Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells
Reexamination Certificate
2001-01-19
2002-03-05
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
Reexamination Certificate
active
06353304
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
(Not Applicable)
BACKGROUND OF THE INVENTION
When rechargeable storage batteries are used in electric systems, the requirements for power to be delivered to the connected loads in discharge and/or the availability of power for charging typically do not have values that allow maximization of life of the batteries and maximization of the performance of the systems of which the batteries are a part.
FIG. 1
shows a typical prior art electrical system which provides uninterrupted electrical power to a load or loads
60
. The system includes a primary power source such as AC source
10
(for example, a stand-alone AC generator or generators, or other source of AC electricity such as an electric utility), a single-throw switch
15
which allows the AC source
10
to be disconnected from the system, double-throw switches
22
and
24
to alternately connect an AC to DC converter
20
or a DC to AC converter
26
between the AC source or load(s) and battery
1
. The converters
20
and
26
with the switches
22
and
24
may optionally be a two-way converter which combines the function of
20
and
26
into a single unit. The battery
1
is shown as consisting of two strings, String A and String B, but may optionally consist of any number of strings equal or greater than 1.
The generic electric system shown in
FIG. 1
may, but not necessarily, also include supplemental generators, such as that shown as DC generator
12
, connected to the battery via power conversion equipment such as the DC to DC converter
14
. In one typical application, DC generator
12
is a photovoltaic array for generating DC from solar power.
In
FIG. 1
, the electric system is shown with the switches
15
,
22
and
24
in a position such that the battery
1
can be charged and the load(s)
60
can be supplied with electric power from the AC source
10
. The direction of current flow in the various legs of the circuit is indicated with the symbol {circle around (→)} in FIG.
1
and in subsequent figures. If electricity is not being supplied by the AC source
10
for any reason, switch
15
is opened, and switches
22
and
24
are put into their alternate position so that the load(s)
60
can be supplied with electricity from the battery
1
via the DC to AC converter
26
. Alternatively, if the DC to AC converter
26
and the AC source
10
are synchronized in a manner conventional in the art, then switch
15
can remain closed. In this event, discharge of the battery can supplement the supply from the AC source so that the battery
1
and the AC source
10
jointly supply the load(s)
60
.
The electrical power requirements of the load(s) and the capabilities of the AC source in electric systems are often such that the battery cannot be charged and/or discharged in the manner required to maximize both the life of the batteries and the performance of the electric system.
For example, batteries based on the zinc/bromine chemistry need to be completely discharged occasionally to maximize their utility. However, such batteries should never be completely discharged when they are used to provide back-up power for critical manufacturing processes, as they otherwise might not be available at critical times.
As another example, certain types of nickel/cadmium batteries exhibit a memory effect which results in an apparent loss of available capacity when repeatedly partially discharged and then recharged. This loss of capacity can be recovered by completely discharging and recharging the battery. These batteries are sometimes used in hybrid electric vehicles where power for recharge is only available during vehicle operation, so the frequent complete discharges these batteries require for optimal life and performance cannot be effected.
As a third example, the state of charge of lead-acid batteries used to help match supply from an electric generator (or electric generators if they are connected into an electricity supply network) and the demand (load requirements) from customers connected to that generator, cannot be optimally managed because the power available for recharge or that required in discharge are determined by the difference between the supply available from the generator(s) and the load demanded by customers. Lead-acid batteries perform best and live longest if each charge is completed (finished) properly and if they are not discharged too deeply. On the other hand, system performance will be maximized if the generator is used only when absolutely necessary. None of these optimization criteria can be strictly adhered to because of the highly variable power available or required in the supply-load matching process. Finishing charge and avoiding overdischarge of lead-acid batteries, and optimizing the performance of systems using lead-acid batteries to help generators match supply and demand, are one of the most important potential applications for the invention disclosed herein.
Most battery manufacturers offer guidelines for ways to optimally charge and discharge their products so as to maximize life and performance. Implementation of these guidelines is made complicated for users by virtue of the fact that most batteries are in fact a collection of individually manufactured units, each of which has slightly different performance characteristics.
The most fundamental unit of batteries is a cell, a unit of 1 to 4 volts depending on the chemistry on which the battery is based. A cell consists of a collection of positive electrodes in parallel and negative electrodes in parallel, juxtaposed so to provide the power and the ampere-hour capacity specified. Sometimes, a few cells (of the order of three to eight) are assembled into modules, with the series electrical connections between the cells being effected during manufacture. Cells or modules are then electrically connected in series at the point of use to form strings.
Other batteries are based on cell-stacks consisting of a number of cells electrically connected in series. In this case, modules are fabricated by connecting a number of cell-stacks in electric series and/or parallel. With some battery chemistries, battery auxiliaries, such as pumps for flow batteries or thermal management hardware for both conventional and advanced batteries, are incorporated with the cells or the cell-stacks into a module.
A storage battery (sometimes referred to as a battery system) consists of a number of cells or modules arranged in series and/or parallel arrays. Cells or modules connected in series are collectively referred to as a battery string. Battery strings may then be electrically paralleled. Occasionally, there is only one module in a string, and infrequently, cells are placed only in parallel in a battery. In these cases, there is no meaning to the term string, but if there were, a string would consist of one module or one cell. Alternatively, a battery consists at times of only one string of cells or modules. As discussed here and after, the current invention does not relate to such single string batteries; at least two strings in parallel are required for operation of this invention, although each of the strings may consist of one or more cells or modules in series.
The number of cells or modules in series in a string or battery is determined by the voltage desired for the battery system, which is in turn set by the requirements of the charging and discharging equipment to which the battery is connected. The charging and discharging equipment is generally referred to as power conversion equipment. The number of strings in parallel is determined by the capacity, i.e., the number of ampere-hours, or the energy rating, i.e., the number of watt-hours, that is desired by the user of the battery system.
In order to standardize the terms used herein,
FIG. 2
shows a charge profile for a lead-acid battery. When a constant current is initially applied to a discharged battery, bulk charging occurs and the voltage across the battery increases in a generally exponential manner to the voltage set-point level of the battery.
Atcitty Stanley
Butler Paul C.
Corey Garth P.
Symons Philip C.
Libman George H.
Sandia Corporation
Tso Edward H.
LandOfFree
Optimal management of batteries in electric systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optimal management of batteries in electric systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optimal management of batteries in electric systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2823501