I have dug up this from the depths of my accumulated info for those suffering from "Flat Battery Syndrome", just to explain how the various battery management systems work.
BATTERY MONITORING SYSTEM (BMS)
When the ignition is switched off, the BMS control module records the battery state of charge and
begins to monitor the battery condition.
If the battery state of charge falls by 7% from the critical start point, the BMS control module will
start to monitor the battery for 5 minutes and sends a 'Warning' message on the LIN bus to the BCM
/GWM assembly. If the battery charge has continued to fall below 50% due to the high quiescent
drain current after the 5 minute monitoring period, the BMS control module will determine that the
control modules are still 'awake'. The BMS control module sends a 'Shutdown' message on the LIN
bus to the BCM/GWM assembly. The BCM/GWM assembly then transmits a 'Shutdown' message on
both MS and HS CAN buses to all control modules, requesting them to shutdown.
The BMS control module will monitor the battery state of charge for a further 15 minutes and
determine if the battery state of charge is still dropping due to the quiescent drain current. The BMS
control module sends a 'Power Disconnect' signal to the BCM/GWM assembly on the LIN bus. The BCM
/GWM assembly then sends a signal to the QCCM via the LIN bus to open its internal relays. When the
QCCM relays are open, the power supply from the battery to non-critical control modules is removed.
The non-critical control modules are any modules associated with the infotainment system and also
the climate control system.
The use of the LIN bus communication ensures that no other control modules are 'woken' during this
process. If CAN bus communication was used, all modules on the CAN bus would be woken by the
BATTERY MONITORING SYSTEM (BMS) LOW BATTERY WARNING AND ENERGY MANAGEMENT
The BMS continuously monitors the condition of the battery. If excessive battery discharge occurs, the
system will begin to shut down non-essential electrical systems in order to protect the battery.
The BMS displays warning messages to inform the driver that the battery is either at a low level of
charge or the engine-off power consumption limit has been exceeded. There are 3 messages that can
be displayed; two on the Touch Screen (TS) and one on the IC message center.
Low Battery - Please switch engine on or system will shutdown in 3 minutes
This message will be displayed as a warning on the TS if the engine is not running. This indicates that
the battery charge has fallen below a predefined threshold. As soon as the battery is charged above
this threshold, the message will be removed.
Low Battery - Please start your engine
This message will be displayed on the IC message center if the engine is not running. This indicates
that the battery charge has fallen below a predefined threshold. As soon as the battery is charged
above this threshold, the message will be removed or it can be manually removed by pressing the
'OK' switch on the steering wheel switchpack.
System will shut down in 3 minutes
This message will be displayed as an energy management message on the TS if the engine is not
running, and system features are causing excessive battery discharge. After 3 minutes the BCM/GWM
assembly will begin shutting down vehicle systems. Normal system operation will resume when the
engine is started.
These messages are based on a percentage of the battery capacity available for the customer to use
the vehicle systems with the engine off. The percentage can change based upon several factors. Once
activated, the resetting of these messages will not occur until the vehicle is driven for 10 minutes with
the engine running (to allow the battery to replace any lost charge). However, if the engine is run for
less than 10 minutes, the messages will only be displayed after an additional 5 minutes with the
ignition is switched on but the engine is not running.
BMS CONTROL MODULE SELF CALIBRATION
BMS CONTROL MODULE SELF CALIBRATION
The BMS control module periodically initiates a self-calibration routine. To self-calibrate, the battery
monitoring system first charges the battery to its full condition.
When the battery is fully charged, the BMS control module will discharge the battery to approximately
75% of its full state of charge, but never lower than 12.2 V. The time taken to complete this part of
the routine is dependent on the electrical load on the vehicle.
When the second part of the routine has been successfully completed, the BMS control module will
return the battery to its optimum level of charge. The optimum level of charge will be between 12.6 V
and 15 V, depending on battery condition, temperature and loading.
The BMS control module also monitors the battery condition with the engine switched off. If a low
voltage condition is detected, the BMS control module can request the infotainment system is
switched off to protect battery voltage.
VOLTAGE QUALITY MODULE (VQM)
The VQM is a DC-DC converter and produce a constant 12V output voltage from a varying (and lower)
input voltage. During a warm engine restart, significant low voltage transients occur in the vehicle
electrical systems, and customer noticeable loads require the voltage to be boosted by the VQM.
There is also a mention that short journeys will not necessarily charge the battery adequately.
This is also mentioned in another section. I am not sure whether it contributes to battery drain or not but it may.
On vehicles with conventional ignition keys , these must not be left in the ignition lock barrel when
the transit relay has been removed, otherwise quiescent current will increase and the battery will
discharge more rapidly.
For keyless vehicles , the Smart Key must be stored at least 5m (16 ft) away from the vehicle when
the vehicle is parked or stored.
Faults and Technical chat for the Jaguar XE