Electricity: battery or capacitor charging or discharging – One cell or battery charges another
Reexamination Certificate
1999-09-16
2001-08-14
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
One cell or battery charges another
C320S104000
Reexamination Certificate
active
06275001
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved dual-battery system and a method for activating a dual-battery system.
BACKGROUND OF THE INVENTION
A known dual-battery system includes a vehicle electrical system battery and a starter battery that are connected to one another when the generator is operating, e.g., see: German Patent 38 41 769. When generator current delivery is low, this has the disadvantage that the vehicle electrical system battery is charged by charge exchange at the expense of the starter battery, if the charge state of the vehicle electrical system battery is less than that of the starter battery. With the known dual battery system, a switch is inserted between the vehicle electrical system battery (which carries the vehicle electrical system loads) and the remainder of the vehicle electrical system. That switch is opened, when the generator is not running as soon as the starter battery voltage falls below a permissible limit value. This feature protects the starter battery from complete discharge only when the generator is not running and also allows partial discharge of the starter battery. This poor condition (partial discharge) increases the initial load on the starter battery as vehicle travel begins and a charge exchange between the two batteries may take place to the detriment of the starter battery. In extreme cases, this can lead to complete discharge or destruction of the starter battery.
German Patent 40 28 242 discloses a dual-battery system in which the connection between starter battery and vehicle electrical system battery can be broken, regardless of the driving status, if the vehicle electrical system battery would otherwise be charged from the starter battery. The two batteries are connected to one another if the vehicle electrical system battery voltage is greater than the starter battery voltage. If the voltage of the vehicle electrical system battery is less than that of the starter battery, the two batteries are disconnected from one another. The disconnection can be performed in various ways, for example by cutting-off the starter battery from the rest of the vehicle electrical system. As a result, the starter battery is not subjected to the load of a discharged vehicle electrical system battery, and in working mode the generator can power the most important loads while charging the vehicle electrical system battery. The two batteries can also be disconnected from the rest of the vehicle electrical system by cutting off the vehicle electrical system battery. If an optional connection is provided between the generator and the starter battery the starter battery can be charged.
A disadvantage of the known dual-battery systems is that activation of the starter battery switch by the control device is very complex. For example, the charge states of the vehicle electrical system battery and the starter battery must be sensed in order to determine the control information.
OBJECTS AND SUMMARY OF THE INVENTION
An object of this invention is to provide a dual-battery system that simply and reliably prevents discharge of the starter battery.
The present invention provides a dual-battery system, comprising a generator, a starter, a starter battery, a starter battery switch, a vehicle electrical system battery, and a control device, in which the starter battery, along with the starter battery switch which is connected in series, is connected in parallel with the generator, starter, and vehicle electrical system battery, and the starter battery switch can be activated by the control device, wherein the starter battery switch is configured as a power MOSFET having a gate connected to the control device.
The present invention further provides a method for activating a dual-battery system as set forth in the preceding paragraph which comprises:
a) sensing initiation of a starting operation upon actuation of the starter (
3
) via the control device (
7
);
b) generating a gate voltage via the control device (
7
) that renders the MOSFET (
6
) conductive;
c) continuously sensing the drain current of the MOSFET (
6
) via the control device (
7
); and
d) shutting off the gate voltage, if the drain current profile indicates termination of the starting operation via the control device (
7
).
Further advantageous embodiments of the invention are evident from the description below and dependent claims. Because the starter battery switch is configured as a power MOSFET, the control device can be of much simpler design. In principle, the MOSFET is always inhibited. When the starter is actuated, this causes a change in the voltage at the MOSFET that can be sensed by the control device via the drain current. Alternatively, the starter can also be connected to the control device, so that initiation of the starting operation is sensed directly. The control device then makes the MOSFET conductive, by way of a gate voltage of correct sign, so that starter current can flow from the starter battery through the MOSFET to the starter. When the starter motor begins to turn current fluctuations occur that are sensed by the control device via the drain current of the MOSFET, so that the control device can inhibit the MOSFET again by shutting off the gate voltage. As a result, the starter battery is on-load only during the actual starting operation. A further advantage of the MOSFET is that it implements a parasitic diode whose cathode is connected via the drain terminal to the starter battery. When the MOSFET is inhibited and the vehicle electrical system battery is discharged, the diode is polarized in the inhibiting direction, and no current (aside from the inhibiting current) flows through the MOSFET. If, on the other hand, the voltage difference between the vehicle electrical system battery and starter battery is greater than 0.7 V, the diode is polarized in the forward direction and a charging current can flow from the vehicle electrical system through the diode to the starter battery. The control device is thus unnecessary for the starter battery charging operation.
In a preferred embodiment, the MOSFET is configured with an integrated current measurement system, making it easier for the control device to pick off the drain current. In addition, the MOSFET is preferably configured with an internal overcurrent protector so that it shuts itself off in the event of a short circuit. A MOSFET of this kind is known, for example, under the trade name PROFET from Siemens AG.
A precrash sensor and/or crash sensor can also be associated with the control device, so that the MOSFET is inhibited when a collision is sensed. For this purpose, the vehicle electrical system battery can also have associated with it a MOSFET that is always conductive in normal operation and is inhibited only in the event of a collision.
REFERENCES:
patent: 5316868 (1994-05-01), Dougherty et al.
patent: 5325038 (1994-06-01), Banzai et al.
patent: 5488283 (1996-01-01), Dougherty et al.
patent: 5726553 (1998-03-01), Waugh
patent: 5764032 (1998-09-01), Moore
patent: 5962928 (1999-10-01), Schenk
patent: 38 471 769 C1 (1990-06-01), None
patent: 40 28 242 A1 (1992-03-01), None
patent: 196 51 612 A1 (1998-06-01), None
patent: 0 583 630 A1 (1994-02-01), None
Kenyon & Kenyon
Tso Edward H.
Volkswagen AG
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