Abstract:
The invention relates to an electrical power supply system for feeding electrical loads, having at least two electrical energy stores, a circuit for connecting the electrical energy stores to an electrical d.c. network with loads and at least one power generator, wherein each energy store can be connected to the d.c. network by activating switching elements by means of d.c. transformers.

Description:
RELATED APPLICATION 
   This application claims priority of German Patent Application No. DE 101 44 282.3, filed Sep. 8, 2001. 
   FIELD OF THE INVENTION 
   This invention relates to an electrical power supply system for feeding electrical loads, having at least two electrical energy stores, a circuit for connecting the electrical energy stores (batteries) to an electrical d.c. network with loads and at least one energy generator. Such an electrical power supply system is preferably installed as a vehicle-mounted electrical system in vehicles. 
   BACKGROUND 
   U.S. Pat. No. 4,056,764 describes a power supply system with two batteries which are connected in parallel and which have a different characteristic and a power-limiting circuit which is connected upstream of a battery. 
   U.S. Pat. No. 5,316,868 describes a circuit for electrically connecting a primary battery and a secondary battery to a load and to a control unit which connects said secondary battery into the circuit if additional electrical power is required. 
   EP 0 363 356 B1 discloses a voltage supply system with a d.c./d.c. transformer for electronic devices, in particular, for safety-related devices in motor vehicles in which groups of devices, or each individual device, is assigned a separate supply circuit with an independent auxiliary voltage source which maintains the power supply of the associated electrical device if there is a fault in the supply circuit. 
   EP 0 398 691 B1 discloses a power distribution system in which an electrical connection is formed between a first and a second power source for a period of time as a function of the presence or absence of physical states, for example, a driver getting into a vehicle. 
   U.S. Pat. No. 5,844,325 describes a multiple battery system and its control in which a switch enables the same poles of two batteries to be connected to one another and a second switch permits the supply to auxiliary loads to be interrupted. 
   Publication DE 19855245 A1 describes a redundant voltage supply for electrical loads in a vehicle-mounted electrical system, in particular, for brakes which can be actuated electrically, having a generator which supplies two separate voltage branches which are d.c. isolated by means of a d.c. voltage transformer and can each be connected to their own voltage store and to the electrical loads, switching elements which are normally in the conductive state being located between each battery and the associated voltage branch, and between each battery and the load. 
   DE 19951128 A1 discloses a method for regulating the voltage in a vehicle-mounted electrical system with two batteries which can be charged by a generator and which are used to supply loads and can be connected to one another by means of a voltage transformer of a control device, it being possible to supply the control unit with information which is typical of the engine or vehicle-mounted network and is processed by a microcomputer, and to adapt the charge power for the starter battery in terms of timing to fluctuating power demands of electrical loads and to power outputs of the generator which fluctuate as a function of the engine speed, and to the capacity utilization of the generator. 
   In these known power supply systems with two batteries, it is impossible or possible only to a limited degree, to operate a targeted battery management system to avoid system faults due to inadequate charging and battery failures, and to ensure the emergency operating capability. 
   It would accordingly be advantageous to provide a power supply system having at least two electrical energy stores, which power supply system has improved reliability and availability by virtue of the possibility of battery management and redundancy in the case of a battery failure. 
   SUMMARY OF THE INVENTION 
   This invention relates to an electrical power supply system for feeding electrical loads including at least two electrical energy stores, a circuit connecting the electrical energy stores to an electrical d.c. network having at least one load and at least one power generator, wherein each energy store is connected to the d.c. network by at least one activating switching element, the switching elements being activated by a control unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the new electrical power supply system is schematically represented in  FIG. 1 . 
       FIG. 2  shows a circuit for monitoring the direction of current in the batteries. 
   

   DETAILED DESCRIPTION 
   The power supply system according to the invention permits energy stores to be decoupled from the d.c. network by the intermediate connection of a d.c. transformer. Each battery can preferably be connected directly to the d.c. network by means of a power switch and to the d.c. transformer by means of a load switch. However, it is preferable to use semiconductor switching elements instead of switches. The switching elements are actuated by a control unit in which measured values of physical variables, for example, temperatures, pressure, current strength or voltage level, determined by sensors, are processed and a value of the load state (SOC, state of charge), a value of the readiness for use (SOH, state of health) or other derived variables for an energy store is determined therefrom. However, the targeted switching off of individual loads and interventions which are compatible with requirements into an engine management system are also provided. A particular advantage of the power supply system is that for the first time each individual battery can be charged in a way compatible with requirements independently of the current voltage level. As a result, inadequate charging of the at least two redundantly connected batteries can be largely avoided. 
   In the power supply system it is, inter alia, possible to monitor the load state of the batteries by means of the control unit. If it is detected at the batteries that a value drops below a critical charge state, a multistage charge strategy is started. If, for example, the power drain is too low due to low temperatures, each battery can be connected to the d.c. network by means of the d.c. transformer. Temperature-adapted charging is possible by controlling the variable output voltage. 
   During individual charging, the operational capability of the batteries is checked by checking the level and duration of the power drain. In the case of a defect, the faulty battery is disconnected completely and its prompt replacement is initiated by means of a message. The emergency operating capability is ensured by means of the second battery. 
   For the particularly dangerous case of a spontaneous short circuit of one of the at least two batteries, the direction of current is detected, which can be carried out either by means of a voltage measurement at shunts connected in series with the batteries or by means of Hall probes or by measuring the contact voltage of the power switches, as only the direction information is important for the actuation of the power switches by the control unit. 
   Turning now to the drawings in general and, in particular,  FIG. 1 , it shows how battery  1  can be connected directly by means of a power switch  4 , or indirectly by means of a load switch  6  and the d.c. transformer  3 , to a d.c. network. Battery  2  is connected directly by means of a power switch  5 , or indirectly by means of a load switch  7  and d.c. transformer  3 , to the d.c. network. Generation of power is preferably carried out with a generator  8 . A starter  10 , which is connected via a switch  11  to the d.c. network, is provided for the operation of starting an engine. Further loads  9  are also connected to the system. The d.c. network and the battery management system are controlled by means of a control unit  12 . The lines (dashed) of action which start from the control unit  12  show that targeted influencing of loads  9  and generator  8  enables those of the d.c. network to be influenced to ensure voltage stability and emergency operating properties. 
     FIG. 2  represents the logical operation of a circuit for monitoring the direction of current in batteries  1  and  2  by measuring currents I 4  and I 5  in closed power switches  4  and  5 . Evaluation of the direction of current in the battery paths is sufficient for the conversion into a switching logic. The directions of current are converted using the signal level converters  13  and  14  so that a positive current (discharge) becomes logic “1” and a negative current (charge) becomes logic “0”. The output signals of the converters are fed to one associated AND element  16  and  17  each and to one exclusive OR element  15 . The output signal of the exclusive OR element  15  is fed to the second inputs of the two AND elements  16  and  17 . The output of the AND elements  16  and  17  is connected to, in each case, one trigger  18  and  19  of a power switch  4  and  5  which, in the event of a fault, disconnects the associated battery  1  or  2  from the d.c. network by opening the closed power switch  4  or  5 . 
   In the following table, a simple case of the monitoring of a vehicle-mounted electrical system with two batteries connected in parallel is described by way of example. In columns I 1  and I 2 , a (+) stands for a discharge current of a battery, and a (−) stands for a charge current. In the columns for the power switches  4  and  5 , a “1” indicates that the corresponding switch is switched on, and a “0” indicates that it is switched off. 
   Actuation of the load switches as a function of the direction of current 
   
     
       
             
             
             
             
             
           
         
             
                 
             
             
               Case 
               I 1   
               I 2   
               4 
               5 
             
             
                 
             
           
           
             
               1 
               + 
               + 
               1 
               1 
             
             
               2 
               + 
               − 
               1 
               0 
             
             
               3 
               − 
               + 
               0 
               1 
             
             
               4 
               − 
               − 
               1 
               1 
             
             
                 
             
           
        
       
     
   
   In case 1, both batteries are discharged: the power switches  4  and  5  are closed. In case 2, battery  1  outputs current and battery  2  takes up current. Switch  5  is opened and battery  2  is disconnected. Case 3 shows the other case in which battery  1  takes up current and battery  2  outputs current: switch  4  is opened and battery  1  is disconnected. In case 4, both batteries are charged directly from the d.c. network and the switches remain closed.