Vertiv Alber Cellcorder CRT-400 User Manual page 141

Application Note CC–001
Measured amounts of energy were methodically removed and each time the internal resistance was
read. It was found that the internal resistance rose no more than 3% after removal of 32% of the cell's
energy. Upon continued discharge, the internal resistance increased rapidly in a non–linear fashion.
Since it is extremely unlikely that the cells' metallic resistance could have changed while the cells were
discharged, it can be concluded that:
•
changes noted in total internal resistance were due exclusively to electrochemical resistance
change
•
electrochemical resistance change is a small part of total internal resistance as the stored energy
drops from 100% to substantially lower levels of charge
In the real testing world, this is good news. It means that a battery can be reliably tested by the
Cellcorder at float, open circuit voltage and even partially discharged.
The "Baseline" and when Best to Get it
The fantasy of a "battery literate" person is that complete capacity and internal resistance test data,
from day zero, is available for analysis. He seeks to determine the typical internal resistance of cells at
100% and 80% as well as the trend from 100% through 80% capacity.
The fantasy begins with the battery's installation. An acceptance capacity test is conducted and the cells'
internal resistances measured. The results should be fairly uniform and he would safeguard this data. He
knows it serves as the ideal baseline, or "reference" per IEEE/ANSI P–1188, for future internal resistance
tests. He will then be able to make cut and dry decisions about maintenance or cell replacement.
Unfortunately, it's a fantasy in most environments because, in the rush and tumult of getting a facility
on–line, establishing a baseline usually has low priority. As a result, we are then left to getting data in
the best ways possible.
The second best time to obtain a set of data is shortly after capacities have been determined on the
same model cells of the same age. Lastly, try obtaining data on the same model cells of any age. Over
time, data to the end–life or 80% capacity level will be accumulated.
Caution
Internal resistance data taken without the benefit of correlated known capacities is often referred to as
"as–found comparative values." This as– found data could be used to set a baseline, but be very
cautious. It is not unusual to find an entire string of cells of uniform internal resistances, and normal
appearance on float, but with failing capacities. In this situation, we highly recommend at least a crude
service/load test be performed to lend some credence to the internal resistance measurements.
The Course Of Action
If you do not have end–life or 80% baselines developed from significant samples of the same cells, age,
etc., you should use the following rough rules–of–thumb based upon Cellcorder user reports and in–
house experience.
If the internal resistance is: 20%–50% above the 100% baseline, then the cell be load tested.... the
sooner the better if it's towards the high end; at 50% or above the baseline, the cell be replaced.
590-2099-501A/SL-29414/4200-070 19- 9 Revision 4