Bosch Motorsport | PowerBox PBX – Wiring Harness Design Recommendations

Technical Bulletin
PowerBox – Wiring Harness Design Recommendations
ABSTRACTThe PowerBox is an intelligent control and distribution unit for theelectric grid in a modern racing car which is seamlessly integrated
into the Bosch Motorsport system architecture. It is capable ofreplacing all conventional relays, fuses and circuit breakers. It
simplifies wiring harnesses and provides diagnostic capabilities.

•Higher ohmic resistance
A smaller diameter of the cable leads to a higher ohmic
resistance and the power loss over the cable increases.

•Higher voltage drop
A higher ohmic resistance will cause a higher voltage
drop across the cable, which results in lower outputvoltage. To compensate the voltage drop the current
must increase to get the same output power. This higher
current through the cable also causes more power loss
in the cable.

•

Higher thermal resistance
Another aspect is the lower thermal conductivity of anundersized cable. The surface of the cable is smaller andtherefore the cable’s ability to transfer heat is reduced.

•Increase of temperature results in increase of ohmicresistance
The three points above show that an undersized cable
will get warmer than a cable that fits the requirements.
This also causes an additional temperature rise,because the temperature coefficient of copper is positive.
That means the resistance of the cable increases when
the temperature rises.

Due to the PowerBox’s high current capabilities, it is necessary totake care of its mounting location in the complete system.

PowerBox PBX 90 and PBX 190

TECHNICAL DETAILS / SPECIFICATIONS
A simplified system overview is shown on the picture on the right.
The input of the PowerBox is connected via main switch to thebattery. The outputs of the PowerBox are used to supply or control
the actuators.
First, let have a look at the wiring harness, especially the input
cable. The following points are equally relevant for output cables
although currents in these cables are lower.
The sum current of all actuator currents flows through the input
cable and also the PowerBox itself. This current is limited to 250A
(200A over connector X2, 50A over connector X4) for PBX 190.
Due to this, high input current it is necessary to choose the correct
wire diameter to minimize voltage drop and power loss of thecable. It is recommended to use 35mm2 cable cross section for
X2 at a high permanent input current. Undersized cables lead toan increase of temperature rising due to some basics of physics:

November 2016

Simplified system architecture

All these facts show that is very important to use the correct cablesize. In the table on the next page some important cableparameters are shown. The resistance of the cable (R) is normallygiven in the datasheet. Pv is the power loss over the cable and ΔT
is the estimated temperature rise of the cable. Imax shows the max.

Bosch MotorsportBosch Motorsport | PowerBox PBX – Mounting Location2|2

current through the cable recommended by Bosch Motorsport(max. temperature rise of cable about 50°C).
RADOX 155 Battery cable parameters
Cablesize

R (@20°C)

Pv @200A(1m cable)

ΔT@PVImax( ΔT=50°C)

16mm2

1.16 mΩ/m46.4W

183 °C

100A

25 mm20.74 mΩ/m

29.7W

75 °C

150A

35 mm20.53 mΩ/m

21.1W

38 °C

220A

As you can see in table on next page the temperature rise of thecable is strongly dependent on its cable diameter, because athicker cable has a smaller wire resistance and a better thermalconductivity.
In the graph bellow the resistance of a 35mm2 cable overtemperature is shown. You can see that at 130°C the resistanceof the 35mm2 cable is the same as of an 25mm2 cable at 20°C
(table above shows R @20°C = mΩ/m for 25mm2 cable).

The conductivity of the housing is very good and also the housing
has a high thermal capacitance, but it is the costumers own
responsibility to use a mounting location where the housing cantransfer the heat to the environment efficiently. An average air
speed of 2m/s is recommended for 250A total input current.

HINTS / SOLUTIONS
Some hints to avoid a thermal incident of the PowerBox and tohave an efficient and robust operation:•Use correct cables for input and output. Be sure thatthe resistance of the cable is low enough, also rated
over the peak temperature which could be reachedduring operation.

•Especially the input cable should be as short as
possible, because a high current flows through thiscable. A longer cable causes a higher voltage drop andtherefore higher power losses.

•A mounting location in a colder area of the vehicle isrecommended for the PowerBox and also for the wire
harness. This minimizes the power loss and provides
the possibility to use thinner cables if cooling issufficient to maintain a low temperature.

•For high total output current it is necessary to have an
airflow over the housing of the PowerBox. Check that
the PowerBox is mounted correctly so that the air flows
in parallel to the fins. Otherwise the effect of the airflow
can be reduced.

•Use the feature of linking outputs for high output
currents to reduce power loss in the PowerBox.

•Spread the outputs and use power output stages on
different connectors to minimize hotspots on the PCB.

Resistance 35mm2 cable over Temperature

Resistance in mΩ/m

0,80,750,70,650,60,550,5050

100150Temperature in °C

Now let’s have a look at the PowerBox itself. The PowerBox hasa power loss due to connector pin resistance, internal wiring andthe resistance of the output power stages. To decrease thisresistance it is helpful at higher currents to link two output stages
rather than leaving it unused in the car. This reduces theresistance of pin, wiring and output power stages by half. It is also
helpful to use power outputs on all connectors to spread thepower losses over the whole device and to minimize thermal
hotspots.

Sender / imprint
Bosch Engineering GmbH
Motorsport
Robert-Bosch-Allee 174232 Abstattwww.bosch-motorsport.com
motorsport@bosch.com

All these resistances inside the PowerBox are also temperaturedependent and increase with a temperature rise. For example theresistance of the output power stages will be nearly doubled whenjunction temperature reaches 150°C instead of 25°C.

November 2016

Bosch Motorsport