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A BETTER BATTERY CHARGER

Part 1: Charging and Termination
by Thomas Richter

Start ı Battery Technologies ı Safe Charging of Batteries ı Termination Methods ı Sources and PDF

BATTERY TECHNOLOGIES

The charging of a battery is made possible by a reversible chemical reaction that restores energy in a chemical system. The battery characteristics depend on the particular chemicals used. When designing a charger, a detailed knowledge of these characteristics is required to avoid damage caused by overcharging.

Modern consumer electronics use mainly four different types of rechargeable batteriesıSLA, NiCd, NiMH, and Li-Ion. Iıll give you some background information on these batteries so youıll be able to select the right battery and charging algorithm for the application.

First letıs look at sealed lead acid batteries which are used in many applications where cost is more important than space and weight. They are typically preferred as backup batteries for UPS and alarm systems. The SLA batteries are charged using constant voltage, with a current limiter to avoid overheating in the initial stage of the charging process. SLA batteries are easy to work with because they can be charged indefinitely, as long at the cell voltage never exceeds the manufacturer specifications (usually 2.2 V).

Nickel cadmium batteries are widely used today because they are relatively inexpensive and convenient to use. Their popularity also stems from the fact that a typical NiCd cell can be fully charged up to 1,000 times. They do have some drawbacks, however. They have a high self-discharge rate, can be damaged from being reversed, and the first cell to discharge completely in a battery pack will be reversed. To avoid damaging discharge of a battery pack, the voltage should be constantly monitored, and the application should be shut down when the cell voltage drops below 1.0 V. NiCd batteries are charged with constant current.

Nickel metal hydride batteries are the most widely used battery type in new lightweight portable applications (i.e., cell phones, camcorders, etc.). Because they have a higher energy density than NiCd batteries, they pack a bigger punch in an equal package. They can, however, be damaged from overcharging. Therefore, it is important to do accurate measurements to terminate the charging at exactly the right time (i.e., fully charge the battery without overcharging). Like NiCd, NiMH batteries are damaged from being reversed.

NiMH has a self-discharge rate of approximately 20% per month, and like NiCd batteries, NiMH batteries are charged with constant current.

Lithium-ion batteries have the highest energy/weight and energy/space ratio compared to the other batteries mentioned. Li-Ion batteries are charged using constant voltage, with current limiting to avoid overheating in the initial stage of the charging process. The charging is terminated when the charging current drops below the lower current limit set by the manufacturer. It is important to note that this type of battery gets damaged from overcharging and may explode when overcharged.

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