TWO-BATTERY SYSTEM

The invention relates to an improved dual-battery system and a method for its operation.

A known dual-battery system includes a starter battery and a vehicle electrical system battery, e.g., see: German Patent 40 28 242 A1. Arranged between the starter battery and the vehicle electrical system battery is a starter battery switch that is closed during a starting operation so that both batteries are connected to one another. Primary and secondary loads are then powered via both batteries. “Primary load” is understood to be loads essential for starting and driving capability or for vehicle safety, and “secondary load” to be for comfort components. In order to prevent any discharge of the starter battery, a control device senses the charge states of each of the starter battery and the vehicle electrical system battery. If the charge state of the starter battery drops below that of the vehicle electrical system battery, so that charging of the vehicle electrical system battery would take place at the expense of the starter battery, the control device will break the connection between the batteries by opening the starter battery switch. The known dual-battery system is disadvantageous firstly because the starter battery is on-load even after the starting operation has taken place, and secondly because the complexity of measuring and sensing the charge states of the two batteries is quite substantial.

An object of this invention is therefore to provide a dual-battery system in which the starter battery is on-load only in the starting phase and the starter battery can nevertheless be charged at any time when the load state permits, and which requires little investment in terms of circuit engineering.

The present invention provides a dual-battery system, which comprises: a starter battery, a generator, a primary load, a vehicle electrical system battery, a starter, a power switch connected between the starter battery and vehicle electrical system battery so that the starter battery and vehicle electrical system battery can be connected together in parallel during operation of the starter, and a controllable electronic switch connected in parallel to the power switch.

The present invention further provides a method for controlling a dual-battery system of the type described in the preceding paragraph, which involves, a) sensing the charge balance of the starter battery and the vehicle electrical system battery by the current profile at the controllable electronic switch, preferably a MOSFET, and b) making the electronic switch conductive whenever a charging current can flow from the vehicle electrical system battery to the starter battery.

Because a controllable electronic switch, in particular a MOSFET, is arranged parallel to the power switch, it is very easy to sense, by way of the internal current measurement capability of the MOSFET, whether the charge state of the starter battery and vehicle electrical system battery allows charging of the starter battery. By generating a gate voltage of the correct sign, in such a situation a charging path can be enabled and the starter battery can be charged. Otherwise, the MOSFET disconnects the starter battery from the primary and secondary load so that the starter battery is on-load exclusively in the starting operation. Since all the control data are already available internally in the MOSFET, separate measurement shunts are unnecessary and the configuration of the control device can also be very simple. Further advantageous embodiments of the invention are evident from the dependent claims.

In a preferred embodiment, there is arranged between the vehicle electrical system battery and the primary load a switch with which, prior to the starting operation, the supply of power to the primary load can be selectably switched over between the vehicle electrical system battery and the starter battery, preferably, this switch is also a MOSFET.

Preferably the two, above-mentioned, MOSFETs are polarized oppositely to one another, so that either the two source or the two drain terminals are connected to one another, so that a short-circuit path cannot occur via the parasitic diodes of the MOSFETs.

Battery system1includes a starter battery2, a generator3, primary load4, secondary load5, a vehicle electrical system battery6, an ignition switch7, and a starter8with an associated switch9. A power switch10with diode11connected in parallel is disposed between starter battery2, and generator3and vehicle electrical system battery6. Prior to the initiation of a starting operation, each of switches7,9, and10, are open. Primary load4, necessary for the starting operation, e.g., an engine control device, is powered exclusively via vehicle electrical system battery6. When ignition switch7is then closed, for example, by turning the ignition key, the current flowing from vehicle electrical system battery6through ignition switch7causes switch9to close, so that the circuit between starter battery2and starter8is closed. A starter current therefore flows from starter battery2to a starter motor associated with starter8; that motor begins to turn and attempts to start an internal combustion engine. The closing of ignition switch7also causes power switch10to close, so that starter battery2and vehicle electrical system battery6are connected in parallel. Depending on the charge state of the two batteries, a charge equalization then takes place between them, and both batteries are available for powering primary load4and starter8. Once the starting operation has ended, each of switches7,9, and10are then opened again, so that starter battery2is not on-load in normal operation. Diode11constitutes a charging path between vehicle electrical system battery6and generator3, and starter battery2, so that a charging current can flow through diode11if the voltage difference between vehicle electrical system battery6and starter battery2becomes greater than 0.7 V. If the voltage difference is less, however, diode11is then inhibited and the starter battery is protected against discharge. The advantage of this arrangement is that the charging path through the diode does not require a separate control system. This arrangement does, however, also have a few disadvantages. On the one hand, the charging voltage for starter battery2is always reduced by a value equal to the voltage drop at diode11. On the other hand, an engine start cannot be accomplished if vehicle electrical system battery6has been discharged to such an extent that it can no longer power primary load4. This would then require that power switch10be closed even before the actual starting operation, in order to charge vehicle electrical system battery6sufficiently via starter battery2. Once again, however, this requires increasing cost for a control device (not shown).

One possible solution to the problem of powering primary load4during the starting operation when vehicle electrical system battery6is discharged is shown in FIG.2. For this purpose, primary load4is connected in parallel with starter battery2, thus ensuring that they are powered even if vehicle electrical system battery6is discharged. A disadvantage of this arrangement is that the primary load4is powered at the expense of starter battery2. If the charging path via diode11is inhibited in normal operation starter battery2then powers primary load4and can thereby be discharged. If the charging path via diode11is enabled, then on the one hand the charging voltage is again reduced by a value equal to the voltage drop through diode11, and on the other hand the charging current is distributed to starter battery2and primary load4, thus degrading the charging of starter battery2in normal operation.

To avoid such discharging of starter battery2, a changeover switch12is associated with primary load4so that primary load4can be switched over between vehicle electrical system battery6and starter battery2, as shown in FIG.3. In conventional configurations, primary load4would be powered via vehicle electrical system battery6. If vehicle electrical system battery6is discharged, however, then primary load4is connected via changeover switch12to starter battery2. A disadvantage of this arrangement is the additional changeover switch12. Moreover, a measurement shunt (not shown) is necessary in order to sense the switchover conditions and the charging voltage for starter battery2is still reduced by a value equal to the voltage drop at diode11.

A comprehensive solution to the problem is realized by way of the circuit arrangement as shown inFIG. 4, in which diode11has been replaced by a power MOSFET13whose drain terminal is connected to starter battery2, and whose source terminal is connected to primary load4. In addition, a further power MOSFET14is arranged between the source terminal and vehicle electrical system battery6. As manufactured, power MOSFETs13and14have a parasitic diode in the inverse direction, which is shown in the circuit arrangement parallel to the actual transistor section. MOSFETs13and14may possess an internal current measurement system, overcurrent protection, and temperature protection. In normal operation, MOSFET14is made conductive by way of a corresponding gate voltage, and MOSFET13is inhibited. In this operational condition the circuit's operation is similar to that of the circuit shown in FIG.1. If, during operation, the voltage of starter battery2drops below that of vehicle electrical system battery6, a charging current can then flow through the parasitic diodes of MOSFET13, if the voltage difference exceeds approximately 0.7 V. The flow of this charging current can be sensed via the internal current measurement system of the control device15(shown in FIG.5). The control device then generates a gate voltage for MOSFET13, so that the latter becomes conductive and is driven inversely. As a result, the charging current can flow from vehicle electrical system battery6via the source terminal to the drain terminal of MOSFET13and charge the starter battery2. Since the internal resistance of MOSFET13when conductive is approximately 10 mΩ, almost the entire voltage of vehicle electrical system battery6is available as charging voltage. Based on the current profile, the control device can then sense the completion of the charging operation and can again inhibit MOSFET13. If vehicle electrical system battery6is discharged prior to a starting operation, the control device can sense this based on the current profile at MOSFET14or the voltage of vehicle electrical system battery6, and inhibit them. MOSFET13is then enabled, so that primary load4is powered via starter battery2. One problem with the circuit configuration, as shown inFIG. 4, is that in the event of a defective switch10that does not close, a short-circuit current can flow from vehicle electrical system battery6through the parasitic diodes of MOSFETs13and14to the starter battery.

To solve this problem, MOSFETs13and14can be polarized oppositely to one another, as depicted in FIG.5. This is done by connecting the source terminal of MOSFET13to starter battery2. As a result, the parasitic diode of MOSFET13is polarized in the inhibiting direction in the event of a defective switch10and prevents any discharge of vehicle electrical system battery6. Given a corresponding potential at primary load4, it is nevertheless still possible for a current to flow from starter battery2to primary load4and discharge the starter battery. The polarization directions of the two MOSFETs13and14are therefore reversed, as depicted in FIG.6. In the event of a short-circuit, the parasitic diode of MOSFET14is inhibited, and the discharge path of starter battery2is inhibited by the parasitic diode of MOSFET13. As schematically indicated, the two MOSFETs13and14and switch10, can be activated by a common control device15of simple configuration.