Datasheet

4.8 DEFINITIONS
Basically
Unless otherwise stated the technical values and
definitions are based on room temperature conditions
(R.T. = 20°C ±2°C).
System – specific data
The gravimetric energy density of the NiMH system
depends on battery size and ranges from approx.
40–55 Wh/kg and the volumetric energy density ranges
from approx.120–180 Wh/l.
Voltage Definitions
Open Circuit Voltage (O.C.V.):
Equilibrium potential 1.25 V to 1.4 V on average,
dependent on temperature, storage duration and state
of charge.
Nominal Voltage of sealed NiMH button cells is 1.2 V.
End of Discharge Voltage (VE):
The voltage at the end of discharging is 1.2 V to 0.9 V
per cell, depending on discharge rate and temperature.
End of Charge Voltage:
Terminal voltage after charge of 14 to 16 hours at the
nominal rate 0.1 CA, about 1.45 V/cell at room tem-
perature.
Capacity Definitions
The Capacity C of a cell is defined by the discharge
current I and the discharge time t: C = I • t
I = constant discharge current
t = duration from the beginning of discharge until the end
of discharge voltage is reached
Nominal Capacity:
The nominal capacity C denotes in quantity of electricity
in mAh (milli-Ampère hours) that the cell can deliver at
the 5 h discharge rate (0.2 CA). The reference tempera-
ture is 20°C ±2°C, and the final discharge voltage 1.0 V.
Typical Capacity:
The typical capacity is the average capacity at a
discharge rate of 0.2 CA to a final discharge voltage of
1.0 V.
Available Capacity:
NiMH cells deliver their nominal capacity at 0.2 CA. This
assumes that charging and discharging is carried out as
recommended. Factors which affect the available cap-
acity are:
■
Rate of discharge
■
End of discharge voltage
■
Ambient temperature
■
State of charge
At higher than nominal discharge rates the available
capacity is correspondingly reduced.
Current Definitions
Charge and discharge rates are given as multiples of the
nominal capacity (C) in ampères (A) with the term CA.
Example:
Nominal capacity C = 100 mAh
0.1 CA = 10 mA, 1 CA = 100 mA
Nominal Charge Current:
The nominal charge current is the charge rate (0.1 CA)
which is necessary to achieve full charge of a cell in 14 to
16 hours, if the cell has been fully discharged.
Overcharge Current:
Permissible current 0.1 CA for occasional overcharging
not exceeding 1 year. Frequent overcharge reduces
cell/battery life. Overcharge is restricted to room tem-
perature. Occasional discharges reinstitute the original
performance.
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