Cypress CY8C24794, AN2309, CY8C29x66 specifications Cell-Balancing Algorithm

AN2309

Figure 10. Two-Cell Battery Charger Firmware Flowchart Part 2

1									2
State	Yes	Charge Off	Check Charge	Yes	Set Initialization
Charge			Restart
		Timers Off			State
Complete			Condition
No			No
State	Yes	Charge Off	Cell Balancing		Check For	Yes	Set Wait For
Wait For					Discharge Stop
		Timers Off	Reset				Temperature State
Temperature					Temperature
No					No
					Check For	Yes	Set Initialization
					Negative Ich		State
					No
							Check For	Yes	Set Initialization
							Charge Restart
									State
							Temperature
							No
									Set Wait For
									Temperature State
State	Yes	Charge Off	Cell Balancing
Error					True
		Timers Off	Reset
No					Yes
State	Yes	Charge Off	Check For	No	Set Initialization
Discharge
		Open LOAD Out	Negative Ich		State
No			Yes
			Check Cell	Yes	Cell Balancing
			Balancing
			Interval
			No
State	Yes	Charge Off	Cell Balancing		Check For	No	Set Initialization
Full Discharge		Timers Off
			Reset		Negative Ich		State
		Close LOAD out
No					Yes

Cell-Balancing Algorithm

At first sight, the cell-balancing algorithm for a two-cell battery charger appears very simple. The criterion for the cell imbalance is the voltage difference between the cells. The cell with a greater voltage must be shunted. But this algorithm can lead to still more imbalance. During cell balancing only intrinsic cell voltage must be taken into account. The voltage portion contributed by the impedance of the cell leads to errors in cell balancing. In the deep discharge battery, where the internal resistance of the battery can be as high as several ohms, the I x R drop dominates the overall cell voltage. For this reason, cell balancing is not recommended when the battery pack is close to deep discharge. Cell balancing during this time can lead to greater imbalance than before cell balancing was conducted.

During the 1-C rate charge, the battery has reached approximately 50 percent of the charged state when its voltage has risen above 3.9 volts.

If the charging current is less than 1C, this threshold can be reduced. At this charge state, the internal resistance drops below 0.2Ω and the distortion level is within acceptable limits. Therefore, some cell-balancing methods can be executed if the cell voltage is above the predefined VMID value (voltage of middle charged state) and the minimum cell-balance parameter consists of the voltage measure error value plus the internal impedance error value.

A better practice, which yields more accurate cell voltage measurements, is to perform the cell sampling operation after suspending or interrupting the charge current - the pulse charge technique. With this technique, the charge operation is temporarily interrupted to permit voltage measurement of the cells in the pack. Such suspension of charging eliminates the contribution of cell impedance to cell voltage measurements and yields more accurate indication of cell mismatches.

When the pulse charge technique is used, the minimum cell- balance parameter equals the voltage measure error value and, therefore, cell balancing can be executed at any time during the full charge cycle. In the present implementation, the pulse charge technique is used. As shown in Figure 11 on page 15, the charge operation is interrupted before voltage measurement.

At the end of the charge process, the shunted current switching on the cells (to achieve cell balance) can result in a premature system shutdown. Therefore, during constant voltage mode of the rapid-charge stage, if the charge current stays below the minimum cell-balance parameter, the balancing process stops. Note in Figure 11 the “Check Out of the Minimum Cell Balancing Current” condition.

Cell balancing during the discharge phase also is executed if the maximum cell voltage is above the predefined VMID value. See in Figure 11 the “Check Out of the VMID Voltage” condition. The discharge VMID value can differ from the charge VMID value (described earlier in this section), and its value is dependent on the discharge rate.

November 25, 2007 Document No. 001-17394 Rev. *B - 14 -

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