Power Management Low-Cost, Two-Cell Li-Ion/Li-Pol Battery Charger with Cell-Balancing Support Specification Sheet

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Power Management - Low-Cost, Two-Cell

Li-Ion/Li-Pol Battery Charger with

Cell-Balancing Support

November 25, 2007 Document No. 001-17394 Rev. *B - 1 -

AN2309

Author: Oleksandr Karpin

Associated Project: Yes

Associated Part Family: CY8C24x23A, CY8C24794, CY8C27x43, CY8C29x66

TGET FREE SAMPLES HERETH

Software Version: PSoC Designer™ 5.0 SP1

Associated Application Notes: AN2107, AN2258, AN2267, AN2294

PSoC Application Notes Index

Application Note Abstract

This application note describes a low cost, two-cell Li-Ion/Li-Pol battery charger. An effective cell-balancing algorithm during

both charge and discharge phases is presented. This charger can be used either as a standalone application to charge a

battery pack with two serial connected Li-Ion/Li-Pol batteries or embedded in residential, office, and industrial applications.

Introduction

A modern portable system requires more operating voltage

than a single-cell Lithium-ion (Li-Ion) or Lithium-polymer (Li-

Pol) battery can provide. A serial connection results in a

pack voltage equal to the sum of the cell voltages. To

increase the battery pack capacity, the cells are connected

in parallel. For many applications, two cells in series are

sufficient, with one or more cells in parallel. This

combination gives nominal voltage and the necessary power

for laptop computers and medical and industrial

applications. Problems can occur when the cells have

different capacities or charge levels. During charging or

discharging, the cells in the battery pack do not have

matched voltage every cell. Therefore, the battery pack is

not balanced. The unbalanced charge between cells causes

the following problems:

 Reduced overall battery pack capacity to the value of

the cell with the least capacity. During the charge

process, this cell reaches the maximum charge level

before the other cells, and during the discharge process

this cell is depleted before the other cells in the pack.

 Reduced overall battery pack life. The charge or

discharge of cells at different values increases pack

imbalance.

 Cell damage, which occurs if the charger monitors only

the summary voltage. For example, if the lower cell has

a capacity deficiency of at least 10 percent, its cell

voltage begins to rise into the dangerous area above

4.3 volts. This can result in additional degradation of the

cell or a safety system response that greatly reduces

pack capacity.

This application note describes a two-cell Li-Ion/Li-Pol

battery charger. An effective cell-balancing algorithm is

designed. It avoids the issues that appear in battery packs

with two cells in series. Through modification of the

configuration parameters, the cell-balancing algorithm can

easily be adapted for various applications and selected

batteries. The unique architecture of the PSoC

device

provides an integrated hardware solution for a two-cell

battery charger and a flexible μC-based, cell-balancing

algorithm with minimal external components at a very

affordable price. The CY8C24x23A PSoC device family

used in this implementation reduces the total device cost

even further.

When you want to use algorithms for the latest charging or

cell-balancing technologies, only the firmware needs to be

modified. PSoC Designer’s in-circuit and self-programming

capabilities make these operations simple.

Specifications for a two-cell Li-Ion/Li-Pol battery charger with

cell-balancing support are listed in Table 1 on page 2.

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Summary of content (21 pages)

PAGE 1
Power Management - Low-Cost, Two-Cell Li-Ion/Li-Pol Battery Charger with Cell-Balancing Support AN2309 Author: Oleksandr Karpin Associated Project: Yes Associated Part Family: CY8C24x23A, CY8C24794, CY8C27x43, CY8C29x66 GET FREE SAMPLES HERE Software Version: PSoC Designer™ 5.0 SP1 Associated Application Notes: AN2107, AN2258, AN2267, AN2294 PSoC Application Notes Index T TH Application Note Abstract This application note describes a low cost, two-cell Li-Ion/Li-Pol battery charger.
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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.
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AN2309  Temperature gradient across the battery pack. Temperature mismatches of 15 degrees Celsius can cause up to 5- percent capacity differential among cells. Such a temperature gradient is relatively common in densely packed products, where multiple heat sources are located close to the battery pack. An example of this is a laptop computer. The balancing circuit is represented by (R1, Q1) and (R2, Q2). These transistors and resistors dissipate energy and control the amount of balancing current.
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AN2309  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: 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.
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AN2309 A two-cell battery charger structure with cell-balancing support is shown in Figure 3. Similar battery charger structures are explained in detail in AN2258, AN2294, and AN2267. Note that the fuel gauge function can easily be added to this project without changing any hardware: It is only necessary to switch from the CY8C24423A to a PSoC device with more program memory. The main fuel gauge calculation parameters are described in AN2294, “The Li-Ion/Li-Pol Battery Charger with Fuel Gauge Function.
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AN2309 Device Schematic The schematics shown in Figure 4 on page 7 and Figure 5 on page 8 constitute a complete two-cell battery charger. A signal from the PWM goes to the RC-filter, which consists of resistor R4 and capacitor C4. A constant voltage signal proportional to the PWM duty cycle value forms at the Q2 gate. Therefore, the PWM and RC-filter is a simple implementation of a PWM-DAC. The bipolar transistor Q2 is driven by an analog signal from the PWM-DAC.
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AN2309 Figure 4. Two-Cell Battery Charger Schematic – CPU, Cell Balancing, and Measuring Equipment Q1 IRLML6402 D1 POWER+ BAT+ C1 R1 0.01uF 10K + C2 47uF C3 MBR360 1uF CER R4 1K Q2 BC817 DRIVE C4 0.1uF R5 15K R7 150K 0.1% V2 Vi2 TP1 Vref V1 5 6 7 8 9 10 11 12 13 BAL1 BAL2 14 R6 50K 0.1% P0[7] P0[5] P0[3] P0[1] P0[6] P0[4] P0[2] P0[0] P2[7] P2[5] P2[3] P2[1] P2[6] P2[4] P2[2] P2[0] SMP Xres P1[7] P1[5] P1[3] P1[1] P1[6] P1[4] P1[2] P1[0] R8 1M 28 Vcc 1 2 3 4 C5 0.
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AN2309 Figure 5. Two-Cell Battery Charger Schematic – Power Supply and User Interface VCC Close to PSoC SW1 + J4 R29 POWER+ 1 2 + C9 POWER 12V DC C10 100u 16V VCC 1K C15 Vbias 0.1u 16V D8 POWER- 0.1u BAS16 D6 VCC BAT+ U2 L78L05/TO 1 R30 IN OUT 3 R28 POWER+ 33 BAT54C C12 + 470 C11 + C13 C14 22u 0.1u 0.
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AN2309 Figure 6. PSoC Internal User Module Configuration Battery Measurement To provide a correct implementation of the charge and cellbalancing algorithms, the charge current, battery voltage and temperature must be measured accurately. These three parameters are measured as the voltage drops on corresponding resistors by using the instrumental amplifier INA. The measurement is implemented as a twostage procedure to eliminate any voltage offset from the INA and ADC inputs.
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AN2309 The voltage measurement also is performed by the INA on the corresponding resistor. The resistive dividers (R7, R6), (R13, R12), and (R18, R19) transform cell voltage into signals suitable for the PSoC device. It is very important to use the high precision resistors in the resistive divider to obtain a high value common mode signal rejection. The recommended R6, R7, R12, R13, R18, and R19 tolerances are 0.1 percent.
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AN2309 Figure 7.
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AN2309 Figure 8. Two-Cell Battery Charger State Diagram 10 Initialization 7 6 9 1 13 Wait For Temperature Activation Discharge 4 8 2 Rapid 11 5 Error 12 Full Discharge 3 Charge Complete Initially the charger is in the Initialization state. After some device preparation, the charger goes to the Activation state (1). When the battery voltage reaches the rapid start voltage, the charger leaves the Activation state and switches to the Rapid state (2).
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AN2309 Figure 9.
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AN2309 Figure 10.
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AN2309 The minimum cell-balance parameter consists of the voltage measure error value plus the internal impedance error value. The cell-balancing algorithm that is implemented here does not significantly lengthen the charge time. The charger monitors all of the cell voltages. Cell balancing is performed during both phases and it is realized in one common module. The cellbalancing algorithm is represented in Figure 11.
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AN2309 Two-Cell Battery Charger Parameters All two-cell battery charger parameters are located in the header file globdefs.h in the project folder. The header file globdefs.h contains the following parameters: Table 2.
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AN2309 Cell-Balancing Parameters All cell-balancing parameters are located in the header file globdefs.h in the project folder. The header file globdefs.h contains the following parameters: Table 3.
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AN2309 Appendix Charge/Discharge and Cell-Balancing Profile Examples Figure 13. Charge/Discharge Manager Profile Cell Voltages Without Charge Interrupt Charger State Charge /Discharge Current Cell-Balancing State COM # Drop-Down Field Start Button Thermistor Resistance Constant Voltage Charge Constant Current Charge Battery Discharge November 25, 2007 Document No. 001-17394 Rev.
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AN2309 Figure 14. Cell-Balancing Activity Profile Cell Voltages With Charge Interrupt Voltage Imbalance Value November 25, 2007 Document No. 001-17394 Rev.
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AN2309 Figure 15. Cell-Balancing Parameter Profile Screen About the Author Name: Oleksandr Karpin Title: Application Engineer Background: Oleksandr received a PhD’s degree in computer science in 2008 from Lviv Polytechnic National University (Ukraine). His interests include embedded systems design and new technologies. Contact: oleksandr.karpin@cypressua.com November 25, 2007 Document No. 001-17394 Rev.
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AN2309 Document History Document Title: Power Management - Low-Cost, Two-Cell Li-Ion/Li-Pol Battery Charger with Cell-Balancing Support Document Number: 001-17394 Rev. ECN Orig. of Change Submission Date ** 1352043 HMT 08/29/2007 *A 1736124 VICK 11/29/2007 *B 2612415 AESA 11/25/2008 Description of change Obtain spec. # for note to be added to spec. system. Update copyright. Add source disclaimer, revision disclaimer, Samples Request Form link, PSoC App. Note Index link. Same title in DMS, .