9

Design of the 48-volt electrical system
Schematic diagram

Engine Control Module
J623

Term. 30 Term. 31

Term. 41

Distributor TV66 in the engine compartment
Starter Generator
C29

Power distributor, term. 30

Term. 30

Term. 31

Term. 30

Term. 31
Battery, 12 V
A

Battery Monitoring
Control Module
J367

x
Term. 30

Data Bus on Board Diagnostic Interface
J533

Key:
Hybrid CAN

Discrete signal lines

FlexRay

48-volt DC positive side

LIN bus
Sub-bus systems
12 volts DC

10

48-volt DC negative side
X

To the 12-volt power consumers

Airbag Control Module
J234

Distributor TV 67 on the 48-volt battery

Term. 30

Term. 41

Term. 40

Voltage Converter, 48 V/12 V
A7

Term. 41

Term. 31

Term. 40
Battery, 48 V
A6

664_062

11

Overview of the 48-volt components
Terminal 40 connection in distributor TV3

48-volt positive cable

Starter Generator
C29

Key:
48-volt DC voltage cables
12-volt DC voltage cables

12

Voltage Converter, 48 V/12 V
A7

Battery, 48 V
A6

664_063

13

Starter Generator C29
General description
Starter Generator C29 is a 48-volt belt starter generator. As
a generator, it charges the 48-volt battery and supplies the
voltage converter with 48 volts DC. As an electric motor, it
can be used both as a starter and as a booster for the TFSI
engine.

It is water-cooled and has an electrical water pump. C29
communicates with the engine control module via a
sub-bus connection (private CAN bus) and activates Starter
Generator Coolant Pump V621 by generating a
pulse-width-modulated (PWM) signal.

Engine Control Module
J623

1
2
3
4
5
6
7
8

Terminal 30

Sub-bus

664_065

PWM
Terminal 31

Terminal 41
Starter Generator
Coolant Pump V621

Terminal 40

664_064
664_066

8-pin connector

Activation of Starter Generator Coolant Pump V621
V621 is activated in an engine-specific fashion. It is connected directly to the engine control module. The integration of V621 into the engine cooling system varies from
one engine type to another. In some engines coolant flow
through the starter generator is assisted by the main water
pump, while in others it is not.

In some cases, V621 performs additional tasks within the
engine cooling system. The conditions under which the
starter generator coolant pump is operated are defined in
the engine control module software. The pump can be
activated using the VAS Scan Tool.

Specifications

1)

14

Designation

Starter Generator C29

Address Word

00CC

Communication

Private CAN to engine control module

48-volt positive / negative terminal designations

40/41

Nominal rpm

6000 rpm

Transmission ratio (starter generator /TFSI engine)

2.72 – 3.40 (depending on engine type)

Rated voltage in motor mode

40 volts

Rated voltage in generator mode

51.5 volts

Rated output in motor mode (boost for the TFSI engine, for maximum
5 seconds)

approximately 8 hp

Rated output in generator mode (recuperation 2, for maximum 5 seconds)

approximately 18 hp

Continuous rated output in generator mode

approximately 6 hp

Maximum torque in motor mode

60 Nm

Recuperation: energy recovery, for example, in overrun or under braking, the kinetic energy of the vehicle is transformed into electrical energy.

Connection to the TFSI engine
The Starter Generator C29 and the TFSI engine are connected by a poly V belt. The starter generator places heavy
demands on the belt tensioner as is the driven element in
generator mode, but is the driving element during engine
operation, for example, during start-up.

The tensioning element of the belt tensioner has been
adapted to meet the requirements of each engine and can
be attached directly to the starter generator or to the
engine block. It is important that the belt tensioner provide
the poly V belt with as large an arc of contact as possible
around the starter generator belt pulley.

C29 operating as a generator

C29 operating as a starter

664_067

664_068

Examples of belt tensioner versions
The illustration shows the starter generator of a 3.0l TFSI
engine. The belt tensioner is attached to the engine block.

664_069

Note
No provision had been made for periodic replacement of the poly V belt at the time of preparation of this eSelf-Study
Program. Always refer to the instructions given in the service literature.
15

48 Volt Battery A6
Installation location
The 48 volt battery is installed in the spare tire well. It is a
lithium ion battery. This type of battery has advantages
over lead acid batteries such as higher energy density and
higher deep cycle resistance. However, lithium ion batteries have to be protected from exhaustive discharge cycles
which can cause the battery cells to short circuit.

Mechanical damage can also cause internal short-circuiting
and chemical reactions due to the ingress of atmospheric
moisture. The battery housing is made from plastic. It is
also covered inside the vehicle by a metal enclosure
attached to the vehicle underbody.

48-volt battery housed in protective
enclosure

664_071

48-volt battery in the
spare wheel well

16

Vent line
(vents battery gases)

Battery components

Connectivity

››
››
››
››
››
››

A6 is a hybrid CAN user and can be accessed with the VAS
Scan Tool using Address Word 21. The positive side of the
48-volt DC voltage system is designated "Terminal 40" and
the negative side is designated "Terminal 41".

Plastic battery housing.
Control module (battery management).
13 lithium-ion battery cells.
1 relay.
1 fuse.
Fans.

The control module in the 48-volt battery measures the
voltage and temperature of the individual cells and transfers this data to Data Bus on Board Diagnostic Interface
J533. In addition to this, it activates the internal fans if a
critical storage temperature is reached.

Specifications

Designation

48 Volt Battery A6

Address Word

21

Communication

Hybrid CAN

Positive/negative terminal designations

40/41

Rated voltage in V

48

Capacitance in Ah

9.6

Number of cells

13

Cell voltage in V

3.68

Operating temperature

- 22 to 140 °F (-30 to 60 °C)

Weight

Approximately 22.0 lb (10 kg)

Cooling

Air (active) via fans

664_073

48-volt negative terminal
Terminal 41
Connection for the
battery-integrated control module

Battery-integrated fuse
The fuse is not designed for
replacement
48-volt positive terminal
Terminal 40

Battery-integrated control module
In addition to the 12-volt power supply and the hybrid CAN
communication lines for the control module, signal wires
running from the Airbag Control Module J234 are still
connected to the battery-integrated control module.

In an airbag deployment situation, the power contactor in
the battery A6 is opened and the 48-volt electrical system
is de-energized.

17

Battery design

14-terminal connector
12-volt side

Battery-integrated
control module
(battery management)

Cell assembly: 13 battery cells

Steel base plate

Battery housing
bottom section

Charging, jump-starting and replacement
When the engine is running, the 48-volt battery is charged
directly by the starter generator. When an external 12-volt
charger is used for charging at the 12 volt jump start terminal, the 48-volt battery is charged via the voltage converter. There is no provision for connecting a charger
directly to the 48-volt battery.
18

The 12-volt jump start terminal in distributor TV 3 is available for jump-starting.

Battery-integrated
fuse cover

Battery housing
Top section

Connection plate with relay,
fuse and 48-volt terminal stud

Plug-in module with a single battery cell

Vent valve (vents battery gases)

664_074

The VAS Scan Tool must be used for battery testing (classification). The battery data is read from the built-in control
module of the 48-volt battery and assessed.

If the battery is not installed in the vehicle, the Scan Tool
can be connected to a battery via high-voltage diagnostic
box VAS 5581 and adapter cable VAS 5581/1A.

19

Voltage Converter, 48V/12V A7
Installation location
A7 is installed behind the right-hand luggage compartment
trim above the 12-volt battery,

Like the 12-volt battery, the voltage converter can be
accessed after removing the protective cap.

664_075

Cover

Voltage Converter, 48V/12V
A7

Function
The Voltage Converter, 48 V/12 V A7 is a bidirectional
voltage converter. This means that the 12 volts generated
by the alternator is converted to 48 volts. It is also converted back to 12 volts in order to charge the 12 volt
battery.

In addition, the 12 volts can be converted to 48 volts under
certain conditions. This happens when, for example, an
external charger is connected to the vehicle via the 12-volt
jump start terminals.

Activation of Control Module Cooling Fan V274
V274 is activated by the voltage converter. The three wires
from the 3-pin fan connector lead directly into the 10-pin
connector at the voltage converter. The fan does not run
continuously when the engine is running, but only when the

20

voltage converter requires active cooling. The fan does not
have a run-on function. To check the fan, technicians have
the option of running an actuator output check for the
V274 using the Address Word of the voltage converter.

Specifications

Designation

Voltage converter, 48 V / 12 VA7

Address Word

C4

Communication

Hybrid CAN

Terminal designations 48V positive/negative

40/41

Terminal designations 12 V

30

Power in kW

Approximately 3

Weight

Approximately 5.51 lb (2.5 kg)

Cooling

Air (passive)

Voltage Converter, 48 V/12 V
A7

3-pin connector
Activation of the fan

10-pin connector
12-volt side

Control
Module
Cooling Fan
V274

664_076

Connector, 48 volts positive
Terminal 40
Threaded stud, 48 volts negative
Terminal 41

Threaded stud, 12 volts positive
Terminal 30

21

Connections

The connector for Terminal 40 as well as the two threaded
studs for Terminals 41 and 30 are protected by a cover flap.
To access the terminals, this flap first has to be unlocked
and then opened. The 10-pin connector prevents the cover

flap from being unlocked (moved). The flap cannot be
unlocked until the connector has been disconnected from
the voltage converter. The flap can then be opened, providing access to the terminals.

Unlocking the flap

Opening the flap

3.
2.
1.
664_077

664_078

Operating scenarios
Even if driving represents the "normal" operating state of a vehicle, other scenarios are possible. These scenarios and their
effects on the voltage converter as well as the 48-volt and 12-volt batteries are summarized in the following table.

22

Engine
operation

Terminal 15

External 12-volt charger

Voltage converter