Safety

Cell Balancing

Outputs

bq6400

management. Temperature is measured by one internal sensor and up to 12 additional multiplexed external

temperature sensors, for a total of up to 13 independent measurement points in the pack. Coulomb counting is

captured continuously by a dedicated 18-bit integrating Delta-Sigma A/D converter. The bq6400 is also

responsible for pack data calculations, black-box forensic data storage and communicating parameters via

SMBus to a host processor as the core of a Smart Battery System (SBS).

Unique in the battery management controller market, the bq6400 simultaneously measures voltage and current

using independent and highly accurate Delta-Sigma A/D converters. This technique removes virtually all systemic

noise from measurements, which are made during all modes of battery operation – charge, discharge, and rest.

Battery impedance and self-discharge characteristics are thus measured with an unprecedented level of

accuracy in real-time. The device applies this precise information to SmartSafety™ algorithms to detect certain

anomalies and conditions which may be indicative of internal cell faults, before they become serious problems.

The bq6400 uses its enhanced measurement system to detect pack faults including cell under and over-voltage,

cell under and over-temperature, pack over-voltage, and pack over-current, over-charge, and short circuit

level safety algorithms will open the in-line chemical fuse and provide permanent, hard protection for the pack

and user. External MOSFET control inputs with programmable polarity can also be used to operate the protection

MOSFETs under control of user supplied circuitry. The bq6400 continuously monitors these inputs. If the

level safety algorithms have programmable time delays to prevent false triggering on noise events.

Patented PowerPump™ cell balancing drastically increases the useful life of battery packs by eliminating the

cycle life fade of multi-cell packs due to cell imbalance. PowerPump™ efficiently transfers charge from cell to

cell, rather than simply bleeding off charging energy as heat the way competitor’s circuits using resistive-bleed

balancing do. Balancing is configurable and may be performed during any combination of 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 and can range from 10mA to 1A depending on component

selection and application or cell requirements.

A variety of techniques, such as voltage or State-Of-Charge balancing, are easily implemented by the bq6400.

By tracking the balancing required by individual cells, overall battery safety is enhanced – often allowing early

detection of soft shorts or other cell failures. Balancing is achieved between all cells within the pack as

dynamically determined by the bq6400.

Charge Control

The open drain outputs CHG and PRE are used to drive MOSFET transistors controlling cell stack charging.

Charge or Pre-charge mode is selected based on the current cell voltage compared to the user-definable cell

pre-charge under-voltage thresholds. When below the limit, or when below the charge temperature minimum, the

PRE signal is active and CHG signal is inactive. This turns on the Pre-Charge MOSFET and is used to charge a

depleted pack through a current-limiting series resistor. When all cell voltages are above the limit and the

temperature is above the charge temperature minimum, then the CHG output also becomes active and enables

the Charge MOSFET to turn on and provide a high current path between the charger and battery cells.

The CHG and PRE MOSFET control outputs are both disabled (low) when a cell reaches any safety cutoff limit

or temperature threshold. During active charging modes (and above cell voltage thresholds), the Discharge

MOSFET is also enabled to avoid excessive heating of the body diode. Similarly, the CHG MOSFET is active

during discharge provided current flow is in the correct direction and no safety violations are present.

The CHG and PRE outputs are intended to drive buffer transistors acting as inverting level shifters.

Discharge Control

The DSG output operates similarly to control cell stack discharging. It is enabled (high) by default. If either a cell

voltage falls below the lower threshold, or excessive current or other safety related fault is sensed, the DSG

output is disabled (low) to prevent damage to the cell or pack.

Copyright © 2008, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Link(s) :bq6400