Application Note CC-001
84
DATA ANOMALIES
Another part of the battery literates’ fantasy is also likely to be shattered:
that
all
the data gathered for the baseline, or test readings taken
afterward, would fall reasonably close to any trend line. That is, higher
capacity cells will have uniformly lower internal resistances and lower
capacity cells will have proportionately higher internal resistances. Even
cells fresh off of a tightly controlled production line fail to completely
conform with this expected behavior.
5
In the course of your own testing, you will find
tested,
high capacity cells
with an unusually
high
internal resistance. Similarly, there will be cells
with the
low
internal resistance characteristic of a good cell that would
fail a capacity test. Unfortunately, the current state of the art does not
permit impedance, conductance, admittance or resistance testing to
definitively identify the cells with anomalies - without the assistance of a
load test.
If you are fortunate to have large amounts of both capacity and
corresponding internal resistance data, developing a baseline should be
no problem. When a program is just starting, or there are few cells of a
particular model, some educated “guess-timating” helps in sorting out
data. If you need baseline data try calling us or the cell’s manufacturer.
INTERNAL RESISTANCE TEST FREQUENCY
The whole purpose of the internal resistance check is to determine
whether a cell has deteriorated and if so, how much and how fast.
Logically, checks are done periodically. At least quarterly, check cells
that have exhibited signs of abnormality either from inspection or an
earlier capacity, integrity or internal resistance test.
Since neither specific gravity nor visual checks can be done on sealed
cells, you would want to do much more frequent testing than on vented
cells. For that reason, it’s no surprise the P-1188 VRLA proposed
Standard recommends internal measurements be taken quarterly.
On the other hand, the standard for vented batteries, 450
1
, doesn’t specify
this type of check. However, it does call for annual connection resistance
readings and you would want to take the internal resistances at the same
time.
