Specifications

the voltage (V) is 1.415 V.
Using equation (4) gives an
internal resistance of 0.16 Ω.
Batteries are complex
nonlinear electrochemical
devices. The two common
electrical measurements of
batteries used to gauge their
state are the DC internal
resistance and the AC internal
impedance. In the "old" days,
AC characteristics of a battery
were not terribly important for
DC operation. However,
modern digital electronic
devices can draw sharp
current spikes from their
power source (e.g., switching
a cell phone to transmit). In a
1 kW car audio system,
currents can exceed 100 A and
the AC behavior of batteries,
stiffening capacitors, and stray
inductances can all play a role.
DC loads can be useful in
helping investigate the
dynamic behavior of these
systems.
The simple model of a battery
as an ideal voltage source in
series with a resistance
captures the first order
behavior (see Figure 8). More
complex models have also
been used. Using DC loads
like B&K Precision's 8500
series, batteries can be
characterized in a manner
which will mirror their
application and construct a
model appropriate for the
design.
PERFORMANCE
TESTING OF DC
LOADS
Just as it is important to have
a good power supply that will
perform accurate
measurements under various
test conditions, it is essential
to have a DC load that is
robust and perform under
required specifications and
test setups. Some of the most
common tests used for
verifying features of a DC load
include trigger delay, switching
time, and slew rate. In the
following section, slew rate
testing for the B&K Precision
8510 DC load will be
highlighted.
Disclaimer: The following
sections include general
setups with some settings
specified for the test
environment. Some details
are ignored, and therefore
results may vary and may not
reflect exactly what is shown
in the following sections.
Slew Rate
The slew rate of a DC load is a
performance measurement
that determines how quickly a
DC load can draw current
within different ranges of
current transition. In general,
the slew rate for low current
transitions, say 0 to 0.5 A, is
significantly lower than slew
rate for current transitions
from 30 to 70 A. Generally,
the appropriate way to test
slew rate is to observe a
portion of the timing during
maximum current transition.
The graph in Figure 10
illustrates this. Between the
10% and 90% region, the
slew rate can be measured by
observing the steepest slope
portion. The indicated
measured time would be used
to calculate the slew rate.
Hence, the slew rate
calculation is simply (rated
max. current – 0 A) / T, where
T is the measured time from
10% to 90% region and rated
max. current is the specified
maximum current of each
load. The following will
demonstrate how to test the
slew rate of B&K Precision's
8510 DC Load.
Approach
Set the DC load to transient
mode and allow it to draw
current from 0 to maximum
rated current. Observe
current transition changes and
timing on an oscilloscope.
Setup
Three B&K Precision's 1796
high current power supplies
are connected in parallel.
10 of 17
Figure 9. Test Setup For Battery
Internal Resistance
+
+ -
DC Load
Setting: Immediate Trigger
Mode: CC at 0.005A
Transient: Pulse 0 to 0.505A
+ -
Remote sense
D size battery -D size battery -