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SLUSAA0 – OCTOBER 2010
The bq78412 is a complete Pb-Acid gas-gauge with a run-time display and warranty information storage. It
supports large batteries up to a maximum capacity of 327 Ahr when measured at the 20 hour rate.
Measurement inputs include the 12-V nominal battery voltage and the battery current. Coulomb counting on
discharge and charge allows a state-of-charge calculation and run-time-to-empty on discharge estimation.
Cumulative usage information is periodically and permanently stored internally and may be retrieved only by a
special sequencing operation performed by the manufacturer.
Operation of the bq78412 requires no user interaction. During charge and discharge, the LED display is
automatically activated when charge or discharge current is detected above a configurable threshold.
Current Sense, Battery Voltage, Temperature, and Time Measurements
The bq78412 measures charge and discharge current using a low-value (between 1 and 3 m
Ω) sense resistor
placed in the negative power path of the circuit. This sense resistor may be as simple as a piece of thermally
stable metal or the lead power post on the battery itself. Calibration of this sense resistor is required in circuit (in
module). The printed circuit board (PCB) designer must consider the impact of drift and/or variation in the sense
resistor value over time and temperature, including self-heating temperature effects. The bq78412 does not
compensate for such changes.
The voltage measured between the RS+ and RS– pins is scaled by the sense resistor value (set in MeasScale
parameter) to calculate the current value. The maximum differential voltage allowed between the RS+ pin and
the RS– pin is 160 mV.
Alternatively, a voltage proportional to the current (derived using means other than a sense resistor, but within
range of the allowable differential) could be applied to the terminals to provide the current measurement.
The bq78412 measures the battery voltage between the VS and AVSS pins
The bq78412 has an on-chip temperature sensor. The battery temperature is assumed to be equal to the on-chip
Time measurement is referenced to an internal oscillator. However, for more accuracy, an external 8-MHz crystal
oscillator or crystal can be used. This is enabled by setting DevConfig2 = 1. The switch-over happens only
after a hardware or software reset.
State-of-Charge (SoC) Gas-Gauging
The bq78412 provides capacity and run-time-to-empty estimates for Pb-Acid batteries using a rate and
temperature compensated coulomb counting algorithm.
The gas-gauging information is used to drive the local LED display with run-time-to-empty information.
Capacity correction is supported based on the discharge current. A 64-byte battery characterization table
contains battery performance data that is used to adjust the remaining capacity and run-time-to-empty as a
function of discharge rate and temperature. This information is unique to each battery model and is programmed
at the battery manufacturing facility based on battery performance data provided by the manufacturer.
Charge Efficiency Compensation
The bq78412 provides a parameter, ChgEff that allows for correction of accumulated charge in the battery due to
charge efficiency. During charge, the passed charge is multiplied by the charge efficiency and the result is added
to the remaining capacity.
For example, if ChgEff is set to 85 (representing 85%), when 100 Ah have been measured, only 85 Ah are
recorded as actually being accumulated. With the default setting for the ChgEff = 100, all charge current is
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Copyright © 2010, Texas Instruments Incorporated
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