Motor vehicle battery disconnect switch circuits

Details Motor vehicle battery disconnect switch circuits Motor vehicle battery disconnect switch circuits

2003-07-01

2004-12-28

Tibbits, Pia (Department: 2838)

Electricity: battery or capacitor charging or discharging

One cell or battery charges another

Vehicle battery charging

Reexamination Certificate

active

06836094

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to electrical systems of motor vehicles. More particularly, the invention relates to battery disconnect circuits for preventing battery drain when the engine does not run for extended periods of time.
BACKGROUND OF THE INVENTION
Certain motor vehicles, heavy trucks that are powered by diesel engines for example, may be parked for extended periods of time. If its engine is allowed to keep running after a vehicle has been parked, the alternator should be effective to keep the battery bank charged. If the engine is shut down, either intentionally by the driver, or automatically by a device such as an idle shutdown timer, while the circuits for electric devices remain on, the battery bank will begin to drain. For example, an ignition switch will typically remain on after an idle shutdown timer has timed out, and certain circuits that are left on may continue to draw current.
Because the battery bank must be able to provide sufficient current for cranking the engine at starting, a diesel-powered heavy truck typically has a battery bank comprising multiple storage batteries ganged together in parallel. If the battery bank is allowed to drain to the point where it cannot deliver the large cranking current necessary for starting the engine, the vehicle cannot be operated. Because a heavy truck may be parked for an extended period of time, it is therefore appropriate to guard against the possibility that its battery bank will be drained due to failure to turn off all its electric circuits.
Consequently, it is known to place a disconnect switch in circuit between the battery bank and the rest of the vehicle electrical system. All that need be done to avoid inadvertent draining of the battery bank while a heavy truck is parked is to turn the disconnect switch off, an act that would typically be performed by the driver upon turning the engine off and departing the vehicle. Because of the particular placement of the disconnect switch in circuit, the disconnect switch is physically mounted at the battery box outside the truck cab. The driver can therefore turn the switch off and on only when he is outside the cab.
A typical installation for a disconnect switch involves the use of two heavy conductors, one to connect the battery terminal of the disconnect switch to a terminal of the battery bank, such as the positive terminal in a negative ground electrical system, and another to connect the load terminal of the disconnect switch to a distribution point for the entire electrical system load, including a cranking motor solenoid through which cranking current is delivered to the cranking motor. Such a distribution point may be a terminal on the cranking motor solenoid.
SUMMARY OF THE INVENTION
The present invention relates to disconnect switch circuits that are believed to offer better solutions for avoiding inadvertent battery draining when a vehicle is parked for an extended period of time. Rather than placing a disconnect switch in circuit between a battery bank and a distribution point for the entire electrical system load, the invention places the disconnect switch between the battery bank and a distribution point for the entire system load except the engine cranking motor. This allows the circuit between the battery bank and the contacts of a cranking motor solenoid that is energized to run the cranking motor to be switch-free. The cranking motor solenoid is often integrated with the cranking motor itself, rather than being a discrete component that is separate from the cranking motor so as to have a battery terminal connected by a heavy conductor to the ungrounded battery bank terminal and a load terminal connected by another heavy conductor to a terminal on the cranking motor. When the cranking motor solenoid is integrated with the cranking motor, the integrated assembly has a terminal connected to the ignition switch start contact, either directly or through a relay, and a terminal to which one termination of such a switch-free circuit is connected. Such a switch-free circuit, whether connected to a cranking motor solenoid that is separate from the cranking motor or one that is integrated with the cranking motor, provides important advantages.
Rather than using one or more heavy conductors from the ungrounded battery bank terminal to the disconnect switch and one or more heavy conductors from the disconnect switch to the cranking motor solenoid for connecting the ungrounded terminal of the battery bank to the cranking motor when the disconnect switch is placed between the battery bank and the cranking motor, the present invention provides a continuous, switch-free conduction path from the ungrounded battery bank terminal to the contacts of the cranking motor solenoid. Although the length of such a continuous, switch-free conduction path may possibly be slightly greater than the combined lengths of the separate conductors it replaces, the number of terminals at the conductor ends are reduced from four to two, and the number of fasteners needed is cut in half. And although an installation that uses the invention may require an extra conductor and associated fasteners from the load distribution point to the battery terminal of the disconnect switch, that cable need not be as heavy as the one from the battery bank to the cranking motor because it does not have to be sized to also carry the amperage of the cranking motor current.
Another advantage of removing the disconnect switch from the battery feed to the cranking motor solenoid is that the electrical resistance between the battery bank and the cranking motor solenoid is significantly reduced. A switch-free path eliminates the contact resistance that is inherently present in a switch. And although some might consider that resistance small, it is significant when the large amperage of the cranking current is considered. Large current flowing through even a small resistance creates a non-trivial voltage drop and attendant heating. Avoidance of such losses is believed especially significant and beneficial when one recognizes the difficult task of starting a diesel engine, which is most noticeable in subzero weather.
Allowing the cranking motor to become a potential battery drain because of the placement of the disconnect switch in accordance with principles of the invention should have no serious adverse consequences because it is quite improbable the cranking motor solenoid circuit that energizes the cranking motor solenoid to operate the cranking motor will be inadvertently left on when the vehicle is parked and the engine turned off. First, operation of the cranking motor is quite likely to be noticed unless a person is totally deaf. Second, ignition switches that are presently in widespread use require maintained contact by the driver against a return spring force when placed in START position for cranking the engine. The return spring force will return the switch to IGNITION position, breaking the START feed, when the driver ceases the maintained contact.
The invention is therefore believed to provide a cost-effective improvement for avoiding unintended battery draining in a motor vehicle when parked for an extended period of time, and the possible inconvenience of having to jump-start the engine, to replace one or more batteries, or perhaps even to tow the vehicle.
One general aspect of the invention relates to an engine-powered land vehicle comprising a chassis containing an engine and a drivetrain through which the engine propels the vehicle on land. A body that provides a compartment for a driver of the vehicle is disposed on the chassis. The electrical system comprises a battery bank having one or more D.C. storage batteries, an electric cranking motor that draws current from the battery bank to crank the engine at starting when a cranking motor solenoid is energized, and a main control switch (commonly known as a ignition switch) that can be placed selectively in any one of multiple positions by the driver to selectively control the delivery of current from the battery ba

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