Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
2000-12-20
2002-06-25
Toatley, Gregory (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
Reexamination Certificate
active
06411097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to an electronic circuit for an energy storage device management system. More particularly, the present invention is directed to an electronic circuit for efficiently and accurately measuring individual voltages in a series connected electrochemical energy storage device which may be utilized with electric and hybrid vehicles.
2. Discussion
In order to commercialize electric and hybrid vehicles on a widespread basis, the energy storage devices or batteries, which are the most expensive component of the vehicle, must operate reliably through the life of the vehicle. In the typical configuration the batteries are formed from a stack of series connected electrochemical cells.
A common requirement for large stacks of electrochemical cells used in electric and hybrid vehicles, particularly in advanced applications such as lead acid, Li-Ion or NiMH battery packs, is the need to measure individual or groups of cell voltages almost simultaneously. In practice, this means the measurements should be taken within a time window of a few milliseconds.
With reference to
FIG. 1
, a common technique known within the prior art accomplishes voltage measurement through the use of a plurality of resistive divider circuits. More specifically,
FIG. 1
shows an exemplary battery pack
10
having fortyeight energy storage cells B
1
through B
48
connected connected in series. A resistive voltage divider circuit
12
is connected between the positive terminal
16
of battery cells B
2
through B
48
and a common ground node
14
. The discrete resistances R
1
, R
2
, . . . , R
n
, are selected such that the output potentials V
m1
, V
m2
, . . . , V
mn
fall below a certain voltage limit, for example 4 volts, suitable for input to a multiplexer and A/D converter. The voltage signals from each resistive divider circuit
12
can then be sampled and digitally processed. The actual nodal voltages V
1
, V
2
, V
3
, . . . , V
48
become increasingly higher towards the top of the battery pack
10
, such that in general:
V
mn
=
V
n
·
k
n
=
V
n
·
R
1
R
1
+
R
n
=
4
V
⇒
V
n
=
V
mn
k
n
;
∀
n
=
1
,
2
,
…
The voltage across each cell segment V
B1
, V
B2
, . . . , V
B48
is then computed as the difference between the nodal voltages measured on either side of the cell according to the formula:
V
Bn
=
V
n
−
V
n−1
For example, the voltage V
B3
of cell B
3
is measured by taking the difference between V
3
and V
2
provided by the respective voltage divider circuits
12
.
The principal problem with this technique of voltage measurement is that a small error in measuring the nodal voltages V
n
translates into a large relative error in the measurement of segment voltages V
Bn
. These errors increase as the nodal voltages V
n
become increasingly larger towards the top or higher potential cells of the battery pack
10
. For example, suppose:
k
48
={fraction (1/48)}
, k
47
={fraction (1/47)}
V
n48
=V
48
·k
48
=4 V,→V
48
=192 V,
V
n47
=V
47
·k
47
=4 V,→V
47
=188 V,
.:V
B48
=V
48
−V
47
=4 V.
If k
48
is in error by=1%, and k
47
is in error by −1%, measurements of the nodal voltages indicate:
V
48
=193·92V; V
47
=186×12V
V
B48
=7.8V., error=95%
Thus, the measurement error associated with this network of resistive divider circuits
12
and measurement technique could be in excess of 95%.
Furthermore, this error is nonuniformly distributed between the cell segments varying from a maximum of 2 percent at the bottom to a maximum of 2n× percent at the top of the battery pack
10
. The latter renders this approach useless in applications where comparison of the cell segment voltages are used for diagnostics or corrective actions such as in cell balancing. Lastly, this conventional resistance network continues draining the cells of the battery pack
10
even when the resistance network is not in use.
While not specifically shown, a matrix of electromechanical relays can also be used for selectively switching across the cell segments of the battery pack. This approach results in slow measurement of cell voltages and is therefore not suitable for modern applications. In addition, such a relay based device also becomes too bulky and heavy for use with an electric or hybrid vehicle. Higher speed and accuracy can be achieved using a separate isolation amplifier for each battery segment, but this approach results in a relatively large and expensive system.
Accordingly, it is desirable to provide an electronic circuit for overcoming the disadvantages known within the prior art. It is also desirable to provide an electronic circuit which allows for a high degree of accuracy when measuring both the lowest potential cell voltages and the highest potential cell voltages. Moreover, it is desirable to provide a highly efficient electronic circuit which minimizes any loss within the circuit. Finally, it is desirable to provide an electronic circuit with various switched components to prevent the leakage of current from the energy storage device when the circuit is not being used.
SUMMARY OF THE INVENTION
According to the teachings of the present invention, a voltage transfer circuit for measuring the individual segment voltages within an energy storage device is disclosed. The circuit includes a plurality of battery segments forming the energy storage device. An amplifier circuit is connected across one of the battery segments for converting a differential voltage to a reference current. A sense resistor is associated with the amplifier circuit to convert the reference current to a voltage signal which is proportional to the voltage across the battery segment. A voltage measurement node associated with the sensing resistor may be used for measuring the voltage signal. In one embodiment of the invention, a multiplexing and sampling circuit provides digitized voltage samples to a processor. The voltage level of each cell within the battery pack can then be monitored by the processor.
REFERENCES:
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patent: 5710503 (1998-01-01), Sideris
patent: 5760488 (1998-06-01), Sonntag
patent: 5808469 (1998-09-01), Kopera
patent: 5824432 (1998-10-01), Currle
patent: 5831514 (1998-11-01), Hilpert et al.
patent: 5945829 (1999-08-01), Bertness
patent: 6020718 (2000-02-01), Ozawa et al.
patent: 6147499 (2000-11-01), Torii et al.
patent: 6166549 (2000-12-01), Ashtiani et al.
Ashtiani Cyrus N
Stuart Thomas A
DaimlerChrysler Corporation
Smith Ralph E.
Toatley Gregory
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