Measuring Resistance
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10 Measuring Resistance
Before testing cell resistance, you should select a site, though not mandatory.
WARNING
: Do not test any cells when the Cellcorder AC Power indicator
is lit. Injury to personnel or damage to equipment could result.
Use the Resistance mode and three clip resistance probes to take internal cell
resistance and intercell connection resistance readings on each cell.
WARNING
: Do not attempt to read internal resistance of an open
cell/module while the battery is on-line. An open cell can be determined by
the unusually high voltage across it and the lower (nominal) voltage across
all other cells/modules.
10.1 Resistance Test Overview
In the Resistance mode, the user connects the leads and initiates testing. The
CRT-300 reads cell float voltage, unless previously stored using voltmeter
mode. Based on cell voltage, the unit determines the correct range, connects
a fixed resistance across the cell and intercell under test, and briefly applies
60 Amps (on 2V cells). Test current changes from 5 to 60 Amps depending
on cell/module voltage (1V to 16V). During load condition, the unit reads
cell voltage, current, and intercell voltage drop and calculates internal cell
and intercell resistances. At completion, the internal cell resistance (Rc) and
the intercell resistance (Ric) is displayed.
The CRT-300 transfers test data to a computer using memory card or IR
(infrared) communication. The Battery Analysis System (BAS) program
analyzes the data, displays data in bar graph format, and generates standard
and custom reports.
10.2 Skewing Effect
When reading resistance, a skewing effect of the cell voltages leads to a
small error. As the first half of the cells are resistance tested, those voltages
become depressed until the charger brings them back to normal float levels.
This means that the voltages of the remaining cells increase to make up the
difference, since the charger controls the overall voltage. The skewing effect
is particularly noticeable on small and weak cells. For large batteries, such as
mini-tank cells, the effect is not as noticeable, but there is still a small error
in the voltage readings.
