Cypress CY8C29x66, AN2309, CY8C24794 specifications Two-Cell Battery Charger Hardware

Cell Balancing Time: If C is the cell capacity and Vb is

the battery voltage, and the requirement is to eliminate

the amount of imbalance (in percent) in one hour of balancing time, then the power dissipation on balancing

circuit P	is:
bal
	Pbal	C	Vb	Equation 10
			100%

For example, balancing the cells for one hour with a battery capacity of 2000 mAh and an imbalance of 15 percent results in the following approximate amount of power dissipation on the balancing circuit:

	2000mAh 4.2V 15%		Equation 11
P		1.26W
bal	100%

Thus, there is a tradeoff between the rate of balancing and power dissipation. Faster balancing provides more options and flexibility, but it also results in increased power dissipation, which increases cost and board space. The one charge/discharge period can be selected as a favorable time for cell balancing.

Cell Capacity: If n is the count of cells connected in

parallel, C is the cell capacity, and is the amount of imbalance in percent (capacity and charge level variation), then the highest required balancing current during one hour is the following:

Ibal	C n	Equation 12
	100%

For example, the initial balancing level is:

Ibal	2000mAh 2	15%	600mA Equation 13
	100%

If the balancing circuit resistance is set to equal 100Ω, then:

Ibal 4.2V /100	42mA	Equation 14
P 4.2V 0.042A	0.1764W	Equation 15

Using a four hour discharge time and a two hour charge time during one complete discharge/charge cycle with full time cell balancing on both phases, 42 mA*(4+2)=252 mA is removed from one unbalanced cell. Therefore, the balancing level from this example can be removed during three discharge/charge cycles with a balancing circuit resistance of 100Ω or during one complete cycle with 40Ω.

For maximum cell balancing, use a balancing circuit resistance of 40Ω to 200Ω and perform cell balancing during both charge and discharge phases. Note that the overnight conditioning cell-balancing algorithm is not implemented in this project. The reason is that the CY8C24xxxA device used in this implementation does not have enough ROM memory space. If you choose another PSoC device family for the same project, the overnight conditioning cell- balancing algorithm can easily be added (see AN2258, “Cell Balancing in a Multi-Cell Li-Ion/Li-Pol Battery Charger”). But

AN2309

for most applications it is not necessary to use this algorithm.

The cell-balancing technique is explained in detail in AN2258, “Cell Balancing in a Multi-Cell Li-Ion/Li-Pol Battery Charger.”

Two-Cell Battery Charger Hardware

Li-based batteries use a two-stage charge profile (activation and rapid-charge). If the battery voltage is less than 2.9 to

3.0volts per cell, the battery must be activated first. In the activation stage, the battery is charged with a constant current (0.05-0.15 CA, where CA is the nominal battery capacity) until the battery voltage reaches a predefined level. The activation charge time-out is set to 1.5 to 2 hours. The activation charge can diagnose battery health and identify troubles such as damaged or shorted cells.

The rapid-charge stage starts after the activation charge finishes without error. This stage consists of two modes: constant current and constant voltage. When the battery voltage is less than the predefined level (4.1V or 4.2V depending on battery type), the charge is processed in constant current mode (0.5-1.0 CA). When the battery voltage reaches this level, the charge source switches to constant voltage mode and the charge process is terminated when the current drops below a predefined limit (0.07- 0.2 CA).

The rapid-charge stage must be protected by time limits. The rapid-charge time is limited to three hours. The charge profile for Li-Ion/Li-Pol batteries is shown in Figure 2. The technique to charge Li-Ion and Li-Pol batteries is explained in detail in AN2107 “A Multi-Chemistry Battery Charger.”

Figure 2. Li-Ion/Li-Pol Battery Charge Profile

Legend:

Ich - Battery charge current

1Iact - Battery activation charge current, 0.1-0.2 CA

2Irap - Battery rapid charge current, 0.7-1 CA Vb - Battery voltage

3Vrs - Rapid start voltage, typically 3 V/cell

4- Constant-current / constant voltage switching point

5Vmax - Emergency shutdown voltage, 4.3 V/cell

6- Rapid charge termination current, typically 0.1 CA

7Trmax - Battery rapid charge maximum temperature, 45 oС

8Trmin - Battery rapid charge minimum temperature, 0 oC Tb - Battery temperature

trch - Rapid charge termination time tcv - Constant voltage charge time

November 25, 2007 Document No. 001-17394 Rev. *B - 4 -

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