Renata SA 3V Lithium Coin Cells - DESIGNER'S GUIDE - Manual (Page 42)

General electrical performance
Frequently Asked Questions (FAQ)
The ageing of the cells at normal conditions (i.e.
room temperature, max. 40% of relative humidity)
will also lead to other physiological increases of
the internal resistance, due to normal ageing
phenomena taking place at the electrodes.
Though of limited extent, these types of increases
of the internal resistance are normally to be
expected and must be also taken into account,
when designing a new application.
Exposing the cells to elevated temperatures, then,
can lead to further grow of the passivation films at
the anode, with an additional increase of internal
resistance. Furthermore, increasing the
temperature above 70°C can cause the internal
resistance to abnormally increase (because of
electrolyte leakages and degradation
phenomena). Abuse conditions such as discharge
at elevated currents and short-circuit can also
increase the internal resistance abnormally,
because of the deterioration of cell internal
components.
Which is the voltage drop of the lithium
cell during current pulse?
The voltage drop during a current pulse (
ͬV) is
the difference between the cell voltage just before
applying the pulse (Voltage-high, V
1
) and the cell
voltage during the pulse (Voltage-low, V
2
):
ͬV = V
2
- V
1
It is also expressed by the formula:
ͬV = Ri x I
peak
,
where Ri (internal resistance) depends on the cell
type and dimensions. In addition, the value of Ri
depends on the temperature and on the discharge
level of the cell (see related section about internal
resistance). Therefore the voltage drop of the cell
will be strongly affected by the temperature and
by the cell's discharge level.
From the above reported formula it also follows
that the voltage drop strictly depends on the
applied pulse itself-particularly on the value of the
pulse-current (I
peak
). The voltage drop is also
affected by the other parameters that define a
pulse-load: the pulse duration (i.e. how long the
pulse current I
peak
is applied), the pulse period
(i.e. the time between two subsequent pulses), the
frequency with which the pulse trains occur
(i.e. how often the pulse trains are applied to the
battery) and -eventually- the basis-current (i.e. the
current applied between two pulse trains). The
last three pulse parameters affect the voltage
drop during pulse, because their settings affect
the value of the cell voltage just before applying
the pulse ( V
1
).
An example of voltage and internal resistance
behaviour during a pulse discharge is reported
below (Figure 2).
42
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1) This curve is intended
as typical data and not
as cell specification.
Figure 2: Pulse-current discharge characteristics 1 of the CR2450N cell.
2.8
50
90
20
2.4
400 300 200 200 500
2.0
Capacity (mAh)
CR2450N Pulse Discharge 10mA/50ms,
Periode 1s. Cut-off voltage: 2.0V
Vo
lta
ge
(
V
)
R
es
is
ta
nc
e
(O
hm
)
V
1
V
2
Ri