Method for monitoring battery cells of a primary on-board electrical system battery, on-board electrical system and motor vehicle

1. A method for monitoring battery cells of a primary on-board electrical system battery of a motor vehicle by galvanostatic impedance spectroscopy, the method comprising:

impressing an AC signal having alternating currents of different frequencies onto the battery cells as an excitation signal,

detecting cell voltage signals of the battery cells as frequency-dependent response signals to the excitation signal,

determining impedance spectra of the battery cells depending on the excitation signal and the response signals, and

dividing the primary on-board electrical system battery into at least two subsections each having at least one battery cell in order to provide the excitation signal, wherein:

the primary on-board electrical system battery is electrically connected to a secondary on-board electrical system battery of the motor vehicle,

two cycles alternating at the different frequencies are carried out, in which at least one of:

two different currents are drawn in alternation from the subsections and a secondary-side constant current formed from the two currents over the cycles is supplied to the secondary on-board electrical system battery, or

a constant current provided on the secondary side by the secondary on-board electrical system battery is provided on the primary side and two different currents formed from the constant current are supplied to the subsections in alternation, and

a differential current formed on the primary side from the two currents is supplied to the battery cells as the excitation signal, wherein the direction of the differential current alternates with the cycles.

2. The method according to claim 1 ,

wherein at least one frequency-dependent characteristic variable of one of the battery cells is determined from the impedance spectrum of the battery cell.

3. The method according to claim 2 ,

wherein at least one of a temperature, a state of charge, or an aging of the battery cell is determined as at least one frequency-dependent characteristic variable of the battery cell.

4. An on-board electrical system comprising:

a primary on-board electrical system battery comprising a plurality of interconnected battery cells,

a secondary on-board electrical system battery,

a vehicle-internal on-board diagnostic device,

a control apparatus, which is configured to impress an AC signal having alternating currents of different frequencies onto the battery cells as an excitation signal, and

a measurement apparatus, which is configured to detect cell voltage signals of the battery cells as frequency-dependent response signals to the excitation signal and to determine impedance spectra of the battery cells depending on the excitation signal and the response signals, wherein:

the primary on-board electrical system battery is divided into at least two subsections each having at least one battery cell and is electrically connected to the secondary on-board electrical system battery via the control apparatus,

the control apparatus is configured to carry out two cycles alternating with the different frequencies,

the control apparatus is further configured to at least one of:

draw two different currents in alternation from the subsections and supply a secondary-side constant current formed from the two currents over the cycles to the secondary on-board electrical system battery, or

provide a constant current provided on the secondary side by the secondary on-board electrical system battery on the primary side and to supply two different currents formed from the constant current to the subsections in alternation, and

the control apparatus is further configured to supply a differential current formed on the primary side from the two currents and the direction of which alternates with the cycles to the battery cells of the primary on-board electrical system battery as the excitation signal.

5. The on-board electrical system according to claim 4 ,

wherein the control apparatus has a DC-DC voltage conversion apparatus, which is electrically connected to the subsections and to the secondary on-board electrical system battery, and the DC-DC voltage conversion apparatus is configured to at least one of:

draw the different currents from the subsections, feed the different currents to the subsections same thereto and feed the constant current to the secondary on-board electrical system battery, or draw the different currents from the subsections.

6. The on-board electrical system according to claim 5 , wherein

the DC-DC voltage conversion apparatus has a first DC-DC voltage converter, the primary side of which is electrically connected to the first subsection, and a second DC-DC voltage converter, the primary side of which is electrically connected to the second subsection, and

the DC-DC voltage converters are electrically interconnectable on the secondary side to one another and to the secondary on-board electrical system battery.

7. The on-board electrical system according to claim 4 ,

wherein the primary on-board electrical system battery has a center tap, via which the primary on-board electrical system battery is divided into the two subsections.

8. The on-board electrical system according to claim 4 , wherein:

the primary on-board electrical system battery is configured as a high-voltage battery for supplying at least one high-voltage component of the motor vehicle, and

the secondary on-board electrical system battery is configured as a low-voltage battery for supplying at least one low-voltage component of the motor vehicle.

9. The on-board electrical system according to claim 4 ,

wherein the primary on-board electrical system battery is configured as a traction battery for driving the motor vehicle.

10. A motor vehicle comprising the on-board electrical system according to claim 4 .