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Primary Charge Termination
Cell Balancing
bq2084-V143
SLUS732 – SEPTEMBER 2006
When charging begins on a depleted battery pack, the voltage is below Voff, and may even stay below Von for
some time. This means the pack is under constant charge, with no pulsing, for some part of the charge cycle. As
the voltage on the cells rises, it crosses the Voff threshold (or the Vmax threshold if Voff is disabled), and the
charge FET turns off. Initially, the off time is short, because the cells are only barely over the threshold and is
quickly relaxed to below Von. As the cell voltages rises, the off times become longer and the on times shorter.
This effect, in combination with the reduced current drawn by the cells, results in a gradually declining charge
current. Eventually, this current falls below the taper current, and the pack detects the full charge condition and
stops charging.
The pulse-charging control operates during normal charging conditions and are overridden in case of a fault
condition. Charging is stopped for any fault conditions which may occur, such as overtemperature or
overcurrent, without regard to the voltage thresholds or time limits.
The bq2084-V143 terminates charge if it detects a charge-termination condition based on current taper. A
charge-termination condition includes the following:
The bq2084-V143 detects a current taper termination when the pack voltage is greater than Charging
Voltage (DF 0x3a-0x3b) minus Current Taper Qualification Voltage (DF 0x4d-0x4e) and the
AverageCurrent() is below the Current Taper Threshold (DF 0x4b-0x4c), but greater than the Charge
Detection Current in DF 0x113-0x114, for a period of Current Taper Window (DF 0x4f).
Once the bq2084-V143 detects a Primary Charge Termination, the bq2084-V143 sets the
TERMINATE_CHARGE_ ALARM and FULLY_CHARGED bits in BatteryStatus(), and turns off the charge
FET via the bq29312. The charge FET is turned on when discharge current is detected and is greater than
Discharge Detection Current (DF 0x115-0x116), to minimize IR losses. The TERMINATE_CHARGE_
ALARM is cleared if charge current is no longer detected or the pack is removed, but returns if charging is
attempted while the FULLY_CHARGED bit is set.
The bq2084-V143 clears the FULLY_CHARGED and TERMINATE_CHARGE_ALARM bit when
RelativeStateOfCharge() is less than the programmed Fully Charged Clear %. See Table 13 for a summary
of BatteryStatus() alarm and status bit operation.
The bq2084-V143 balances the cells during charge by discharging those cells above the threshold set in Cell
Balance Threshold DF 0xe8-0xe9, if the maximum difference in cell voltages exceeds the value programmed in
Cell Balance Min DF 0xec. For cell balancing, the bq2084-V143 measures the cell voltages at an interval set in
Cell Balance Interval DF 0xed. On the basis of the cell voltages, the bq2084-V143 either selects the appropriate
cell to discharge or adjusts the cell balance threshold up by the value programmed in Cell Balance Window
0xea-0xeb when all cells exceed the cell balance threshold or the highest cell exceeds the cell balance threshold
by the cell balance window.
Cell balancing only occurs when charging current is detected and the cell balance threshold is reset to the value
in Cell Balance Threshold at the start of every charge cycle. The threshold is only adjusted once during any
balance interval.
If the cells are severely imbalanced during charging, where VCELL
(MAX)
- VCELL
(MIN)
> Cell Imbalance Threshold
AND SBS. Current()> Balance IMAX for a period of Cell Imbalance time then the CIM bit in PF Status is set.
Table 6. Cell Balancing and Cell Imbalance Programming
NAME DF ADDRESS DESCRIPTION
Cell Balance Threshold 0xe8-0xe9 Sets the maximum voltage in mV that each cell must achieve to initiate cell balancing.
Programming Cell Balance Threshold to 65,535 disables cell balancing.
Cell Balance Min 0xec Sets in mV the cell differential that must exist to initiate cell balancing
Cell Balance Window 0xea-0xeb Sets in mV the amount that the cell balance threshold increases during cell balancing
Cell Balance Interval 0xed Sets the cell balancing time interval in seconds.
Cell Imbalance Threshold 0xee-0xef Sets the severe imbalance fault limit for cell imbalance detection
Balance IMAX 0xf2-0xf3 Sets the charge current required to allow a cell imbalance to be detected
Cell Imbalance Time 0x134 Sets the time period during which a cell imbalance must be selected for the bq2084 to enter
PF mode.
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