Enhanced battery "gas gauge" keeps its data through glitches

Texas Instruments' BQ2010 battery-"gas-gauge" IC offers a convenient method for recording available charge stored in a nickel-cadmium or nickel-metal-hydride battery. However, even though plenty of charge remains available, under certain circumstances, transient-current spikes can fool the BQ2010 into registering a discharged battery. For example, spikes can occur when you connect a heating element or a switched-mode regulator containing a high-value input capacitor, or if you momentarily short-circuit the battery's terminals while making a connection.

During a current spike, the battery voltage decreases by the voltage drop across the battery's internal resistance plus the voltage drop across the circuit's current-sense resistor. The BQ2010 misinterprets the voltage decrease as a low-cell voltage condition normally seen during discharge. The device then loses data on the remaining battery capacity, and, depending on the application, of the BQ2010's Empty output, the load may inadvertently disconnect. Finally, the battery must contain a partial charge to unlatch the BQ2010's Empty output.

The circuit in Figure 1 improves the BQ2010's current-spike immunity in several ways and adds several useful features. First, a 3.3V, current-limited low-dropout voltage regulator, IC₄, supplies power to IC₁, the BQ2010. Second, an LTC1477 short-circuit-protected, high-side FET switch, IC₅, limits battery-to-load current to a maximum of 2A. To prevent IC₁'s SB (single-cell voltage) monitor pin from sensing an invalid Empty state, current-compensation amplifier IC_3B makes the voltage to the SB pin current-independent. The negative rail of IC_3B connects to the active side of the ground-referenced current-sense resistor, R₄. With no load current, op amp IC_3B's output should rest at 0V, but few rail-to-rail op amps provide outputs that go to 0V. The solution is to bias the positive side of the op amp enough to set the output above V_OL and compensate by lowering the gas-gauge-voltage sense-resistor ratio.

Additional features include a short-circuited load shutoff to prevent IC₅ from going into thermal-protection mode (Figure 2). Also, the entire circuit shuts off when the battery provides no current to the load or when the battery is discharged. A timer circuit consisting of IC₇, IC₈ and IC₉ provides an additional shutoff option. You can set the turnoff delay from minutes to days by changing R₃₃ and C₇ to reduce the clock frequency, or by selecting other taps on binary ripple counters IC₈ and IC₉. Pressing switch S₂ or starting a recharging cycle turns the controller back on. The parallel-connected sections of Schottky diode D₆ provide a current path for recharging the battery.

Although the resistor values shown in Figure 2 apply to a specific application, you can customize the circuit for a battery's chemistry, capacity, internal resistance, cell count, and timer and display options. All of the circuit's low-profile, surface-mounted components fit on one side of a 1.8-sq-in., four-layer board. The switches and LED readout connect to the pc board's underside.

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