Atec Megger-MBITE - Manual (Page 2)

Why Measure Batteries at all?
Measuring batteries is not for the battery but for the
equipment the battery is supporting whether to ensure
revenue streams as in wireless phone service or to support
critical plant equipment such as oil priming pumps. In a
sense, it is insurance that the MBITE provides and peace-
of-mind knowing that the batteries are in good health.
APPLICATIONS
The MBITE measures impedance values and dc voltage for
flooded and sealed lead-acid and nickel-cadmium cells.
This information aids the operator to determine cell
replacement criteria based on impedance trends. The
MBITE also identifies weak cells in a battery string and
pinpoints unsatisfactory intercell and/or intertier
connections. A schematic diagram of a typical test setup is
shown in Figure 2.
Test Procedure
The MBITE base measurement unit applies a capacitively
coupled ac current to the battery under test by way of the
current source leads. Current sensors monitor source
current. The standard CT has a 2-in. (50-mm) internal
dimension while the optional CT is 0.5 in. (13 mm). The
CT is clamped around a convenient battery intercell or
intertier connection within the battery’s current loop. The
potential probes are placed across the cell under test. The
base measurement unit displays cell impedance and dc
float voltage as well as intercell connection resistance.
Figure 3 shows the value of taking direct intercell
connection resistance measurements. The information can
be verified by the operator and then stored by pressing the
data send button on the potential probe before moving
onto the next cell. Intercell resistance measurements also
can be made and
stored using the
cell/strap mode.
Single cell/module
applications
can be
accommodated
using one of the
optional Factory
Probe Lead Sets.
MBITE
Miniature Battery Impedance Test Equipment
Interpretation of Readings
Data produced by the MBITE can be interpreted in both
short- and long-term time frames. It is recommended that
MBITE measurements be made part of a battery
maintenance
program, with
readings taken
and recorded
quarterly or semi-
annually. Figure
4 shows an
example of how
impedance
changes as cells
weaken over time
and cycle life.
Short Term Interpretation
Impedance readings for individual cells can be used in the
short term to compare with the average impedance
reading for the entire battery. Individual cell values
varying by more than ±40% of the (sealed) battery average
typically indicate a problem with that cell (and ±20% for
flooded). There are two different methods of evaluating
impedance data in Excel
® : 1) use the bar graph of the
MBITE printout in Cell # order and 2) by rearranging the
data into ascending impedance as shown in Figure 5 using
a spreadsheet. The cells on the right side of the graph are
weaker than the others. The dramatic increase in
impedance is a clear indication of questionable cells.
Further investigation of such cells is recommended,
including a
verification of
intercell
connections,
specific gravity,
if appropriate,
ambient
temperature, and
perhaps, a single
cell load-cycle
test.
Long-Term Interpretation
Impedance readings can be used in the long term to
determine replacement criteria. Battery cell impedance
values should be recorded and compared to previous
readings to determine
the cell’s position on
the curve of
impedance versus cell
life. Individual cell
values varying by
more than ±40% from
the initial baseline of
the battery average
typically indicate a
problem with that cell
(and ±20% for
Figure 2. A typical test set-up.
(-) (+)
Figure 3. Intercell Connection
Resistance.
Figure 4. Impedance increases with
battery age (and weakness.)
Figure 5. Ascending Impedance.
Figure 6. Ascending Impedance
compared to Load Test,