Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
1999-09-10
2001-06-05
Wong, Peter S. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S127000
Reexamination Certificate
active
06242889
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is battery powered devices, including battery powered vehicles.
BACKGROUND OF THE INVENTION
Considerable resources have been invested over the last several years in the development of battery powered devices. For many such devices, especially small consumer items such as portable electric shavers and toothbrushes, in which the power storage and delivery requirements are not terribly demanding, the known technologies are quite adequate. For other devices, including portable power tools and computers, golf carts and fork lifts, the demands are such that presently available technologies are relatively satisfactory, but inconvenient. For still other devices such as all-electric automobiles, the demands are such that implementation of commercially acceptable embodiments has been significantly impeded by excessively long charging duration, and limited battery capacity. Thus, there is still a need to improve power storage and delivery in battery powered devices.
There are four systems of especial importance to the overall efficiency of battery powered devices, namely the battery or battery pack, the battery charging system, the battery discharging system and the load. Previously, these systems were almost always developed and implemented independently, resulting in additional inefficiencies even if the underlying systems were themselves relatively efficient. Thus, there is a continuing need to integrate two or more of these systems in ways that improve the overall efficiency.
Batteries
Batteries can generally be divided into two types, those in which the electrolyte is maintained largely in a liquid phase, as exemplified by common lead-acid automobile batteries, and those in which the electrolyte is maintained largely in a solid or semi-solid phase, as exemplified by Nickel-Cadmium (NiCad) batteries. Within each type of battery, there are numerous possible electrolytes and electrode materials available.
Both liquid electrolyte and solid electrolyte batteries suffer from significant drawbacks. Liquid electrolyte batteries tend to leak, and to experience electrolysis involving gas formation at boundaries of the electrodes. These phenomena increase the apparent impedance of the battery and cause current related heating that may result in failure of internal structures. Extreme cases may result in explosion. Even without damage or danger of explosion, the gasses may require venting and are generally hazardous. Electrolysis may also cause loss of electrolyte which is deleterious to the battery chemistry, causing reduced battery life and increased maintenance costs.
Solid and semi-solid electrolyte batteries are more resistant to leaking, but are still prone to electrolyte degradation. Moreover, such batteries generally have significantly reduced power density and relatively limited number of recharge cycles.
The problems discussed above with respect to different types of batteries can be exacerbated through the use of known charging technologies. For example, both lead-acid and Nickel Metal Hydride batteries can become explosive during rapid charging, especially where the charger does not protect against overcharging.
Battery Chargers
Battery chargers generally fall into two categories—(1) direct current (D.C.) chargers and (2) pulsed current chargers. Direct current chargers typically utilize either a constant voltage mode in which the voltage is fixed and the current varies, or a constant current mode in which the current is fixed and the voltage varies. D.C. chargers give rise to several problems, many of which can be reduced or eliminated by limiting the maximum charging current to a low-value, and thereby extending the charge cycle up to several hours. A typical low-value charging current would be one-tenth battery capacity, i.e., where the charging current falls at the battery's nominal amp-hour capacity divided by 10 hours. Thus, a ten amp-hour battery charging at a rate of 1 amp would employ a low-value charging current. Such chargers, known as trickle chargers, are advantageous in that they obviate the need for complex control schemes, and minimize the danger of reaching an overcharge condition. This is especially true in the constant voltage mode since current will reduce even further as battery voltage approaches the voltage of the charging source. The main drawback of trickle chargers is the inconvenience of being unable to use the battery for the 8 to 18 hours that are required to recharge the battery, or alternatively, the expense of procuring additional battery packs to act as replacements during the recharge cycle. These disadvantages are especially relevant with respect to electric vehicles where the batteries cannot readily be swapped in and out of the vehicle.
In pulsed battery chargers, the charging current is turned on and off periodically, thus allowing gases and separated ions sufficient time to recombine in the electrolyte solution. A further improvement involves utilizing the period of recombination to apply short discharge pulses to the battery to “clean-up” the newly plated material, thereby eliminating contaminants and nodules in the plated matrix. This technique was originally developed and patented by G. W. Jernstedt (assigned to Westinghouse Electric) between 1948 and 1954, and adapted to battery chargers by W. B. Burkett and others (assigned to Christie Electric Corp) around 1971.
An added benefit of pulsed charging is that it allows much higher current density in the charge pulse, which may significantly reduce the charge time. There are practical considerations such as current carrying capacity of the internal battery structure that must be observed, so extremely short charge cycles (less than 0.1 hour) are rarely practical, but still may be possible. Major concern of a high rate charging system centers around when to stop charging, since even a moderate overcharge will cause battery temperature to rise drastically, and can cause explosion. Traditional approaches have been to stay on the safe side and terminate charging well before peak capacity has been achieved. More complex control schemes have been devised (e.g. U.S. Pat. No. 4,746,852 to Martin), but are largely limited to specific battery types where the charge curve is predictable. Many of these approaches depend on further instrumentation of the battery pack through addition of temperature or other sensors. In the case of the example above, identification modules are used to select a specific control mode based upon the signaling of a specific battery type.
It has been known for several years to vary the rate or end-point of battery charging as a function of 0
th
, 1
st
, 2
nd
and 3
rd
order sense parameters. 0th order sense parameters are those which do not vary over time. Examples include the expected maximum (reference) voltage of a particular type of battery, the maximum safe temperature of the battery during charging, or the maximum safe charging current. 1st order sense parameters are those which do vary over time. Examples include power supply voltage (V
ps
), battery voltage upon application of a given load (V
load
), battery voltage without any load (V
unload
), and the three corresponding currents (I
ps
), (I
load
) and (I
unload
). 2
nd
order sense parameters are time derivatives of the 1
st
order sense parameters, and 3
rd
order parameters are time derivatives of the 2
nd
order parameters. Examples include the 2
nd
order parameter V′
ps
, (which is dV
ps
/dt), and V″
ps
, (which is dV
2
ps
/d
2
t).
Known battery chargers have employed relatively simple combinations of 0
th
, 1
st
, 2
nd
and 3
rd
order sense parameters. For example, chargers are known which modify one or more charging parameters as a function of two different 1st order sense parameters, power supply voltage (V
ps
) and temperature (T), but no second or third order parameters. Other chargers are known which modify one or more charging parameters as a function of one 1st order sense parameters such as temperature (T) and two different 2nd order s
DAX Industries, Inc.
Fish Robert D.
Fish & Associates, LLP
Luk Lawrence
Wong Peter S.
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