Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2002-04-19
2002-11-26
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S128000
Reexamination Certificate
active
06486636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of battery systems, and particularly to rechargeable battery systems for mobile computers.
2. Description of the Related Art
Many electronic products include rechargeable batteries which enable them to operate without connection to an AC power source. The status of the battery is often of critical importance to the product's user, as the product can operate only as long as the battery retains some useful life. For example, mobile computers, which are powered by a removable battery pack when not connected to an AC power source, typically provide a battery status screen to a user which includes an estimation of the remaining life for the battery housed within the pack.
One parameter which is often of interest is the charge level on the battery. Battery capacity in ampere-hours is typically calculated by measuring current flow in to and out of the battery, and integrating the current over time. Some recent mobile computers have embedded a microcontroller or complex finite state machine (FSM) integrated circuit inside the battery pack as a sub-system of the mobile computer. Using this device in combination with a number of sensors, voltage, temperature, current and various other variables are measured with circuitry housed within the battery pack itself. This results in a very complex and expensive battery pack. Nevertheless, this approach has been adopted as an industry standard by the Smart Battery System Implementers Forum (SBS-IF).
A block diagram of a battery system for a mobile computer which complies with the SBS-IF specification is shown in
FIG. 1. A
battery pack
10
includes a rechargeable battery and the associated sensors and circuitry mentioned above. Current is provided to and drawn from the pack via a line
12
, and the pack is connected to a common point via a line
14
.
When connected to an AC power source, an AC/DC converter
18
provides a DC supply voltage for the battery system via a line
19
. When the DC supply voltage is present, it powers the system host
20
(which includes a mobile computer's microprocessor and associated circuitry), typically via one or more DC/DC converters
22
. A power detection circuit
23
detects the presence or absence of the DC supply voltage; when present, a switch S
1
is operated such that power is provided to a battery charger
24
, which in turn provides charging current to battery pack
10
. When the DC supply voltage is absent, switch S
1
connects battery pack
10
to DC/DC converters
22
, so that system host
20
is powered by the rechargeable battery.
Communications between battery pack
10
, system host
20
, and battery charger
24
are handled with a serial bus referred to as an “SMBus”
26
, which complies with the requirements of the SBS-IF. The system also requires that battery pack
10
provide a “safety signal”
28
to charger
24
, to prevent overcharging.
Various conventional implementations of battery pack
10
are shown in
FIGS. 2
a
-
2
d.
Each battery pack includes the battery itself
30
, a current sense resistor
32
, a pair of FET switches
34
and
36
, and circuitry
38
. The FET switches are controlled by circuitry
38
to prevent either overcharging or over-discharging the battery
30
. In each figure, the current through the sense resistor is designated as I
S
, and the current required to power circuitry
38
is designated as I
C
; I
S
, is determined by measuring the voltage across resistor
32
and dividing by its resistance. The current into or out of the battery pack is designated as I
BP
.
To accurately measure I
BP
, it is preferable to calibrate circuitry
38
. Ideally, this requires the ability to measure the voltage across sense resistor
38
with no current flowing in it, to determine how much zero signal offset is in circuitry
38
. However, when pack
10
is configured as shown in
FIG. 2
a,
circuitry
38
always puts a small current drain (I
C
) on the battery. Even when I
BP
equals zero, I
S
=−I
C
and as I
S
flows through resistor
32
, a zero-current condition cannot be achieved; thus, some current measurement inaccuracy is inevitable with this approach.
In
FIG. 2
b,
the arrangement of battery
30
and current sense resistor
32
is changed. Here, the current I
C
required by circuit
38
does not pass through sense resistor
32
, enabling the voltage across sense resistor
32
when I
S
is zero to be measured. Now, however, the I
C
current drain is never accounted for, thereby introducing a different standard error in the system.
Yet another arrangement is shown in
FIG. 2
c,
in which current sense resistor
32
is referenced to ground. However, with resistor
32
connected between ground and battery
30
, I
S
cannot be made zero, and thus the system cannot be calibrated for zero offset.
In the arrangement shown in
FIG. 2
d,
a separate “battery ground”
40
separate from system common (
14
) is employed, which allows the system to measure zero current. Here, however, I
C
is not measured and continually drains the battery. Furthermore, the lack of a common ground between battery and system induces a ground shift which is proportional to I
S
. For a large current load, this ground shift significantly reduces the noise margin of digital signals between system host
20
and circuitry
38
.
SUMMARY OF THE INVENTION
A rechargeable battery measurement and calibration system is presented which overcomes the problems found in the prior art approach described above. The current measurement and battery status intelligence is moved outside of the battery pack, resulting in a system which has both higher accuracy and lower cost than prior art systems.
In accordance with the present invention, the battery pack is greatly simplified: the current sensing element is moved outside of the battery pack, the FET switches can be eliminated, and the battery status intelligence is moved from the battery pack to the system host; these steps significantly reduce the cost, complexity, and power consumption of the battery pack. The system host controls the measurement and calibration of the battery system; the host can command a zero current flow through the current sensing element, enabling the acquisition of accurate calibration data. In a preferred embodiment, calibration values are determined under both zero-current and non-zero current conditions, enabling linearity errors that might otherwise be present in the current measurements to be reduced.
The present system complies with the SBS-IF specification for Smart Battery systems, and is well-suited for use with mobile computers.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 5541489 (1996-07-01), Dunstan
Smart Battery System Specifications, Smart Battery System Manager Specification, Revision 1.0, Dec. 15, 1998, Release Candidate b, pp. i-22.
Maxim Digitally Controlled Fuel-Gauge Interface, Max1660, Rev 1; Oct./1998, pp. 1-20.
Unitrode, Gas Gauge IC With SMBus-Like Interface, bq2092, Jun./1999, pp. 1-28.
Benchmarq Products From Texas Instruments, Gas Gauge IC With SMBus Interface, bq2040, Jun. 1999, pp. 1-32.
Austin Steven E.
Stolitzka Dale F.
Analog Devices Inc.
Koppel, Jacobs Patrick & Heybl
Luk Lawrence
Tso Edward H.
LandOfFree
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