e019026.PDF

GENERAL INTEREST

Lithium Ion Cells

Charging techniques and circuits

By G. Kleine

Where applications demand the greatest energy density and minimum

weight, Lithium Ion (Li-Ion) rechargeable cells have quickly become the

battery of choice despite their continued high cost. Recent advances in

the cells and recharging methods are described in this article.

as a better alternative. They have

the advantage of not containing

harmful heavy metals and offer a

better energy density (cell size to

stored energy ratio). We have seen a

doubling of their energy density

since they were ﬁrst introduced with

the prospect of a 2.0 Ah AA sized

cell not far off.

The clear winner on the energy

density front at the moment is the

Lithium-Ion (Li-Ion) cell. These are

relatively expensive and sensitive

to mis-use but are the first choice

for applications such as laptops,

camcorders, mobile phones and

portable equipment where weight

(lithium is the lightest known

metal) and capacity of the battery

pack is of critical importance.

Recent developments have pro-

duced high current cells suitable

for powering passenger vehicles

and in one case a full-size glider

with motor assist! The German air-

craft manufacturer Lange has

taken advantage of the low weight

of Li-Ion cells to power its ‘Antares’

electric motor glider. The prototype

was developed for Ni-MH cells but

replacing them with Li-Ion will

give the 500 Kg aircraft a climb

rate of 885 ft/min, carrying the

pilot to an altitude of almost

10,000 ft with its 57 hp (42 KW)

brushless electric motor.

The ideal rechargeable battery is still a long

way off. For high current application the

established Nicad (Nickel Cadmium or NiCd)

cell has always been preferred. Recent tests

have established that the so-called memory

effect (see box) is no longer a problem with

this type of cell. From a cost standpoint

Nicads are the cheapest cell but they do incur

an environmental cost. They contain

the heavy metal cadmium and

should therefore not be consigned to

landfill sites at the end of their life.

For this reason Europe is committed

to cease production of Nicads by

1998. Nickel metal Hydride (NiMH)

cells are becoming increasingly seen

Elektor Electronics

9/2001

GENERAL INTEREST

Cell Structure

and Characteristics

Returning to earth for a moment we

will now delve into the internals of

the cell. A significant advantage of

Li-Ion is that the cell potential is 3.6

or 3.7 V. This means that two to three

cells of Ni-MH or Nicad (which have

a cell potential of 1.2 V) can be

replaced by just a single Li-Ion bat-

tery.

The Li-Ion cell is composed of a

graphite anode and a lithium cobalt

oxide or lithium manganese oxide

cathode in a organically fluid elec-

trolyte containing dissolved lithium

salt which supplies the lithium ions.

Manganese oxide cathodes give a

cell potential 3.7 V while cobalt

oxide produce 3.6 V. As in all things

in life there is no gain without pain.

In the case of the Li-Ion cell it is its

sensitivity to improper use. The re-

charging voltage is 4.20 V for cells

with manganese oxide cathodes and

4.10 V for cobalt oxide cathodes. This

voltage level must be maintained to

within 50 mV if the cell is not to be

permanently damaged. During dis-

charge it is also important to ensure

that the cell voltage does not fall

below 2.4 or 2.5 V, otherwise cell life

will be seriously compromised.

will lead to internal gassing, over-

heating and eventual explosion. A

voltage increase of just 1% above

this optimum level can cause the

lithium ions to begin to convert to

metallic lithium in the cell. This in

turn reacts violently with water in

the electrolyte and at this point I

think we should all retire to a safe

distance or risk a close encounter

with the cell and its contents. On the

other hand if the charging voltage is

below its optimum level it leads to a

significantly under-charged cell. A

voltage level of just 100 mV below

the optimum will result in a 7%

reduction of the stored capacity. If

this were not bad enough, Li-Ion is

also sensitive to how low the cell

voltage is allowed to fall during dis-

charge. Deeply discharging the cell

leads rapidly to an irreversible

decline in cell capacity.

As you will appreciate, Li-Ion

cells are not the most fault tolerant

on the market. For this reason they

are not currently available for gen-

eral-purpose use and are not in stan-

dard battery package outlines (AAA,

AA and C cell etc.). Li-Ion cells tend

to be used in custom battery packs

for specific equipment i.e. laptops,

mobile phones, camcorders etc.

where the equipment is specially

designed to accept a Li-Ion pack and

correct charging equipment will be

guaranteed.

NTC

010058 - 11a

Figure 1a. Circuit diagram of a Li-Ion pack with

temperature sensor.

B1P

B2P

Protector-IC

DIS

CHG

010058 - 11b

Figure 1b. Circuit diagram of a Li-Ion pack with

a protector IC.

The ‘Ion’ part in the battery name

simply refers to the fact that the

Lithium should never occur in its

metallic form in the battery. During

charging Lithium ions (Li+) are col-

lected on the graphite anode from

the electrolyte.

B1P

B2P

Data

R SENSE

SDA

SMBus

Battery Manager

Clock

Battery Packs

SCL

SENSE

DIS

CHG

Li-Ion battery packs are usually fit-

ted with some form of electronic pro-

010058 - 11c

Cell Chemistry (charging):

Figure 1c. Circuit diagram of a Li-Ion pack with

SM-Bus battery management IC.

positive Pole:

LiCoO 2

➝ Li1-n CoO 2 + n Li+ + n e–

negative Pole:

C + x Li+ + x e– ➝ CLin

The essence of rechargeable cells is that this process is reversible so as you would

expect the opposite process occurs during discharge.

potential is measured by a protector IC and

the charge or discharge current can be inter-

rupted by turning off MOSFET T2 or T1

respectively. In each case the inherent MOS-

FET body diode is used to allow current to

flow through the unswitched MOSFET.

Switching off both MOSFETs will effectively

disconnect the battery pack.

The Protector IC prevents over-voltage dur-

ing charging and deep discharge. The pro-

tector IC itself consumes less than 1 µA in

standby mode.

tection to prevent mis-use. Figure 1

shows some typical circuits. One of

the simplest methods ( Figure 1a )

uses an in-built NTC sensor to con-

vey cell temperature to the con-

nected equipment. A more sophisti-

cated circuit giving protection

against over and under charging is

shown in Figure 1b here the cell

The consequences of

improper use

Charging with too high a voltage can

easily damage Li-Ion cells. If the

charging voltage is increased above

its optimum value of 4.1 V or 4.2 V it

9/2001

Elektor Electronics

GENERAL INTEREST

Figure 1c shows a battery pack with a

built-in battery manager IC using a System

Management Bus (SMB) interface. The IC

measures each cell potential and also the cur-

rent drain via R sense , a low resistance (a few

10’s of milliohms) sense resistor. This allows

the management IC to determine the amount

of charge remaining in the battery pack and

send this information to the charging circuit

of the equipment over the clock (SCL) and

data (SDA) lines of the SMB two-wire inter-

face. Further information of this interface is

given later.

For safety reasons Li-Ion batteries have an

over-pressure valve fitted to each cell. This

allows pressure in the cell caused by high

environmental temperature (e.g. fire) to be

vented to the atmosphere. A low resistance

Positive Temperature Coefficient (PTC) ele-

ment is also included in the cell. If high cur-

rents are drawn, this element warms up and

its resistance increases thereby reducing the

short circuit current.

Tr1

+11V

≈

IC2

LM317T

230V

50Hz

I = 1V25

2200µ

16V

100n

PBYR745

zie tekst

SB550

see text

siehe Text

100n

LED

voir texte

GATE

CHG

IC1

TSEL

BSP171P

MAX1679

ADJ

BATT

THERM

Li Ion

Li-Ion charging

Single

Cell

- Θ

NTC

100n

4µ7

35V

Li-Ion cell recharging is carried out using a

constant voltage, current limited source and

requires close monitoring of the cell voltage.

Incorrect charging will quickly lead to per-

manent loss of cell capacity or worse. A typ-

ical recharging cycle will begin with the

charging equipment measuring the no-load

cell potential. If this is below 2.5 V then the

cell is in a deep discharge condition and

requires a ‘prequalification’ charging phase.

This is performed by trickle charging at

5 mA until the cell potential reaches 2.5 V.

At this voltage level the charger will start

the fast charging stage. The current is lim-

ited to a value from 1C to 2C (where C is the

Ah rating of the cell) until the cell potential

reaches 4.1 V (for cobalt oxide) or 4.2 V (for

manganese oxide). Now the charger begins

the constant voltage phase, it maintains

this voltage (+/– 5 mV) and monitors the

current until it falls below a pre-defined

level. This top-off phase ends when the

charging current falls to less than 5% of the

current supplied during the constant cur-

rent phase, i.e., 0.05 C to 0.1 C. The cell is

now fully charged.

It is permissible as long as the cell is not

fully charged to use pulsed charging with a

voltage greater than 4.2 V.

For safety reasons it is necessary to use

two timers to limit the charging times. One

timer limits the fast charging phase whilst

another limits the total charging time. If the

fast charge timer runs out before the cell

potential reaches 4.2 V the charging process

is terminated and an error message will be

10k

010058 - 12

Figure 2. Simple Li-Ion charger using the MAX 1679.

Table 1. LED readout with MAX1679

LED state

Function

Flashing

Qualifying (Vbatt < 2.5V)

Charging (fast charging or top-off charging)

Flashing

Fast charging ﬁnished

Flashes every 3.5 s

Charging ﬁnished

ICs for

charging Li-Ion cells

sent indicating a faulty cell. The

‘total time’ timer will terminate the

top-off phase if it is not switched off

during the normal cycle.

Li-Ion cells have a very low self

discharge so they do not require a

maintenance charge in fact this

would lead to an over-charge condi-

tion of the cell. An NTC sensing ele-

ment is used to ensure that the cell

temperature stays within its operat-

ing limits of +2°C to +45°C. Cell

overheating will cause the controller

to switch off the charging process

until the temperature has returned to

normal.

The circuit in Figure 3 shows a sim-

ple charger suitable for re-charging

single Li-Ion cells. At the heart of the

circuit is the MAX 1679 (IC1) from

MAXIM. IC1 measures the cell

potential at its BATT input and

switches MOSFET T1 using a pulsed

waveform with a variable mark-

space ratio the so that the charging

current from the constant current

source formed by IC2 and R1 can be

controlled at will by IC1. During the

fast charge phase T1 is on constantly

Elektor Electronics

9/2001

GENERAL INTEREST

Tr1

U/V

+4V2

230V

50Hz

BAT

IC1

MIC79050

BT1

GND

678

470µ

4µ7

I/A

Li-Ion

battery

0,2

0,4

0,6

0,8

1,0

010058 - 13

Figure 3a. Simplest Li-Ion charger using a mains adapter and the MIC 79050.

Figure 3b. Load line characteristics of an unreg-

ulated mains adapter.

Memory effect?

What memory effect?

Recent tests by a German consumer magazine compared the performance of AA

format rechargeable cells. Their test results can be summarised as:

the type of battery used. The jumper at input

TSEL selects the maximum charging time

(this time can be between 2.8 and

6.25 hours). At the end of a charging cycle the

MAX 1679 will switch itself into low power

mode so that the IC draws less than 1

from the battery.

A simpler Li-Ion charger can be con-

structed using a mains unit adapter. The

output characteristic of a typical unregu-

lated adapter is shown in Figure 3b . You

would expect an ideal voltage source to

have a flat load line i.e., it would produce the

same voltage irrespective of load. However

the transformer winding impedance of a typ-

ical unregulated mains unit causes the out-

put voltage to drop as load current

increases. The circuit shown here in Fig-

ure 3a puts this property to good use. The

transformer will need to produce +4.5 V at

a current of 0.5 to 1.0 times the capacity of

the cell in A/hr. In figure 3b this output cur-

rent is 0.4 A. During the constant current

phase of charging the battery is switched

directly to the output of the transformer. The

charge current is limited by transformer

impedance and the cell voltage gradually

rises. When 4.2 V is reached the MC79050

switches to constant voltage charging and

the MC79050 will monitor the cell current

until it is fully charged.

Memory Effect

The testers could not ﬁnd any evidence of this effect (for NiCd and NiMH

cells). Cells were fully charged and then discharged to half capacity. After

repeating this cycle 50 times they still delivered their full capacity! This was

found true for both NiMH and NiCd cells.

Self discharge

In contrast to earlier ﬁndings, self discharge was less of a problem in NiMH

cells than in NiCd. The best NiMH cell tested was given a ‘good’ rating with

only a small loss in capacity after 80 days at 20

C. Three other NiMH cell

manufacturers scored ‘satisfactory’. NiCd cells were all rated ‘poor’ in this

category.

Conclusions:

The memory effect seems to be a thing of the past. Nicads are now ‘out’ for most

applications. They are useful mainly for power tools and model builders but this

situation is changing slowly.

The Rechargeable Alkaline (RAM) cells (distributed principally by Rayovac)

were found to be a relatively poor alternative. They are expensive and can only

be recharged a limited number of times (25 max). Testers swapped the RAM cells

for standard alkaline cells (so called non-rechargeable primary alkaline cells) and

found that they had exactly the same properties as the expensive RAM cells! But

regular Elektor Electronics readers are already aware of this phenomenon since

our article in December 1996.

Figure 4 shows a Li-Ion charging circuit

using the IC LM 3622 from National Semicon-

ductor. This circuit uses a PNP transistor as a

linear regulator to maintain the voltage on the

cell at 4.1 or 4.2 V. The charging current is

related to the value of R sense and is given by

the equation:

and during the top-off phase T1 will

be pulsed. The charging method

used by this IC adheres closely to

the charging method described

above. The MAX 1679 can measure

cell potential with an accuracy of

less than 1%. An LED indicates the

status of the charger (see Table 1).

Schottky diode D1 ensures that

the Li-Ion cell does not discharge

through the body diode of T1 after

the charging process is ﬁnished. An

NTC thermistor R3 is connected to

the THERM input of IC1 and must be

in close physical contact with the

cell to sense its temperature. The

ADJ input switches an internal volt-

age reference that allows selection

of the terminal charging voltage of

either 4.2 V or 4.1 V depending on

Charge current = 0.1 V/R sense

A heatsink should be used to dissipate the

energy in transistor T1.

9/2001

Elektor Electronics

GENERAL INTEREST

Smart battery systems

Table 2. Protection ICs for Li-ion battery packs.

Manufacturer

Type

Function

No. of cells

With the advent of laptop PCs and

mobile phones it is important to

know accurately just how much life

is left in the batteries. There are few

things more annoying when trying

to save work after a low battery

warning to ﬁnd that the battery has

too little charge to make the back-up.

Intel and Duracell are developing a

so called ‘intelligent’ battery pack,

these packs send information over a

two wire interface to the equipment

identifying the type of cell in the

pack (NiCd, NiMH, Li-Ion etc.) and

the amount of charge left. This Smart

Battery System (SBS) will therefore

ensure optimum cell life by providing

information to the equipment about

the battery and the best way to

recharge it. The battery pack effec-

tively controls its own charging! The

message and command format of the

SBS is well deﬁned and can be used

licence-free by all

( www.sbs-forum.org ).

Using information from the SBS

interface, equipment will at last be

able to accurately predict remaining

run time. ICs using the SBS are pro-

duced by MAXIM (MAX1645,

MAX1647/1648 and MAX1667) and

Linear Technology (LTC1759). Also

noteworthy for Li-Ion cell monitoring

is the DS2760 from Dallas Semicon-

ductor – this device uses its own

Dallas 1-wire bus.

Maxim

MAX 1665

Lithium-Ion Battery Pack Protector

2, 3 or 4 *

Maxim

MAX 1666

Advanced Lithium-Ion Battery Pack Protector

2, 3 or 4 *

ON Semiconductor

MC33348

Lithium Battery Protection Circuit

ON Semiconductor

MC33349

Lithium Battery Protection Circuit

ON Semiconductor

MC33351A

Lithium Battery Protection Circuit

Philips Semiconductors

SAA1502

Safety IC for Li-ion

TI / Unitrode

UCC3952

Li-ion Battery Protection IC

* version of IC dependent on number of cells

Table 3. Simple charger ICs for Li-ion batteries.

Manufacturer

Type

Function

No. of cells

Maxim

MAX1679

Single Cell Li+ Battery Charger

Single Cell Li+ Battery Charger for Current-

Limited Supply

Maxim

MAX1736

Micrel

MIC 79050

Simple Lithium-Ion Battery Charger

Linear Technology

LTC1730

Lithium-Ion Battery Puls Charger

Linear Technology

LTC1731

Lithium-Ion Linear Battery Charger Controller

Linear Technology

LTC1732

Linear Lithium Battery Charger Controller

Linear Technology

LTC1734

Lithium-Ion Linear Battery Charger

National Semiconductor

LM3420

Lithium-Ion Battery Charge Controller

1 ... 4 *

National Semiconductor

LM3620

Lithium-Ion Battery Charge Controller

1 ... 2 *

National Semiconductor

LM3622

Lithium-Ion Battery Charge Controller

1 ... 2 *

TI / benchmarq

bq2400

Linear Li-Ion/Li-Polymer Charger

1 ... 2

TI / benchmarq

bq2057

Li-Ion Charge Management IC

1 ... 2

* version of IC dependent on number of cells

Table 4. Universal charger ICs.

no. of cells

NiMH/NiCd

no. of cells

Li-Ion

Recognition

end of charge processs

Manufacturer

Type

cell type

current and voltage setting

via controller

Maxim

MAX1772

2...4

Lead-acid, Li-ion, NiCd, NiMH, universal

National

LM3647

2...7

1...4

Li-ion, NiCd, NiMH, universal

U, voltage, temperature, time

∆

Microprocessor-Controlled Battery Management System

for lead-acid, Li-ion, NiCd and NiMH batteries

Linear Technology

LTC1325

1...8

1...3

voltage, temperature, time

New products

In the past year many new ICs have

appeared on the market speciﬁcally

for monitoring and supervising the

recharging of Li-Ion cells. The main

players are Maxim, Linear Technol-

ogy, ON Semiconductor (A branch of

Table 5. Lithium-Ion current-source controllers.

Manufac-

turer

no. of cells

Li-Ion

Recognition end of charge

process

Type

Function

MAX1737

MAX1757

MAX1758

1...4

1...3

1...4

Stand-Alone Li-Ion Charger

Controller

Voltage and Current Limit,

Thermistor, Max Time

Maxim

Elektor Electronics

9/2001

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