Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2000-11-27
2002-09-24
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S128000
Reexamination Certificate
active
06456042
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains generally to battery chargers and more particularly to a method for charging batteries which utilizes a variable voltage lid which is responsive to the estimated charge acceptance level to thereby prevent overcharging of the battery as it nears a fully charged state.
2. Description of the Background Art
The process by which a battery is charged determines the relative usable capacity of that battery and to a large degree the service life that can be expected from the battery. Insufficient charging of a battery results in a requisite reduction in battery capacity, wherein the available ampere-hours are inadequate in consideration of the weight, size, and cost of the battery. In contrast, overcharging a battery leads to a reduction in service life for the cells of the battery. Determining a proper charge rate for any battery is complicated by the fact that a fully depleted battery can accept a higher charge rate than a battery which is approaching a state of full-charge, therefore, batteries are typically charged at a variable rate. Unfortunately, the situation is further complicated by the fact that as the battery approaches a fully charged state the charge acceptance drops and charge voltage rises to create an overcharge potential which produces damaging effects on the battery.
Numerous charging methods have been developed, therefore, to provide a charge rate which can fully charge the battery while introducing a limited amount of overcharging. For example, constant current chargers typically generate a constant charge current held within a limited voltage, such that the current drops off as the battery approaches the upper voltage limit of the charger output.
FIG. 1
depicts a battery under charge
10
, wherein a battery
12
is connected to a voltage source
14
with upper limit V
MAX
which drives a constant current through the constant current regulator
16
to provide charge current. The charger illustrated in
FIG. 1
is a typical example of a CI/CV charger employed in a variety of applications where it supplies a constant current limited by a constant upper voltage limit.
Vehicles often employ CI/CV charging systems which are typically designed to maximize service life by maintaining the battery state-of-charge (SOC) at a moderate level, so as to reduce the deleterious overcharging effects. In battery charging literature and practice, a number of algorithms concerning battery charging provide compromises between service life and performance. A common approach is to maintain the battery at a nominal level of about an 80% state-of-charge (SOC) at all times such that the vehicle power system operates within a narrow SOC range from about 70% to 90%. However, in view of the demands for increases in energy density it is prudent to attempt to maximize the SOC operating window and utilization of the battery.
Numerous misconceptions exist with regard to battery charging which have been promulgated within typical battery charging systems. An application engineer may posit the question “at what voltage should a specific battery be charged?” The question is understandable in relation to
FIG. 1
, however, it is misleading, as are many similar questions and does not lead toward establishing mechanisms for proper charging. To advance the art of charger design toward maximum battery utilization requires re-examination of the underlying charging concepts. Considered in a strict sense, a battery may not be charged by a “constant-voltage” source as it is the concomitant charging current associated with the driving force of the voltage that forces energy storage to occur within the battery. The “constant-voltage” is more correctly the upper limit of the charging voltage which is not exceeded during charging. It will be appreciated that charging at a “constant-voltage” would force unrealistic charge current levels into a depleted battery.
It is beneficial to understand the factors relating to a battery being charged. While undergoing charging, the voltage seen at the terminals of the battery is substantially the sum of three components represented as:
Measured Voltage=Equilibrium Voltage+Polarization Voltage+OhmicVoltageDrop (1)
wherein the equilibrium voltage is commonly referred to as the open-circuit battery voltage, V
OC
; the polarization voltage describes the combined effects of concentration and ion/charge-transfer; while the ohmic voltage drop is the voltage drop associated with the ohmic resistance at the given charge current. In contrast to typical electrical components, a battery is an energy storage device that absorbs and provides electrical energy according to an internal electrochemical balance which has an associated reaction voltage that is a dynamic reflection of the “driving force” function and depends strongly on the past operating history, or time derivatives, experienced by the battery.
FIG. 2
depicts basic charging effects, wherein the battery voltage profile is shown as a function of state-of-charge (SOC) for a series of charging currents
20
b
through
20
f
in reference to an equilibrium voltage
20
a
. The equilibrium voltage
20
a
is the voltage which would be measured across the open-circuit battery at that point in the charge cycle as represented by the voltage curve if the applied charging current were interrupted or disconnected and equilibrium established. Battery charging current is often expressed as a ratio, C-rate, which expresses the ratio of charging current to nominal battery capacity, I/Q
N,
so that the charge rate may be expressed independently of battery capacity. Charging current curves
20
b
through
20
f
identify increasing levels of charging current applied to the battery with
20
b
at a 0.05C-rate,
20
c
at a 0.10C-rate,
20
d
at a 0.33C-rate,
20
e
at a 0.67C-rate, and
20
f
at a 1C-rate. It can be seen that during charging, the induced battery voltage exceeds the equilibrium voltage
20
a
as one would expect in order to force energy into the battery. The curves also indicate that as the battery nears full charge (100% SOC), the battery voltage increases more readily than the equilibrium voltage so as to cause the voltage curves to diverge. In literature, the divergence characteristic of the charge curve from the equilibrium voltage is commonly interpreted as an increase in battery internal resistance as a function of SOC, and simple equivalent circuits and mathematical models are derived accordingly. However, the rationale of such internal resistance concepts are contradictory to the actual chemical and electrochemical nature of a battery. As active materials are converted from lead sulfate, PbSO
4
(insulator) in the discharged state in both electrodes to lead dioxide, PbO
2
(1.2×10
−6
to 2×10
−5
&OHgr;/m) within the positive electrode and metallic lead Pb (10
−7
&OHgr;/m) within the negative electrode, the overall cell resistance decreases rather than increases. The attendant increase in sulfuric acid concentration that accompanies charging generally causes a minimal increase (less than 10%) on the conductivity of the electrolyte. The electrolyte concentration is typically in the range from 1.250 to 1.280 kg/L. Furthermore, changes to the resistance of metallic parts, e.g., terminals, cell interconnects, lugs, during a single charge cycle is negligible such that ohmic resistance is largely unchanged. Finally it should be appreciated that temperature increases caused by ohmic and joule heating result in further decreases in ohmic resistance within the battery.
It will be appreciated, therefore, that the concept of increasing internal resistance during battery charging is misleading, since resistance levels within the battery do not significantly increase as the state-of-charge increases. In real
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Luk Lawrence
Tso Edward H.
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