Microchip Technology Inc. WebSeminar March 23, 2004

Lithium-Ion Battery Charging: Techniques and Trade-offs

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Hello everyone, my name is Scott Dearborn. Today we’re going to talk about Lithium-Ion battery charging.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Topics for WebSeminar ●

Battery Overview ● ●

●

Li-Ion Battery Market & Trends Battery Terminology Primer

Lithium-Ion Charging Methodologies ● ●

Charge Algorithm MCP7384x Stand-Alone Charge Management Controllers ● Important Features and Why

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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We’re first going to give a quick overview on the Lithium-Ion battery and where it’s heading in the market. Then we’re going to go into Lithium-Ion charging methodologies and in particular we’re going to talk about the new MCP7384x stand alone charge controllers that Microchip has recently introduced.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Li-Ion Battery Market & Trends ●

Li-Ion is the fastest growing rechargeable battery chemistry ● ● ● ● ●

About 760M cells in 2002, CAGR of 15% Over 70% aimed on 1- and 2-cell applications Li-Polymer will account for 7% and Li-Ion for 50% of worldwide market in 2004 Energy density will be growing 10% annually Battery thickness is moving towards 3mm

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Lithium-Ion has become the fastest growing rechargeable battery chemistry in the market place today, especially for handheld battery powered application. As today’s devices need more and more energy, they are turning to the Lithium-Ion because it has the highest energy density by weight and by volume. So, as more and more features are added to these portable devices, to get the same amount of runtime, you need more energy. So the turn and focus has been toward Lithium-Ion.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Battery Terminology ●

Battery Capacity “C” ● ●

●

“C” Rate ● ●

● ●

measured in mAh (milli-Amp hour) higher the number, the greater the capacity a measure of the amount of current going into or out of the battery charge and discharge rates are measured in C

Cell voltage - voltage considered “full” End Voltage - voltage considered “empty”

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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There are a couple of quick terms we would like to discuss. Batteries are typically rated with a capacity (C) in milliamp-hours (mAh). Larger batteries are rated in “C” hours. Obviously, the higher the number the greater the capacity. The “C” rate is a measure of the amount of current going into or out of the battery. Typically, the mAh rating of a battery is at the C divided by 5 rate. So in other words, with a 1000 mAh battery, you could draw 200mA out of that battery for 5 hours. If you are going to draw 1000mA out of a 1000 mAh battery, you probably could not run for one hour. The reason is there is some inefficiencies while you are discharging the battery. So the C rate as we’ll define it, is just the one C rate where milliamps equal to one mAh. So the 1000 mAh battery will have a C rate of 1000 mA.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Lithium Ion Stage 1: Trickle Charge

Stage 2: Fast Charge (CC)

Stage 3: Constant Voltage 4.2V

VCELL VCELL ( V )

2.8V

ICH

1.0C

ICH ( A ) T ( °C ) 0.1C

T ( Cell Temperature ) 0.07C

1 hr

1.5 hr

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Depicted here is a typical Lithium-Ion charge cycle. The left axis has the battery or cell voltage, the charge current and temperature, all rising up the left-hand axis. The x-axis across the bottom is time. It is not proportioned to scale, but there to give you an idea. The battery voltage or cell voltage is the top line, the charge current is the middle line and the cell temperature is the bottom line. The preferred charge algorithm for the Lithium-Ion battery chemistry is a constant-current, constantvoltage algorithm that can be broken down into three stages: Stage one the trickle charge; stage two the fast or bulk charge; stage three the constant-voltage charge. In stage one, the trickle charge is employed to restore charge to deeply depleted cells. When the cell voltage is below approximately 2.8 V, the cell is charged with a constant current of 0.1C maximum. After the cell voltage has reached the 2.8 V, the second stage, the fast charge or the bulk stage is entered. The fast charge current should be less than or equal to 1C. Often in linear chargers, the current is ramped up as the cell voltage rises in order to minimize heat dissipation in the path element. The drawback to this is an extended charge cycle time. The cell voltage then reaches its pre-determined value, typically 4.2 V. Some of the older LithiumIon is 4.1 V. It generally depends on the type of anode material that is used in the cell. So when this voltage is reached, we go into a constant voltage mode and we hold the voltage that is charging the battery at that voltage to 4.2 V. A charge is terminated when the charge current has either diminished below about 7% of the fast charged current or when the safety timers are employed and it is just a lapsed timed to terminate the charge. Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Battery Charging Applications Stand-Alone Single Cell Lithium-Ion Charger Unregulated Input

RSENSE

+ Reverse Blocking Protection Diode

VDD STAT1

SENSE

DRV

VBAT

MCP73841 EN TIMER

C1

+ C2 Li-Ion

THREF THERM GND

CTIMER

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Here we have configured a schematic of the newly introduced charge controllers from Microchip. These are linear constant-current, constant-voltage chargers for single or dual series cells. Your fast, or bulk charge current is set by changing the values of the sense resistor, Rsense. Typically, the voltage drop across this resistor is about 110 mV. So 110mΩ sense resistor will give you about 1A bulk or fast charge current. The Ctimer capacitor in this picture sets up the safety timers. The Thermistor (THERM) reference provides a 2.5V reference to an NTC (negative temperature coefficient) or PTC (positive temperature coefficient) Thermistor, generally located in the pack and will not charge the battery outside a pre-set window. This helps to increase the reliability in the performance of the battery. There is a status indicator which is a fixed current so you don’t need resistors with the LED. You can attach this so you can get some indication of where you are in the charge cycle.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Battery Charging Applications ●

KEY FEATURES ● ● ● ● ● ● ● ● ●

Output voltage regulation accuracy Fast charge current regulation accuracy Charge termination current accuracy Preconditioning current control Wide input voltage range Automatic recharge Safety timers Temperature monitoring Reverse leakage current

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Some of the key features that we want to talk about and why they are key to the MCP7384x family is the output voltage accuracy. This is one of the big, key features of charging the Lithium-Ion batteries. You want to charge them to no worse than plus or minus 1% of their cell voltage or 4.2 V. This is where you will get the most capacity out of the battery; if you charge to a tight voltage regulation. The fast charge current is important to keep your charge cycle times as low and short as possible, to know what they are going to be and to control power dissipation and thermal issues in your system. Charge termination current accuracy is another thing that is very important. Again this has to do with how much capacity you are getting into the battery on every charge cycle. Pre-conditioning current control is another key feature that minimizes heat and pressure built-up inside the battery and in this manner you are increasing the reliability of the battery. A wide input voltage range allows the charger to be used with a variety of input sources and lowers your overall system cost. Automatic recharge is nice to have and it will automatically recharge your battery if your input is still present and the battery has been discharged below approximately 4 V. Safety timers are implemented to give your system safety and reliability. Temperature monitoring again also improves the reliability and overall performance of the battery, as well as reverse leakage current. When the input to the charger is not present, but your charger is still hooked up to the battery, you do not want to be discharging your battery. The current draw that is caused by the charger with the MCP7384x is typically around 200 nA. It is very, very low.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Li-Ion Charge Voltage Regulation Capacity vs. Charging Voltage

Capacity Loss vs. Undercharge Voltage

17 50

C a p a c it y L o s s - %

Capacity - mAh

17 00 16 50 16 00 15 50 15 00 14 50 4.2000

4.1875

4.1750

4.1625

10% 9% 8% 7% 6% 5% 4% 3% 2% 1% 0% 0.0%

4.1500

Charging Voltage - V

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

1.4%

UnderCharge Voltage - %

A small decrease in voltage accuracy results in a large decrease in capacity! © 2004 Microchip Technology Incorporated. All Rights Reserved.

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Here we see two graphs that depict the capacity of the battery verses the charging voltage on the left, and capacity loss versus undercharged voltage on the right. Again, this goes back to your tolerance on your charge voltage during the constant voltage regulation phase. You can see that just a slight under voltage charge of 1%, you are loosing about 8 % of your capacity. The MCP7384x family is typically about 2%, so you are loosing roughly 0.2% capacity at full charge, if you are at that typical number. We have a maximum set at 0.5%, or a little less than 5 % capacity loss if you are on the low end.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Li-Ion Termination Current 102%

Full Charge Capacity

98% 96%

At a fixed Imin cut-off of 10% of the charge rate, the full charge capacity of the battery can vary by 10%

10%

8%

94% 6% 92%

A compensated Imin threshold provides the maximum stored capacity under any charging temperature.

90% 88% 86% 84%

Imin Cutoff

100%

12%

4%

2%

0% 10 Degree s

25 De gr ees Av ailable Capacity

45 De gr ees

Imin for full charge

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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Charge termination current is another big part of how much capacity you are getting into the battery. If you have a fixed Imin cut-off or your parts termination of 10%, that is going to change the amount of capacity you get in your battery based on temperature. The full charged capacity on the left y-axis, goes with the upper trace line. You’ll see that at a fixed Imin cut off of 10% at 45 °C, you are getting 100% of the battery capacity. At 25 °C, you are only getting about 97% of the capacity. And at 10 °C, it’s down to about 90%. With the MCP738xx, ideally, you want to compensate the Imin threshold based on temperature. The way the MCP7384x family handles this is if we start with a typical number that is about 6%, so we set it at about 6%. The MCP7386x family, on the right hand y-axis, would be ideal if you compensated that charge termination current based on temperature. We set it somewhere around 6%. So at 25 °C you are nominally getting your 100% capacity back into your battery. This will move slightly with temperature, we do not follow this curve quite as much, we will only move about a half percent in either direction. But we do center it at the 25 °C based on a fast charge current of 1C. It will be half that number at a fast charge current of 0.5C, again out of a 1000 mAh battery that will be at a 500 mA fast charge.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Wall Adapter Input Supply Unregulated Input Supply Example LEAKAGE FLUX ΦL1

LEAKAGE REACTANCE L1 = ( N1ΦL1 / I1 ) L2 = ( N2ΦL2 / I2 )

ΦL2

IRON CORE

AC INPUT

DC OUTPUT TO CHARGER N1

N2

+ PTC

MUTUAL FLUX ΦC PRIMARY WINDINGS

SECONDARY WINDINGS

TRANSFORMER RATIO = a = ( N1 / N2 ) © 2004 Microchip Technology Incorporated. All Rights Reserved.

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A wide input supply range allows compatibility with a variety of low cost input sources. Many applications use very inexpensive wall cubes for the input supply. Due to its unregulated nature, the output voltage is highly dependant on the AC input voltage and the current being drawn by the charger.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Wall Adapter Input Supply Simplified Output Voltage (Input to Charger) DC OUTPUT = ( 1.414 ) * ( VIN / a ) - IOUT * ( REQ + RPTC) - CORE LOSS - 2 * VFD

Example: NOKIA Type: ACP-7U Input: 120VAC/60Hz/4.8VA Output: 3.7VDC/340mA Maximum Output: 700mA

DC Output Voltage (V)

16.00

132 VAC INPUT 120 VAC INPUT 90 VAC INPUT

14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0

100

200

300

400

500

Output Current (mA) © 2004 Microchip Technology Incorporated. All Rights Reserved.

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A typical AC mains input voltage can vary from 90 volts rms to132 volts rms for a standard wall outlet in the U.S. Assuming a nominal input voltage of 120 volts rms, the tolerance is plus 10%, minus 25%. The charger must provide proper regulation to the battery independent of this input voltage. The input voltage to the charger will scale the corner of the AC main and the charge current. You can see in the upper line on this Figure. The output voltage of this wall cube is nearly 14 volts at high line and only about 9 volts at low line. As you add current, you have drops across your resistances and core loss in the transformer, causing that voltage to decrease. The MCP7834x can interface directly to this type of wall adapter, unlike some of the older chargers that require a 5volt input.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

End-of-charge Detection ●

●

Accurately detecting end-of-charge condition extends pack’s cycle life and maximizes cell capacity usage Voltage Detection ● ●

●

Charge Current Level ●

●

Simplest and lowest cost implementation Very inaccurate (65% to 80% of capacity) Minimum charge current indicates full charge: 7% of I CHARGEMAX (typical)

Safety Timers © 2004 Microchip Technology Incorporated. All Rights Reserved.

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Now overcharging Lithium-Ion batteries produces oxygen which results in increased pressure, higher temperature, reduced reliability in the life of the battery. Therefore the charge needs to be terminated at the end of the charge, unlike your nickel based batteries where you can leave a trickle charge on them. They have an oxygen recombination process in their internals, so they can deal with the extra oxygen that is being applied. The Lithium-Ion batteries have nothing to do with that. As soon as it reaches full charge, you’re done. They should not remain on a trickle charge. There are several methods that are implemented in the industry today; one is a simple voltage detection which is very inaccurate. Once you reach that constant voltage phase of the charge, you only have about 65 to 80% capacity of the battery. At that time, many people will reach that constant voltage phase, start a timer, time for a certain period of time and then terminate the charge. The second approach that the MCP7834x family employs is to measure a minimum charge current to indicate flow charge. You will reach that constant-voltage phase and then you watch the charge current until it diminishes to a certain point. Again, reset this to about 6 or 7% of the bulk or fast charge current. So at one speed this would be on our 1000mAh battery this would be about 70 mA. If you are at half speed, this would be 35 mA or 3.5% of the C rate of the battery. In addition, safety timers are also employed; the 7384x family has an elapsed timer that will time out to determine the charge if no other conditions have terminated the charge. This is another technique that is employed by some other people in the industry. They will detect this minimum charge current threshold and then continue the charge for another 20 or 22 minutes.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

MCP73841 Charge Profile MCP73841 Charge Cycles, 1000mAh Battery VBAT @ 1C

1.20

Voltage (Volts)

4.00

1.00

VBAT @ 0.5C

3.50 3.00

0.80

IBAT = 1C

2.50

0.60

2.00 1.50

IBAT = 0.5C

1.00

0.40 0.20

0.50 0.00

Current (Amps)

4.50

0.00 0.0

50.0

100.0

150.0

200.0

Time (minutes) © 2004 Microchip Technology Incorporated. All Rights Reserved.

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Here we can see the MCP73841’s charge profile charging a 1000 mAh battery. The left y-axis is the battery cell voltage and the right y-axis is the actual charged current. Across the bottom is time. The upper curve in this figure is the battery voltage charged at the 1C rate, your fast charge. It corresponds with the third line down or the Ibat equals 1C. That battery voltage and current goes together and the other battery voltage at the half C rate in the current at Ibat equals 0.5C charge rate. As you can see, charging at that lower rate increases your charge cycle time by almost 40%. The charge termination again is occurring at a lower level than where it was at the 1C rate. This is charge termination point is based off your bulk charge current.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Summary ●

How is energy properly restored to Lithium-Ion batteries? ●

constant current - constant voltage with an Imin or timer charge termination

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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So, in summary, we restore energy to the Lithium-Ion batteries by a constant current, constant voltage charge algorithm with the charge termination method looking at the minimum charge current for the last time.

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Microchip Technology Inc. WebSeminar March 23, 2004

Lithium-Ion Battery Information ● ● ● ● ●

http://www.powercellkorea.com http://sanyo.com/batteries/lithium_ion.cfm http://lgchem.com/en_products/ electromaterial/battery/battery.html Handbook of Batteries, Third Edition, David Linden, Thomas B. Reddy ADN008: Charging Simplified for High Capacity Batteries, Bonnie Baker, Microchip Technology Inc. © 2004 Microchip Technology Incorporated. All Rights Reserved.

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The more information on Lithium-Ion batteries, these first three references are some battery manufacturers websites. They have a lot of good information on them. The fourth reference is a book by David Linden and Thomas B. Reddy; the Handbook of Batteries, this is probably the best reference manual I’ve seen on batteries; it’s very good, very complete. We also have ADN008, written by our own Bonnie Baker, “Charging Simplified for High Capacity Batteries”. This gives some of the advantages of the MCP7384x family. Since they are controllers and not fully integrated chargers, you can control the power dissipation better in a pass element and charge higher capacity batteries faster.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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Microchip Technology Inc. WebSeminar March 23, 2004

Lithium-Ion Battery Charging: Techniques and Trade-offs

© 2004 Microchip Technology Incorporated. All Rights Reserved.

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That is the end of the session for today, if you have any questions please type them in and we will respond to them.

Lithium-Ion Battery Charging: Techniques and Trade-offs

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