Electricity: battery or capacitor charging or discharging – Battery or cell charging – Pulsed
Reexamination Certificate
2001-07-11
2002-12-03
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
Pulsed
C320S162000
Reexamination Certificate
active
06489750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an appropriate apparatus for charging rechargeable batteries.
Various rechargeable batteries, such as lead-acid or lithium-ion rechargeable batteries, need to be charged using a voltage source whose current is limited, in which the voltage is regulated very accurately and, in some circumstances, must also be varied as a function of the temperature of the rechargeable battery cells, in order to compensate for temperature-dependent fluctuations during the charging process. In this case, the voltage source makes use of current limiting, which limits the charging current supplied from the voltage source. The current limiting is in this case particularly effective when a completely discharged rechargeable battery is connected to the voltage source.
FIG. 2
illustrates a conventional method for charging rechargeable batteries, and the way in which the current limiting operates. As shown in
FIG. 2
, a charging voltage U
L
which is less than the maximum permissible charging voltage U
max
is initially applied to the rechargeable battery. As the charging process progresses, the charging voltage is raised and increases up to the regulator voltage U
max
setting. The voltage from the voltage source U
L
is then limited to this preset value U
max
by means of the voltage regulation setting. The internal rechargeable battery voltage, that is to say the no-load voltage of the rechargeable battery, which is produced as a result of the charging process, rises as the charging process progresses. Conversely, this means that, since the charging voltage U
L
is now kept constant and, to a first approximation, the internal resistance of the rechargeable battery is assumed to be constant, the current drawn or the charging current I
L
of the rechargeable battery falls continuously from the time T
2
at which the voltage limiting starts. The charge state of the rechargeable battery can thus be deduced from the magnitude of the charging current I
L
. Conversely, the charging current I
L
must be monitored in order to detect when it falls below a predetermined lower limit value, in order in this case to terminate the charging process, and to switch off the charging voltage source, immediately or after a specific time.
The charging voltage source may be in the form of a high-precision voltage source (in the form of a linear or switched-mode regulator) with current limiting. This high-precision voltage source is accommodated in the immediate vicinity of the rechargeable battery, that is to say very close to the rechargeable battery being charged, or can be connected to the rechargeable battery via so-called sense lines. These sense lines are lines that are provided in addition to the current-carrying lines which are used for charging the rechargeable battery. The sense lines do not carry current, and are intended exclusively for monitoring the charge state of the rechargeable battery. However, this procedure has the disadvantage that, despite its space requirement and its heat losses, the charging voltage source must be accommodated close to the rechargeable battery to be charged, or the wiring complexity and/or the complexity of the plug connections are/is increased by the use of the sense lines.
The charging current is measured, in a known manner, by inserting a shunt resistor into the charging circuit and by measuring the voltage dropped across this resistor. The use of such a shunt resistor in turn results in the disadvantage that it causes an additional voltage drop in the charging circuit, which slows down the charging process. Furthermore, the monitoring of the voltage dropped across the shunt resistor represents a level of complexity which is not insignificant for measured-value detection. This is particularly true if the shunt resistor needs to be inserted in the positive line rather than in the ground line (for example if the rechargeable battery is coupled to ground via the appliance to be operated).
Furthermore, the conventional procedure described above results in the problem that different types of rechargeable batteries (for example NiCd/NiMH or Li-Ion rechargeable batteries) cannot be charged at the same time since their final charging voltages are of different magnitude. Generally, NiCd/NiMH rechargeable batteries are charged with a constant current, independently of the charging voltage.
A further known option for charging rechargeable batteries is to use a regulated, that is to say variable, current source, in which case the charging current supplied to the rechargeable battery is regulated by varying the duty ratio of a pulsed signal which is applied to a switch provided in a current-carrying conductor. In particular, the internal rechargeable battery voltage of the rechargeable battery is monitored, and the duty ratio, that is to say the pulse-pause ratio of the switch, and hence the charging current as well, are regulated as a function of this. A battery charger of this type is described in U.S. Pat. No. 4,885,522. In the circuit described there, the emitter-collector path through a transistor which is used as a line circuit-breaker, and an inductance in series with it, are connected in the charging circuit. The base of this switching transistor is connected to the output of a Schmitt trigger. In operation, this circuit represents an oscillator, which uses the switching transistor to produce a pulsed charging current. The duty ratio of the pulsed charging current is governed by the hysteresis of the Schmitt trigger, which is in turn governed by the switching state of a temperature filter connected to the battery to be charged. This circuit allows high switching frequencies to be achieved, and thus makes it possible to reduce the amount of heat produced in the switching transistor. This concept admittedly allows different rechargeable battery technologies (for example NiMH or Li-Ion rechargeable batteries) to be charged without any additional complexity arising from variable current limits in one and the same appliance. However, current overshoots occur at the switching time of the switch and can lead to a short-term overvoltage being applied to the rechargeable battery at the switching times, and because of the regular pulsed operation, this would lead to the rechargeable battery being damaged over a period of time. Increased efforts are therefore required for the design of the electronics for the current source used, in order to limit the previously described current overshoots at the switching times, although this is associated with increased costs.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and an appropriate apparatus which overcome the above-mentioned disadvantageous of the prior art methods and apparatus of this general type, and which enable rechargeable batteries to be charged reliably with little complexity.
In particular it is intended to be possible to charge rechargeable batteries based on different rechargeable battery.
With the foregoing and other objects in view there is provided, in accordance with the invention a method for charging a rechargeable battery, that includes steps of: charging a rechargeable battery with an unpulsed rated current supplied from a variable current source; detecting at least one operating parameter of the rechargeable battery; providing a pulsed control signal having a specific duty ratio and producing the duty ratio as a function of the operating parameter; and adjusting the unpulsed rated current supplied from the current source as a function of the duty ratio of the pulsed control signal.
In accordance with an added mode of the invention, a rechargeable battery voltage that is produced as a consequence of charging the rechargeable battery is detected; and the rechargeable battery voltage is used as the operating parameter.
In accordance with an additional mode of the invention, the duty ratio of the pulsed control signal is increased if the detected rechargeable battery voltage of the rec
Hebding Uwe
Koch Georg
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