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the swiss power source

General electrical performance

Frequently Asked Questions (FAQ)

Which values of open circuit voltage do

ithium cells typically show?

The CR-type coin cells, based on the lithium/

anganese dioxide electrochemical system, have

a nominal voltage of 3 V. In practice, a fresh

lithium cell will typically show an OCV (Open

Circuit Voltage) between 3.10-3.40 V. This range

of values is intended for measurements

performed at room temperature; in fact, the OCV

values depend on the temperature of the

measurement.

After storage periods the cells may also show

values outside this range, due to ageing effects

(see the recommended storage conditions for

lithium coin cells, also reported in this document).

What is the internal resistance of a cell?

How does it affect the performance of the

cell?

From an electrical point of view, a cell is a

combination of an energy source and a

resistance. The internal resistance (Ri) is a key

parameter for a cell, as it determines its high-

power capability (i.e. its ability of delivering its

energy in a short time). The internal resistance

reduces the useful voltage in applications and

leads to internal heat, thus loss of energy, which

increases with the square of the current.

The internal resistance of lithium cell is a sum of

both ohmic contributions and of resistive

contributions coming from electrochemical

phenomena taking place during the discharge of

the cell. By accurate selection and quality control

of materials, Renata manufacturing process

inimizes the resistive factors contributing to the

internal resistance of the lithium cells.

As the internal resistance includes a number of

resistive contributions coming from electro -

chemical phenomena, each of them being

characterised by a time constant, the value of

internal resistance is pretty much depending from

the measuring method and conditions. A simple

and inexpensive method for measuring the Ri is to

apply a resistive load (R1) to the cell and to

measure the value of the cell voltage under load

(CCV, Closed Circuit Voltage). The internal

resistance is then calculated as:

Ri = ( OCV - CCV ) x R1 / CCV.

Does the internal resistance changes with

time, or during the cell discharge?

Generally speaking, there is a limited, physiolo -

gical increase of the internal resistance of a

primary cell during its service-life. In the case of

lithium coin cells, the normal increase during the

cell discharge is due both to ohmic factors (the

distance between the electrodes increases during

discharge) and to electrochemical phenomena

taking place at the lithium anode (growing of

interface films between lithium metal and

electrolyte solution).

The increase of the overall internal resistance with

increasing discharge level is reported in the figure

below.

3.0

300

150

1.5

400200600

Time (hrs.)

Voltage (V)

Internal resistance (Ohm)

Voltage

Internal resistance

Figure 1 Characteristic curve

of a CR2450N cell. Discharge load:

R1=3.32 kOhm. Measurement of internal resistance

during discharge: by applying the load R2=150 Ohm for

1s, every 3 hrs.

1) This curve is intended

as typical data and not

as cell specification.

1 2 3 4 5

Inhaltsverzeichnis

Seite 1

41the swiss power source40General electrical performanceFrequently Asked Questions (FAQ)Which values of open circuit voltage dolithium cells typically

Seite 2 - 1) This curve is intended

General electrical performanceFrequently Asked Questions (FAQ)The ageing of the cells at normal conditions (i.e.room temperature, max. 40% of relative

Seite 3 - Application proper design

43the swiss power source42General electrical performanceFrequently Asked Questions (FAQ)What is the maximum pulse current thelithium coin cells can ha

Seite 4 - (40-20)/10

Influence of temperature on electrical performanceFrequently Asked Questions (FAQ)The operating temperatures of lithium coin cellsare given on page 7.

Seite 5 - General FAQs

45the swiss power source44Influence of storage / ageing on electrical performanceInfluence of contact materialFrequently Asked Questions (FAQ)Which ar

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