Ford Trucks BMS Battery Monitoring Systems

https://www.f150forum.com/f38/replacing-battery-new-way-231938/

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"I've been changing batteries for 40 years, I know what I'm doing! I don't need somebody to tell me how to change a battery."

Ok, you still change the battery the same way, but if your 2011 or newer F150 has this or something similar on the negative battery cable then you have what Ford calls a Battery Monitoring System (BMS).Starting in 2011, Ford started putting this on the trucks. It monitors the state of charge of the battery and can power systems (displays, htd seats, etc) down to conserve power, and usually gives a message on one of the displays when it does. This is only one reason that you may get that message, so if you have not replaced the batteryand get this message, there is probably something else wrong. Whenever you replace the battery or recharge it externally, the BMS has to learn the new state of charge of the battery. It takes 8uninterrupted hours for it to do so. So, after you connect the battery, shut the doors and hood, you let it sit, uninterrupted for 8 hours, do not drive it, use the remote or anything else that changes the battery voltage or you could have issues. They probably won't happen right away due to you having a new battery. Reading some of the posts with replacing batteries and then having dead batteries out of the blue, makes me think the BMS may be trying to energize the alternator (I know, truck is off) and charge the battery. Crazy, but who knows what it will do. The ones after battery replacement that have systems powering down, this could be why. The BMS has to learn the new battery for 8 continuous hours. Again, not all the trucks have it, look and see. If you have it and replace the battery at the dealer, they should be able to reset it with their computer so you don't have to wait 8 hours. Not sure all dealers know about it, so make sure to ask them.

Here is what the owners manual says.

Battery Management System (If Equipped)

The battery management system (BMS) monitors battery conditions
and takes actions to extend battery life. If excessive battery drain is
detected, the system may temporarily disable certain electrical features
to protect the battery. Those electrical accessories affected include the
rear defrost, heated/cooled seats, climate control fan, heated steering
wheel, audio and navigation system. A message may be shown in the
information displays to alert the driver that battery protection actions
are active. These messages are only for notification that an action is
taking place, and not intended to indicate an electrical problem or that
the battery requires replacement.

Electrical accessory installation
To ensure proper operation of the BMS, any electrical devices that are
added to the vehicle should not have their ground connection made
directly at the negative battery post. A connection at the negative
battery post can cause inaccurate measurements of the battery
condition and potential incorrect system operation.

Note: Electrical or electronic accessories added to the vehicle by the
dealer or the owner may adversely affect battery performance and
durability, and may also affect the performance of other electrical
systems in the vehicle.

When a battery replacement is required, the battery should only be
replaced with a Ford recommended replacement battery which matches
the electrical requirements of the vehicle. After battery replacement,
or in some cases after charging the battery with the external charger,
the BMS requires eight hours of vehicle sleep time (key off with doors
closed) to relearn the new battery state of charge. Prior to relearning the
state of charge, the BMS may disable electrical features (to protect the
battery) earlier than normal.

This is from a workshop manual

Charging System

The charging system is a negative ground system consisting of:
• a generator with an internal voltage regulator
• a charging system warning indicator
• a battery
• circuitry and cables
• a PCM
• a radial arm adapter (serviced separately from the generator)
• a battery current sensor (this is what's in the picture)
• a generator current sensor

The generator is driven by the accessory drive belt. When the engine is started, the generator begins to generate AC voltage which is internally converted to DC voltage. The DC voltage is controlled by the voltage regulator (located on the rear of the generator) and supplied to the battery. The PCM controls the voltage regulation set point, working with the generator internal voltage regulator over 2 control and communication circuits.

This vehicle is equipped with load shed strategy. The Body Control Module (BCM) monitors the battery state of charge using the battery current sensor
attached to the negative battery cable and the battery open circuit voltage as measured by the BCM during 8 continuous hours of vehicle sleep time (ignition off with doors closed).

With the engine off and the ignition in the ACC, RUN or delayed accessory position, when the BCM determines the battery state of charge is low, a message is sent to shut down the audio/navigation system in order to save the remaining battery charge. Under this condition:

• the Front Display Interface Module (FDIM) displays SYS OFF TO SAVE BATT (without navigation) or BATTERY SAVER — SYSTEM OFF
PLEASE START THE ENGINE (with navigation).
• the Instrument Panel Cluster (IPC) message center may also display TURN POWER OFF TO SAVE BATT (base message center) or TURN POWER
OFF TO SAVE BATTERY (optional message center) to notify the driver that battery protection actions are active.
• the battery indicator may also illuminate.
If the vehicle battery has been charged or battery replaced, it takes approximately 8 hours for the BCM to learn the new battery state of charge. During this 8 hour period, the vehicle must be undisturbed, with no doors opened or keyless entry buttons pressed. If the vehicle is used before the BCM is allowed to learn the new battery state of charge, engine off load shedding may occur earlier than normal and a message may be displayed.
If the vehicle has been jump started, engine off load shedding can still occur and a message may be displayed until the BCM determines the battery state
of charge is above 40%.

With the engine running, when the BCM and Power Steering Control Module (PSCM) voltage is low, a message is sent from the BCM to either minimize or
shut down the climate controlled seats, rear defrost, heated mirrors and Dual Automatic Temperature Control (DATC) blower motor to improve system
voltage. Under this condition, the IPC message center displays either LOW BATTERY LESS FEATURES (base message center) or LOW BATTERY
FEATURES TEMPORARILY TURNED OFF (optional message center) to notify the driver that battery protection actions are active.

The BCM gets its info from the BMS. 
Alright, I think that about does it. Sorry for being long, but it is new to me so I thought I'd share what I found out, this is about all I can tell you about it. Not here to start a war about is it good or bad, I think it's crazy, but others may like it. There are several threads here with similar issues as described here, this may or may not be their problem, but something to look into.