As a rule, the energy for motor-vehicle electrical systems is supplied by at least one battery and one generator. Under normal conditions, the battery has a sufficient storage and buffer effect to keep the system voltage stable, that is to say, the battery supplies a newly switched-in consumer with energy in the meantime, until the generator is adjusted to a higher power level.
Increasingly, systems such as electrohydraulic braking systems or electrical power steering, for whose operation a stable electrical energy supply is important, are being used in vehicles. In particular, the functioning of safety-relevant systems must be ensured, and must not be jeopardized, for example, by unacceptable voltage dips. At full operativeness of the battery, sufficient buffer effect is present to meet this requirement.
However, if, for example, the battery is in a discharged state before driving begins, and the vehicle is started by an external starting aid, then the buffer effect of the battery is not sufficient. The same also holds true for a weak or defective battery which is able to provide less reserve power.
If, at this point, an electrical consumer is switched in with pronounced starting current, then, because of the relatively large exciter-circuit time constant (approximately 50–100 ms), the generator is only able to react comparatively slowly, and the vehicle system voltage falls briefly below acceptable values. Control units of safety-relevant systems may also thereby fail for a short time. Using an electrical steering or braking system as an example, this represents a very critical state for the controllability of the vehicle. Therefore, the goal must be to achieve rapid stabilization of the voltage.
From the related art, it is known, when the main battery is weak or has failed, to switch in an auxiliary battery until the main battery has been charged or exchanged. A voltage stabilization from some other quarter is therefore not necessary. On the other hand, an additional buffer battery with monitoring electronics and a switching element must further be provided.