AVR Battery Charger for SLA, NiCd, NiMH and Li-Ion Batteries.pdf

AVR450: Battery Charger for SLA, NiCd,

NiMH and Li-Ion Batteries

Features

• Complete Battery Charger Design

• Modular “C” Source Code and Extremely Compact Assembly Code

• Low Cost

• Supports Most Common Battery Types

• Fast Charging Algorithm

• High Accuracy Measurement with 10-bit A/D Converter

• Optional Serial Interface

• Easy Change of Charge Parameters

• EEPROM for Storage of Battery Characteristics

8-bit

Microcontroller

Application

Note

Description

The battery charger reference design is a battery charger that fully implements the lat-

est technology in battery charger designs. The charger can fast-charge all popular

battery types without any hardware modifications. It allows a full product range of

chargers to be built around a single hardware design; a new charger model is

designed simply by reprogramming the desired charge algorithm into the microcon-

troller using In-System Programmable Flash memory. This allows minimum time to

market for new products and eliminates the need to stock more than one version of

the hardware. The charger design contains complete libraries for SLA, NiCd, NiMH,

and Li-Ion batteries.

Figure 1. Battery Charger Reference Design Board

Rev. 1659B–AVR–11/02

The battery charger reference design includes two battery chargers built with the high-

end AT90S4433 microcontroller and the highly integrated low-cost 8-pin ATtiny15

microcontroller. However, it can be implemented using any AVR microcontroller with

A/D converter, PWM output and enough program memory to store the desired charging

algorithm.

Introduction

As more and more electronic equipment becomes portable, the rush for better batteries

with higher capacity, smaller size and lower weight will increase. The continuing

improvements in battery technology calls for more sophisticated charging algorithms to

ensure fast and secure charging. Higher accuracy monitoring of the charge process is

required to minimize charge time and utilize maximum capacity of the battery while

avoiding battery damage. The AVR microcontrollers are one step ahead of the competi-

tion, proving perfect for the next generation of chargers.

The Atmel AVR microcontroller is the most efficient 8-bit RISC microcontroller in the

market today that offers Flash, EEPROM, and 10 bits A/D converter in one chip. Flash

program memory eliminates the need to stock microcontrollers with multiple software

versions. Flash can be efficiently programmed in production just before shipping the fin-

ished product. Programming after mounting is made possible through fast In-System

Programming (ISP), allowing up-to-date software and last minute modifications.

The EEPROM data memory can be used for storing calibration data and battery charac-

teristics, it also allows charging history to be permanently recorded, allowing the charger

to optimize for improved battery capacity. The integrated 10-bit A/D converter gives

superior resolution for the battery measurements compared to other microcontroller-

based solutions. Improved resolution allows charging to continue closer to the maximum

capacity of the battery. Improved resolution also eliminates the need for external op-

amps to “window” the voltage. The result is reduced board space and lower system

cost.

AVR is the only 8-bit microcontroller designed for high-level languages like “C”. The ref-

erence design for AT90S4433 is written entirely in “C”, demonstrating the superior

simplicity of software design in high-level languages. C-code makes this reference

design easy to adopt and modify for today’s and tomorrows batteries. The reference

design for ATtiny15 is written in assembly to achieve maximum code density.

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Theory of Operation The charging of a battery is made possible by a reversible chemical reaction that

restores energy in a chemical system. Depending on the chemicals used, the battery will

have certain characteristics. When designing a charger, a detailed knowledge of these

characteristics is required to avoid damage inflicted by overcharging.

The AVR 8-bit RISC MCU The reference designs includes two separate battery chargers. One using AT90S4433

AVR microcontroller and one using the ATtiny15 AVR microcontroller. The AT90S4433

design demonstrates how efficient a battery charger can be implemented with C-code.

The ATtiny15 design shows the highest integrated and lowest cost battery charger avail-

able in today’s market. The AT90S4433 can be used for voltage and temperature

monitoring with UART interface to PC for data logging. Table 1 shows the differences in

the design.

Table 1. Design Differences

AT90S4433 Design

ATtiny15 Design

Programming Language

Assembly

Code Size (approximately) 1.5K Bytes

<350 Bytes

Current Measurement

External Op-Amp Gain Stage Built-in Differential Gain Stage

PWM Frequency

14 kHz, 8-bit Resolution

100 kHz, 8-bit Resolution

Clock Source

External Crystal, 7.3 MHz

Internal Calibrated RC

Oscillator, 1.6 MHz

Serial Comm. Interface

Yes

In-System Programming

Yes

Battery Technologies

Modern consumer electronics use mainly four different types of rechargeable batteries:

• Sealed Lead Acid (SLA)

• Nickel Cadmium (NiCd)

• Nickel Metal Hydride (NiMH)

• Lithium-Ion (Li-Ion)

It is important to have some background information on these batteries to be able to

select the right battery and charging algorithm for the application.

Sealed Lead Acid (SLA)

Sealed Lead Acid batteries are used in many applications where cost is more important

than space and weight, typically preferred as backup batteries for UPS and alarm-sys-

tems. 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 can be

charged infinitely, as long at the cell voltage never exceeds the manufacturer specifica-

tions (typically 2.2V).

Nickel Cadmium (NiCd)

Nickel Cadmium batteries are widely used today. They are relatively cheap and conve-

nient to use. A typical NiCd cell can be fully charged up to 1,000 times. They have a high

self-discharge rate. NiCd batteries are 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 shutdown when the cell voltage drops below 1.0V. NiCd batteries are charged with

constant current.

1659B–AVR–11/02

Nickel Metal Hydride (NiMH)

Nickel Metal Hydride batteries are the most widely used battery type in new lightweight

portable applications (i.e., cell phones, camcorders, etc.). They have a higher energy

density than NiCd. NiMH batteries are damaged from overcharging. It is therefore

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%/ month. Like NiCd batteries, NiMH

batteries are charged with constant current.

Lithium-Ion (Li-Ion)

Lithium-Ion batteries have the highest energy/weight and energy/space ratio compared

to the other batteries in this application note. Li-Ion batteries are charged using

constant voltage, with current limiter 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. The battery takes damage from over-

charging and may explode when overcharged.

Safe Charging of

Batteries

Modern fast chargers (i.e., battery fully charged in less than three hours, normally one

hour) requires accurate measurements of the cell voltage, charging current and battery

temperature in order to fully charge the battery completely without overcharging or oth-

erwise damage it.

Charge Methods

SLA and Li-Ion batteries are charged with constant voltage (current limited). NiCd and

NiMH batteries are charged with constant current and have a set of different termination

methods.

Maximum Charge Current

The maximum charge current is dependent on the battery capacity (C). The maximum

charge current is normally given in amounts of the battery capacity. For example, a bat-

tery with a cell capacity of 750 mAh charged with a charging current of 750 mA is

referred to as being charged at 1C (1 times the battery capacity). If the charging current

for trickle-charge is set to be C/40 the charging current is the cell capacity divided by 40.

Overheating

By transferring electric energy into a battery, the battery is charged. This energy is

stored in a chemical process. But not all the electrical energy applied to the battery is

transformed into the battery as chemical energy. Some of the electrical energy ends up

as thermal energy, heating up the battery. When the battery is fully charged, all the elec-

trical energy applied to the battery ends up as thermal energy. On a fast charger, this

will rapidly heat up the battery, inflicting damage to the battery if the charging is not ter-

minated. Monitoring the temperature to terminate the charging is an important factor in

designing a good battery charger.

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Termination Methods

The application and environment where the battery is used sets limitations on the choice

of termination method. Sometimes it might be impractical to measure the temperature of

the battery and easier to measure the voltage, or the other way around. This reference

design implements the use of voltage drop (-dV/dt) as primary termination method, with

temperature and absolute voltage as backup. But the hardware supports all of the below

mentioned methods.

t – Time

This is one of the simplest ways to measure when to terminate the charging. Normally

used as backup termination when fast-charging. Also used as primary termination

method in normal charging (14 - 16h). Applies to all batteries.

V – Voltage

Charging is terminated when the voltage rises above a preset upper limit. Used in com-

bination with constant current charging. Maximum current is determined by the battery,

usually 1C as described above. Current limiting is crucial to avoid thermal damage to

the battery if charge current is too high. SLA batteries are normally charged infinitely by

setting the maximum voltage above the actual charge voltage. Used for Li-Ion as pri-

mary charging algorithm/termination method. Li-Ion chargers usually continue with a

second phase after the maximum voltage has been reached to safely charge the battery

to 100%. Also used on NiCd and NiMH as backup termination.

-dV/dt – Voltage Drop

This termination method utilizes the negative derivative of voltage over time, monitoring

the voltage drop occurring in some battery types if charging is continued after the bat-

tery is fully charged. Commonly used with constant current charging. Applies to fast-

charging of NiCd and NiMH batteries.

I – Current

Charging is terminated when the charge current drops below a preset value. Commonly

used with constant voltage charging. Applies to SLA and Li-Ion to terminate the top-off

charge phase usually following the fast-charge phase.

T – Temperature

Absolute temperature can be used as termination (for NiCd and NiMH batteries), but is

preferred as backup termination method only. Charging of all batteries should be termi-

nated if the temperature rises above the operating temperature limit set by the

manufacturer. Also used as a backup method to abort charging if voltage drops below a

safe temperature – Applies to all batteries.

dT/dt – Temperature Rise

The derivative of temperature over time can be used as termination method when fast-

charging. Refer to the manufacturer’s specifications on information on the exact termi-

nation point (Typically 1C/min for NiCd batteries) – Applies to NiCd and NiMH.

DT – Temperature over

Ambient Temperature

Terminates charging when the difference between ambient (room) temperature and bat-

tery temperature rises over a preset threshold level. Applies to NiCd and SLA as primary

or backup termination method. Preferred over absolute temperature to avoid battery

damage when charged in a cold environment. As most systems have only one tempera-

ture probe available, the ambient temperature is usually measured before charging is

initiated.

dV/dt = 0 – Zero Delta Voltage This termination method is very similar to the -dV/dt method, but pinpoints more accu-

rately when the time voltage no longer rises. Applies to NiCd and NiMH batteries.

1659B–AVR–11/02

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