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loss, at varying degrees. However, a skin temperature above 85 °C is likely to cause immediate harm to the cells
and should be avoided at all costs.
Recommended Discharge Currents for Strings
Determine the maximum continuous discharge current for a string of cells by multiplying the number of parallel
cells in the string by the maximum continuous discharge current for a single cell. Note that this calculation does
not take into account limitations imposed by any protection electronics or any other features of the battery pack
assembly.
Eq 8.
Number of cells in parallel x Max Discharge Current / cell = Max Discharge Current / string
It is important that the cell-to-cell current collection tabs are correctly sized to carry the maximum design current.
Currents that are higher than the tab can handle, may cause damage to these tabs and overheat the cells.
Additionally, the design of the cell-to-cell interconnections must insure that the current is equally shared between
multiple parallel cells. The internal resistance of A123 cells is low enough to make the task of balancing the current
using a less-than-ideal connection material challenging. A123 engineering regularly employs FEA (finite element
analysis) to simulate the currents flowing through a pack to meet the design’s current sharing specifications.
Voltage Limits
During the end of a discharge, the cell voltage will start to fall precipitously when it has less than 5% of its storable
charge in it. A well-designed pack will never allow any cell in the pack to fall below the absolute minimum voltage
limits in Appendix A. If the cell voltage falls below these limits, the cell can be damaged immediately. The longer
this condition is maintained, the more damage the cell suffers, and the more dangerous it is to operate the cell
subsequently. A123 recommends that if any cell falls below the absolute lower limit, that the pack be taken out of
service and recycled.
Cut-Off Voltage Limits for Strings
The discharge of a cell or battery should be terminated whenever any cell in the string reaches its lowest
recommended discharge cutoff voltage.
The system shall be designed to stop discharging the battery whenever any of the following conditions is true:
•
The string of cells reaches the recommended discharge cut-off voltage
•
Any one cell in the series connection reaches its minimum allowable cut-off voltage
•
The cells exceed the maximum allowable cell temperature
Determine the recommended discharge cut-off voltage for a string of cells by multiplying the number of series
elements in the string by the recommended discharge cut-off voltage for a single cell.
Eq 9.
Number of cells in series x Recommended discharge cutoff voltage / cell = Cutoff Voltage / string