Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
2000-12-22
2001-10-23
Wong, Peter S. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C324S426000
Reexamination Certificate
active
06307378
ABSTRACT:
BACKGROUND
This invention relates to techniques for measuring impedance in electrochemical cells. More particularly, the invention is directed to apparatuses and methods used for taking internal impedance measurements of electrochemical batteries and cells with improved sensitivity and noise/electromagnetic immunity as compared to currently existing methods.
Electrochemical batteries and cells have very low internal impedance. This is true in different types of cells, including those based on either lead acid or nickel cadmium chemistries for which impedances can be on the order of milliohms (m&OHgr;). For this reason, an effective method for measuring impedance must be highly sensitive to small impedance values while being immune to noise and electromagnetic circuit interference. Prior methods of impedance measuring normally utilize one of five different types of electrical circuits: (1) bridge circuits; (2) voltage dividers; (3) 4-wire connections; (4) short circuits; and (5) time constant circuits. However, each of these methods is limited by the inherent characteristics of the particular circuit type used in performing the impedance measurement.
Bridge circuits are commonly used to sense impedance changes in batteries. Such a bridge circuit
20
is depicted in
FIG. 1
, which shows the basic configuration of a circuit of this type which is powered by an AC voltage source V
i
. These circuits generally include impedance elements
22
that are located along first and second current paths
24
and
26
, the impedance elements
22
being located on either side of voltage divider points where the voltages V
A
and V
B
can be measured. For battery impedance measurements, one of the impedance elements
22
in the bridge represents the battery being measured. The output of the bridge, V
o
, is the potential difference between V
A
and V
B
. The voltages V
A
and V
B
are related to the input voltage, V
i
by the relation
V
A
=
V
i
[
Z3
Z1
+
Z3
]
and
V
B
=
V
i
[
Z4
Z2
+
Z4
]
,
under the condition that V
o
is equal to zero (i.e. V
A
=V
B
), so that
(
Z
1
)(
Z
4
)=(
Z
2
)(
Z
3
).
For example, one way of using this circuit is to make one of the impedance elements
22
adjustable and adjust the value of the impedance until V
o
is equal to zero. The problem with this type of operation is that it requires continuous adjustment of the element for each frequency at which the measurement is made. This is because battery impedance is not constant over the frequency spectrum of interest.
An automated system for handling such a procedure is complex and difficult to implement. This circuit is typically used by picking nominal values of the three known impedance elements
22
to maximize the output voltage swing as the battery impedance changes through the sweep of frequencies and usable life. The sensitivity of the output is maximized when Z
2
=Z
3
and Z
1
=Z
4
. This implies that one of the impedance elements
22
must have a value that is the complex conjugate of the battery impedance.
Another limitation of bridge circuits relates to the fact that since internal impedance is very low for most cell types, voltage drops across the battery will also be very low. Fixing the values of all but one impedance element
22
and allowing only this battery impedance element to change implies that either V
A
or V
B
will remain constant. The bridge
20
reduces to a voltage divider for changes in the battery impedance. The output voltage is inversely proportional to changes in the battery impedance. Thus, as the impedance of the battery increases, output voltage becomes smaller. To get sufficiently large voltage drops at the output, a large amount of current is required. For example, if the magnitude of the battery impedance were 5 m&OHgr;, a 1 A current would be required to produce a 5 mV drop at the output.
Such a condition would place a high gain requirement on any sensing amplification equipment connected at the output of the bridge circuit. For example, the input impedance of such an amplifier would be the impedance of the bridge circuit
20
and would be very low due to the low battery impedance. Where such low input impedances are involved, such as those below 1&OHgr;, amplifiers become highly susceptible to electrical field noise, whether self-generated or from other sources. This condition is compounded where the input signal is also very low. Adverse interference effects can be expected regardless of whether BJT or FET input stages are used. Although the addition of a transformer across V
o
is typically recommended in cases of low input impedance, the addition of such a device tends to contribute to circuit impedance, lowering the circuit's sensitivity. Alternatively, where a sufficiently high turns ratio is present, an added transformer can reduce the bandwidth of the output signal produced.
Since bridge circuits do not easily permit impedance sensing without adjustment of the known impedance elements
22
, the circuit has no more sensitivity than the voltage divider circuit. Thus, such circuits are normally only usable in laboratory settings where the impedance elements can be adjusted.
A second commonly used technique for impedance measuring uses a voltage divider circuit, which is typically preferred over bridge circuits when adjustment of impedance is not required. A voltage divider circuit
28
used for battery impedance measurements is shown in FIG.
2
. The circuit
28
, like most designs of this type, is driven by an AC current source
30
since voltage levels are typically in the range of millivolts and current in the range of amperes and thus amperage is easier to regulate than voltage. The circuit includes a sensing impedance Z
s
and a battery impedance Z
b
in a series loop
29
with the AC current source. Each sensing and battery impedance has a respective sensor
31
that connects to the series loop
29
at the respective impedance's point of positive and negative potential. Each sensor
31
is separated from the series loop
29
by capacitors
32
used to block the battery's DC signal. This technique involves two measurements: (1) measurement of the voltage V
s
across a sensing impedance Z
s
, permitting measurement of the loop current given the known size of Z
s
; and (2) measurement of the voltage V
b
across the battery
34
being measured.
Voltage divider circuits used to measure battery impedance are limited by the same disadvantages as bridge circuits. Like bridge circuits, voltage measurements are taken in the millivolt signal level since batteries have very low impedance. Thus, voltage divider circuits, like bridge circuits, are susceptible to electrical field noise and have limited sensitivity.
A third technique utilizes a circuit known as a 4-wire or “Kelvin” connection. This is among the most frequently used techniques for measuring battery impedance and has been described in numerous patents and other references. The general configuration of a 4-wire connection
36
is shown in FIG.
3
. In principle, this circuit is very similar to a voltage divider circuit, being driven by a current source
37
. But the 4-wire connection
36
does not have a sensing impedance Z
s
. A battery
38
is interrogated with a current signal, and the voltage drop V
b
across the battery
38
is measured with a sensor
31
separated from the battery nodes by capacitors
32
. As indicated above, for most lead acid and nickel cadmium cells, the internal impedance Z
b
is very low. This means that the battery
38
will be driven with amperes of current, and output signals will be on the order of millivolts of potential.
Most of the problems associated with bridge circuits and voltage dividers also apply to 4-wire connections. In fact, U.S. Pat. No. 5,821,757 to Alvarez et al. specifically addresses the problem of reducing electromagnetic interference (EMI) that adversely affects the 4-wire connection described in U.S. Pat. No. 5,281,920 to Warst with the addition of tw
Goebel, Jr. Esq. Edward W.
Luk Lawrence
MacDonald Illig Jones & Britton LLP
The Penn State Research Foundation
Wong Peter S.
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