Figure 9-8 compares the three methods of
measuring the internal resistance of a battery and observe
the accuracy. In a good battery, the discrepancies between
methods are minimal. The test results deviate to a larger
degree on packs with poor SoH.
Impedance measurement alone does not provide
a definite conclusion as to the battery performance. The mW
readings may vary widely and are dependent on battery chemistry,
cell size (mAh rating), type of cell, number of cells connected
in series, wiring and contact type.
Figure 9-8: Comparison of the
AC, DC and Cadex Ohmtest™
were obtained using the Cadex 7000 Series battery
analyzer by applying a full charge/discharge/charge cycle.
The DC method on the 68% SoH battery exceeded 1000mW.
When using the impedance method, a battery with
a known performance should be measured and its readings used
as a reference. For best results, a reference reading should
be on hand for each battery type. Figure 9-9kl; provides
a guideline for digital mobile phone batteries based on impedance
The milliohm readings are related to the battery
voltage. Higher voltage batteries allow higher internal resistance
because less current is required to deliver the same power.
The ratio between voltage and milliohm is not totally linear.
There are certain housekeeping components that are always
present whether the battery has one or several cells. These
are wiring, contacts and protection circuits.
Temperature also affects the internal resistance
of a battery. The internal resistance of a naked Li-ion cell,
for example, measures 50mW at 25°C (77°F). If the temperature
increases, the internal resistance decreases. At 40°C (104°F),
the internal resistance drops to about 43mW and at 60°C (140°F)
to 40mW. While the battery performs better when exposed to
heat, prolonged exposure to elevated temperatures is harmful.
Most batteries deliver a momentary performance boost when
Figure 9-9: Battery
state-of-health based on internal resistance.
The milliohm readings relate to
the battery voltage; higher voltage allows higher milliohm
Cold temperatures have a drastic effect on all
batteries. At 0°C (32°F), the internal resistance of the same
Li-ion cell drops to 70mW. The resistance increases to 80mW
at -10°C (50°F) and 100mW at -20°C (-4°F).
The impedance readings work best with Li-ion
batteries because the performance degradation follows a linear
pattern with cell oxidation. The performance of NiMH batteries
can also be measured with the impedance method but the readings
are less dependable. There are instances when a poorly performing
NiMH battery can also exhibit a low mW reading.
Testing a NiCd on resistance alone is unpredictable.
A low resistance reading does not automatically constitute
a good battery. Elevated impedance readings are often caused
by memory, a phenomenon that is reversible. Internal resistance
values have been reduced by a factor of two and three after
servicing the affected batteries with the recondition cycle
of a Cadex 7000 Series battery analyzer. Of cause,
high internal resistance can have sources other than memory
The terms ‘internal resistance’ and ‘impedance’
are often intermixed when addressing the electrical conductivity
of a battery. The differences are as follows: The internal
resistance views the conductor from a purely resistive value,
or ohmic resistance. A comparison can be made with a heating
element that produces warmth by the friction of electric current
Most electrical loads are not purely resistive,
rather, they have an element of reactance. If an alternating
current (AC) is sent through a coil, for example, an inductance
(magnetic field) is created, which opposes current flow. This
AC impedance is always higher than the ohmic resistance of
the copper wire. The higher the frequency, the higher the
inductive resistance becomes. In comparison, sending a direct
current (DC) through a coil constitutes an electrical short
because there is only a very small ohmic resistance.
Similarly, a capacitor does not allow the flow
of DC, but passes AC. In fact, a capacitor is an insulator
for DC. The resistance that is present when sending an AC
current flowing through a capacitor is called capacitance.
The higher the frequency, the lower the capacitive resistance.
A battery as a power source combines ohmic, inductive
and capacitive resistance. Figure 9-10 represents these
resistive values on a schematic diagram. Each battery type
exhibits slightly different resistive values.
Figure 9-10: Ohmic, inductive and
capacitive resistance in batteries.
= ohmic resistance
= constant phase loop (type of capacitance)
- L = inductor
= Warburg impedance (particle movement within the electrolyte)
= transfer resistance