Datasheet

OPERATION
Cell-Voltage Measurement
Cell-Temperature Measurement
Cell Balancing
bq76PL102
SLUS887A – DECEMBER 2008 – REVISED OCTOBER 2009 .......................................................................................................................................... www.ti.com
Voltage measurements are made using one-per-cell precision delta-sigma analog-to-digital converters (ADC). An
internal calibrated band-gap voltage reference is provided with each part. Measurements are performed when
commanded by the bq78PL114 master-gateway battery-management controller via the one-wire PowerLAN
serial communications bus. This allows all cells to be measured at exactly the same time under the same load
conditions.
Temperature measurements can be obtained using one internal and up to two external sensors. Each external
sensor consists of one (or two for increased accuracy) series-connected diodes and a capacitor for filtering. The
use of dual diodes in a single SMT package is recommended (MMBD4148SE or equivalent). The diode can be
located up to 6 inches (15 cm) from the circuit board. The RF filter capacitor should be co-located very close to
the diode to minimize unwanted noise coupling.
The temperature measurement subsystem uses the same dual ADCs that are used for measuring voltages.
Temperature measurements are fully independent of voltage readings, and are ordinarily interleaved at a
fractional rate of the voltage readings by commands from the bq78PL114 master-gateway battery-management
controller.
Balancing is provided among any number of supported cells. The bq76PL102 and PowerLAN family of
master-gateway battery controllers is optimized for designs using more than four cells in series.
The patented PowerPump reactive cell balancing dramatically increases the useful life of battery systems by
eliminating the cycle life fade of multicell batteries due to cell imbalance. PowerPump efficiently transfers charge
from cell to cell, rather than simply bleeding off charging energy as heat. Charge is moved from higher-capacity
cells to lower-capacity ones, and can be moved as needed between any number of series cell elements.
Balancing is performed during all battery operational modes – charge, discharge, and rest. Compared to resistive
bleed balancing, virtually no energy is lost as heat. The actual balance current is externally scalable with
component selection and can range from 10 mA to 1 A (100 mA typical) depending on application or cell
requirements. (See the reference schematic, Figure 7 .)
Algorithms for cell balancing are centrally coordinated by the bq78PL114 PowerLAN master-gateway
battery-management controller and directed across the array of bq76PL102 dual-cell Li-Ion battery monitors.
Balancing is done in both directions by the bq76PL102s within the cell stack array: north or up the cell stack and
south or down the cell stack. Each bq76PL102 node provides the circuitry to transfer (pump) the charge from cell
to cell to provide balancing. The balancing algorithm is implemented in the bq78PL114 master-gateway battery
controller, and commands are communicated to the bq76PL102s via the PowerLAN communications link. By
tracking the balancing required by individual cells, overall battery safety is enhanced – often allowing early
detection of internal micro-shorts or other cell failures.
Cell balancing pumping, or charge transfer from one cell to another, is accomplished using a circuit that forms a
simple flyback converter under control of the bq76PL102, which is in turn controlled by the master gateway. The
outputs of PUMP nd (cell number, direction) control MOSFET transistors which charge an inductor from one cell
and then discharge the inductor into an adjacent cell through the intrinsic body diode of the other MOSFET.
• PUMP1S: Pumps charge from cell 1 to the next lower cell (closer to battery negative). This signal is unused
by the first or lowest cell in the string.
• PUMP1N: Pumps charge from cell 1 to cell 2.
• PUMP2S: Pumps charge from cell 2 to cell 1
• PUMP2N: Pumps charge from cell 2 to the next higher cell in a pack (closer to battery positive). This signal is
unused by the highest cell in the string.
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