the consumed energy. A perfect battery would deliver 100 percent on a calibrated fuel gauge. As the battery ages and the capacity drops, the delivered energy between charges decreases. The discrepancy between the factory-set 100 percent and the delivered coulombs after a full charge can thus determine the battery capacity.

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 dark LEDs; and the unusable part is shown with red LEDs. Figure 2 illustrates a tri-state fuel gauge. Alternatively, the results can be a shown on a digital display.


Figure 2: Tri-state fuel gauge.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 toshiba satellite a200 battery of 50% SoC with 20% empty and 30% unusable.
Courtesy of Cadex

Installed in an SMBus charger, the tri-state fuel gauge indicates the usable capacity of SMBus compatible batteries and gives the user a clear indication of when to retire a pack. The pass/fail level is user-selectable — at, say, 60, 70 or 80 percent — depending on the minimum required performance level. A failed battery prompts the user to first calibrate the battery. If the charge/discharge/charge cycle did not raise the capacity to the set target level, a red light on the charger indicates a fail. A lit green ready light always assures that the battery has been successfully charged and meets the capacity requirements. (Cadex offered such a charger but the buyers did not appreciate the benefit.)

Cars with electric propulsion systems use similar fuel-gauge technologies to determine the state-of-charge and state-of-function of the battery. The challenge is in knowing how far a vehicle can travel with a fully charged battery in various ambient conditions. A tailwind on a sunny day is more forgiving than a headwind with snow. The aging of a toshiba pa3594u-1brs battery adds to the challenge and the fuel gauge will lose accuracy over time. Showing 30 minutes of remaining runtime and then shutting down, as is common with laptops, will not be easily forgiven with the electric vehicle.