Battery Power - March/April 2012 - (Page 8)

Feature Lithium-Ion Battery-Emulation Circuit Using a Microcontroller Mark Pallones, Senior Applications Engineer Advanced Microcontroller Architecture Division Microchip Technology, Inc. Battery-powered devices, such as mobile phones, laptops, PDAs and other portable consumer media devices are quickly evolving to include sophisticated functionality in a small form factor, with ever-increasing demands on battery power. For this reason, battery-powered devices have created demand for high energy-density batteries, which has lead to the popularity of lithium-ion (Li-Ion) batteries. Charging Requirements Lithium-Ion Battery Characteristics Aside from superior energy density, Li-Ion batteries have a low self-discharge rate. Improved self discharge increases the shelf life of the batteries and permits them to initially have nearly a full charge when put in use. Additionally, Li-Ion batteries do not exhibit memory effect, which means that they do not need to be cycled periodically in order to maintain capacity. Amidst these appealing characteristics, Li-Ion batteries also have a few drawbacks. For example, in comparison to other battery chemistries, they have a high internal resistance that prohibits their use in high discharge-current applications. Li-Ion batteries also have little tolerance for abuse. Excessive charging may result in rapid disintegration and cause permanent degradation of battery performance. This is the reason why Li-Ion battery packs employ an internal protection circuit. This circuit prohibits operation beyond the normal operating range of the batteries. However, this protection circuit should be viewed as the last line of defense; the battery charger should not utilize this circuit during normal operation. Refer to Figure 1 for a detailed description of the Li-Ion battery voltage charge range. Properly restoring energy to a Li-Ion battery cell involves certain charging algorithms and accuracy requirements. To explain this algorithm and accuracy, let’s begin with a deeply depleted battery cell. The voltage across the deeply depleted battery cell is below the precondition voltage threshold. This threshold is typically 67 percent of a battery cell’s full-charge voltage. The full-charge voltage for a Li-Ion battery cell is 4.2 V. At this preconditioning stage, the battery charger should apply a continuous Constant Current (CC), with a typical maximum value of 0.1 C (10 percent of the fast-charge current) until the voltage across the battery cell reaches its precondition voltage threshold. This will properly restore a charge to the deeplydepleted battery cell and prepare it for the next fast-charging action. Forcing a fast-charge current while the battery cell is still in precondition stage will likely cause the battery to go into an irreversible short-circuit condition. When the voltage across the battery cell is raised above the precondition voltage threshold, the charge current will change from an initially low- to a high-value constant current to perform fast charging. The fast-charging current of a Li-Ion battery cell should be less than a 1.0 C rate. The recommended fast-charge current should be obtained from the battery manufacturer, in order to avoid degradation of the battery’s performance or life. The fast-charging stage will continue until the full battery voltage is achieved. After fast charging ends, a constant voltage (CV) stage is initiated. At this stage, the charge current diminishes as the output voltage of the battery charger stays at 4.2 V. A battery charger’s voltage regulation should be better than +- 1 percent. A small decrease in voltage results in a large decrease in capacity, and a small increase in voltage will reduce its useful life. It is not recommended to continue to trickle charge the Li-Ion battery cell. Charging should be terminated when the charge current is below 0.07 C. This is essential in order to avoid stressing the battery due to overcharging. Figure 2 illustrates the typical charging profile of a Li-Ion battery cell. Figure 1. Li-Ion Battery Voltage Range Figure 2. Li-Ion Battery Cell Charge Profile 8 Battery Power • March/April 2012 www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - March/April 2012

Battery Power - March/April 2012
Nissan and ABB Link to Evaluate Second-Life Storage Applications for Nissan LEAF Battery Packs
Nichicon Develops 50 kW Quick Charger for Evs
Lithium-Ion Battery-Emulation Circuit Using a Microcontroller
High-Efficiency Switching-Mode Charger System Design Considerations with Dynamic Power Path Management
Making Lithium-Ion Abuse Tests Meaningful
Batteries
Charging & Testing
ICs and Semiconductors
Power Supplies
Components
Industry News
Marketplace
Calendar of Events
Research and Development

Battery Power - March/April 2012