Automotive Engineering - June 2023 - 30

A connected approach to
AUXILIARY
BATTERY
CONTROL
Fig. 3: Flowchart
of the proposed
strategy.
Fig. 2: A high-level illustration of the proposed strategy.
percent of APU load over its maximum load. It is observed that the
maximum efficiency is 92% between 50% and 60% of the maximum
APU load. Therefore, the operating range of the APU is selected so it
works at its maximum efficiency whenever possible.
For instance, if the sum of the loads requires the APU to operate
above 60% of its maximum load, the low priority loads will be temporarily
turned off to allow the APU to charge the auxiliary battery
while operating at its maximum efficiency. If the sum of the loads is
less than the current provided by the APU, the APU will also work
at its most efficient operating point. As a trade-off, the charging
time is extended.
Once the SoC of the auxiliary battery reaches approximately 95%,
the APU turns off. The SoC high is selected to be less than 100% because
the APU works in constant voltage mode. Thus, the APU load
is reduced as the auxiliary battery gets fully charged. To avoid operating
at low efficiency, the SoC high is selected at the point where
the APU load hasn't dropped dramatically.
Low SoC detection for key-off use case
In a vehicle key-off use case, the low SoC condition can result from
the vehicle being parked for a long time or some fault of the continuous
loads. Also, the low SoC condition can happen if some of the
ECUs don't 'fall asleep' after completing OBD tests. Whether an ECU
falls asleep can be determined by its communication status.
Unlike the low SoC detection for the key-on use case, the detection
for the key-off use case begins once the vehicle shuts down. As illustrated
in Fig. 3, the PCM may get a message from the cloud indicat30
June 2023
ing how many days the vehicle will be left in park. The
park duration may get updated later if the owner of
the vehicle changes his or her plan. With the park duration
and the current SoC, the PCM determines if the
energy left in the auxiliary battery is enough to get the
vehicle ready to drive when the owner returns. If not,
the PCM continues monitoring CAN signals such as
auxiliary battery SoC and voltage from the ABSM.
As the auxiliary loads draw power from the auxiliary
battery, the SoC decreases over time. Once the SoC
drops below the SoC low threshold (e.g., 10%), the low
SoC condition is determined. Otherwise, the PCM continues
monitoring these CAN signals.
AUTOMOTIVE ENGINEERING
BOTH IMAGES: KUMAR/FORD

Automotive Engineering - June 2023

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