MACS Monthly Newsletter - 2021 - JAN2

BMW & Eberspaecher

Figure 4: Graphic illustrating three different cabin heater configurations.

Figure 2: Illustration showing components of a Chevrolet Bolt
BEV that comprise the typical cabin heating system.

Quarto Technical Services, LLC

designs are very similar. Heaters currently in the field will contain heating elements (one or more) and control circuits.
The HV Distribution Box is also a point in which the HV Battery Charger can connect, that permits battery charging while
powering of other devices (i.e., Cabin Heater) when the high
voltage battery is being charged. However, high voltage components can be independently initialized by the control systems
(i.e., powered ON) while the vehicle is parked (i.e., powered
OFF) to permit specific functions to operate while the vehicle is
being charged. HV Distribution Module (Box) locations are typically very accessible, because the connection points for many
components are centralized for ease of diagnostics and repair
(Figure 5).

January 2021

Quarto Technical Services, LLC

Figure 3: Pictures showing the location of the Chevrolet
Bolt cabin heater.
From the perspective of simplicity of system design, it doesn't
get much simpler than a BEV system. The only moving part in
the passenger compartment heating system (not including the
blower motor and coolant) is the 12V pump. From the perspective of a diagnostician, the BEV system appears to be very simple, that is, until supporting or ancillary systems are considered.
The Cabin Heater in a BEV is an electrically powered Positive
Temperature Coefficient (PTC) device, and it receives this power from the HV Battery Pack. The resistance of a PTC is very low
when cold and increases resistance as temperature is increased
when electrical Current is passed through it. Most BEVs are
mechanized with a HV distribution box/center that permits the
HV battery pack to distribute electrical power to the HV electric
Cabin Heater from a central point of the vehicle. For communications (COM), the HV Battery Pack will use the High-Speed
Controller Area Network (HSCAN) Bus to communicate with
the HV Battery Charger. The HV Cabin Heater will typically be
connected to the Low Speed (LS) CAN Bus or the much slower
Local Interconnect Network (LIN) Bus. The LIN Bus connection is typically via the HVAC controller. The HV Cabin Heater
is not a critical functioning system, therefore its communication
(COM) speed does not warrant a high-speed network speed.
Cabin Heaters can be designed in many different configurations
(Figure 4). Although the configurations may differ, the internal

Figure 5: Picture indicating the location of the Chevrolet
Bolt high voltage distribution module.

From the perspective of an electrical block diagram, the connection of the HV Cabin Heater will typically connect to an HV
Distribution Module/Box, or to a component that serves as an
HV Distribution point in the system. Not shown in the block
diagram is the HV Fuse for the Cabin Heater. That would be
located in the HV Distribution Module/Box or in another HV
component that serves as the power connection point for the
Cabin Heater. See Figure 6.

MACS Service Reports

MACS Monthly Newsletter - 2021

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