Renata SA Frequently Asked Questions - Manual

41
the swiss power source
40
General electrical performance
Frequently Asked Questions (FAQ)
Which values of open circuit voltage do
lithium cells typically show?
The CR-type coin cells, based on the lithium/
manganese 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
minimizes 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
400 200 600
0
Time (hrs.)
Vo
lta
ge
(
V
)
In
te
rn
al
r
es
is
ta
nc
e
(O
hm
)
Voltage
Internal resistance
Figure 1 Characteristic curve 1 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.