22
Advanced features
Advanced features
Simulating battery impedance
The electronic resistance of batteries varies according to a variety of factors such as
chemistry, cell construction, number of charge/discharge cycles, temperature, and depth of
discharge. If a battery is used as a source in a circuit with a dynamic load, changes in the voltage
across the load will be produced proportional to the electronic resistance of the battery and other
sources of resistance in the circuit. If the peak load current is high enough or the electronic
resistance of the battery and the resistance between the source and the DUT is large, the voltage
drop compromises the performance of the device.
This phenomenon occurs in TDMA and GSM cellular handsets where the magnitude of the
“ON/OFF” current during transmission varies by as much as a factor of 20. In the absence of
any filtering capacitance between the battery and the RF power amplifier, the handset will shut
off if the supply voltage is below the operating threshold for periods as short as several
microseconds.
Figure 10
shows a simple schematic of a battery. This battery is represented by an ideal
voltage source, V
cell
, with internal impedance, R
i
(t), and is connected to a DUT with
interconnects having resistance R
interconnect
.
Figure 10
Battery schematic
If R
interconnect
is small compared to R
i
(t) and if R
i
(t) is relatively constant during the length
of the pulse, R
i
(t) ~ R
i
,
then the voltage across the DUT may be expressed as:
where I(t) is the time varying current through the battery.
R
interconnect
R
interconnect
V
cell
R
i
(t)
V(t)
I(t)
Cell or Battery Pack
DUT
V t
( )
V
i
I t
( )
R
i
–
=