Simulating Battery Impedance; Variable Output Impedance Control On Channel #1 - Keithley 2302 Instruction Manual

E-2 Applications Guide

Simulating battery impedance

Variable output impedance control on channel #1

The electronic resistance of batteries varies according to a variety of factors such as but not
limited to 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 will compromise 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 shutoff if
the supply voltage is below the operating threshold for periods as short as several microseconds.
Figure E-1
V , the internal impedance, R
cell
R
interconnect
Figure E-1
Battery schema.tic
If R
the pulse, R
where:
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shows a simple schematic of a battery, represented by an ideal voltage source,
(t), connected to a DUT with interconnects having a resistance
i
.
R
interconnect
R (t)
i
Cell or Battery Pack
V
cell
R
interconnect
is small compared to R
interconnect
(t) ~ R , then the voltage across the DUT may be expressed as:
i i
V t ( )
I(t) is the time varying current through the battery.
I(t)
V(t)
DUT
(t) and R (t) is relatively constant during the length of
i i
I t ( )R
V
= –
i i