Cypress CY8C24794, AN2309 Cell-Balancing Foundation, Qcell 1 Qcell, Ccell 1 Vcell 1 Ccell 2 Vcell

AN2309

Table 1. Specifications for Two-Cell Li-Ion/Li-Pol Battery Charger with Cell-Balancing Support

Item			Item Value
Battery Charger Parameters
Built-In Battery Charger Type		Two-cell Li-Ion/Li-Pol battery charger
Power Supply Voltage		10…14V
Power Consumption		35 mA
Battery Current Measurement Error (Not Calibrated)		5 percent
Battery Voltage Measurement Error (After Calibration)		0.5 percent
Battery Thermistor Resistance Measurement Error		5 percent
User Interface		2 LEDs
PC Communication Interface		RS232
PC Communication Speed	115200
Cell-Balancing Parameters
Cell-Balancing Algorithms	1.		During charge phase
	2.		During discharge phase
			Cell-balance circuit resistors nominal
			Cell-balance interval parameter
Cell-Balancing Configuration Parameters			Minimum cell-balance parameter for charge phase
			Minimum cell-balance parameter for discharge phase
			Minimum charge current value when cell balancing is allowed
			VMID value for discharge phase (voltage of middle charged state)
Minimum Cell Balancing During Charge Phase		Equal to the voltage measurement error value (15 mV-30 mV)
Minimum Cell Balancing During Discharge Phase		Equal to the voltage measurement error value (15 mV-30 mV) plus the
		internal impedance error (10 mV-30 mV)

Cell-Balancing Foundation

This section describes the fundamentals of cell-balancing techniques. Cells are considered balanced when:

Qcell1 Qcell2 Equation 1

The value QcellN is the charge of cell N. The equation for the charge is:

Q I t C V Equation 2

Therefore, Equation 1 can be transformed into the following equation:

Ccell1 Vcell1 Ccell2 Vcell2 Equation 3

The value VcellN is the electrochemical potential of the fully charged cell. The VcellN potential is fixed for a given set of

electrodes is fixed and does not change from cell to cell. When two cells are unbalanced, the following is true:

Q	Q		Equation 4
cell1	cell2
Ccell1 Vcell1	Ccell2	Vcell2	Equation 5

However, Vcell does	not change from cell to cell.
Therefore, the cells are unbalanced if:
Ccell1	Ccell2	Equation 6

Equation 6 shows two cells that have different capacities, which is one cause of cell imbalance. A difference in cell- charge levels, which can be identified by using Equation 4, is the second cause of cell imbalance. For both kinds of mismatches in the battery pack – different cell capacities and difference cell charge levels – the highest voltage cell shows relative charge redundancy and must be shunted during the charging/discharging process. This is the heart of the cell-balancing issue.

The main reasons for variation in cell capacity are:

Variations in cell assembly. Today’s factory manufacturing of cells produces Li-Ion battery backs with cell capacity matched to three percent.

Different rates in cell degradation. The self-degradation rate is 30 percent at 500 cycles, which equals 0.06 percent per cycle. But individual cells degrade differently depending on temperature, charge voltage, and the particular self- degradation process. For example, a cell with a lower capacity is exposed to a higher charge voltage, which degrades it faster, further reducing its capacity and increasing the pack imbalance.

November 25, 2007 Document No. 001-17394 Rev. *B - 2 -

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