Application Note CC-001
76
However, little evidence has evolved that ties capacitance with the actual
capacity of a cell. It’s worth noting that one researcher felt capacitive
reactance “obscures” the predominant indicator of cell capacity, elec-
trochemical resistance. More so, as the test frequency is increased.
Impedance and admittance measurements include both capacitance and
inductance in their readings.
THE NATURE OF INTERNAL RESISTANCE
At Albér, we have centered our efforts on cell internal resistance. A
refined cell model has been developed which subdivides the internal
resistances of a cell, as shown in Figure 2, into its metallic and
electrochemical components. These resistances do not change with test
frequency. Indeed, researchers have concluded that a zero frequency, DC,
or extremely low frequencies, yield the best results when studying
capacity relationships.
4,5
Another group focusing on internal resistance is prestigious Nippon
Telephone and Telegraph. Their research and field personnel have shown
that a major result of the lead acid battery deterioration mechanism is an
increased internal resistance. A 500 microsecond duration pulse
technique is used to measure cell internal resistance. Test results lead
them to conclude that battery deterioration (capacity) can be estimated
within
10% by internal resistance measure.
Note that Capacity vs. Normalized R
INTERNAL
curve, shown in Figure 3, is
somewhat curved.
Some feel the relationship between most cell internal
parameters and capacity is not a simple straight line as has been
described in recent times but rather a more complex
curvi
-linear
function.
6
