House Battery Charging

This relates to a 2010 Thor Class C motorhome with a Ford E350 chassis.
I had noticed that on checking the house battery voltage after a driving a few hours it was only 12.5v.  It should be 12.7v for a fully charged battery. And it seemed to vary.  I checked the voltage after starting the engine and it did not change to indicate charging.  After tracing out the charging equipment and checking the manufacturers website I found this is normal operation for the Battery Isolation Manager installed.  The BIM controls a solenoid that connects the engine and house batteries together so they both recieve charge current from the altenator.  It is made by Precision Circuits Inc, part number 00-10028-200, and here is the description of how it works.
  • Overview:
    The Battery Isolation Manager (BIM) monitors the Battery
    Voltage of both the Chassis and Coach Batteries over long
    periods of time. If it senses a charging voltage, it connects
    the two batteries together. If the charging system is
    drastically overburdened, the batteries will be isolated,
    however, if the BIM sees a long term charging of both
    batteries it will allow the batteries to remain connected and
    allow the charging system to do its job. Once the batteries
    have reached a Float Charge state for one hour, the BIM
    will isolate the batteries to prevent overcharging, and will
    only reconnect the batteries for charging if one of the
    Battery drops to approximately 80% charge, and the other
    is being charged. If the batteries are not being charged,
    BIM isolates the two batteries to prevent an electrical draw
    in one system from depleting the other battery. The long
    term monitoring of the batteries prevents the annoying
    Relay clicking that exists in simpler Isolation Modules today.
Detailed Operation:
1) Relay is turned on if:
a) Ignition is on for 20 seconds &
i) 2 minutes have passed since Relay last turned off &
Coach Battery voltage low and needs charging
Chassis Battery has a charging voltage &
Chassis Battery is Less than 15.5V &
Generator is off
b) Ignition is off &
i) 10 minutes have passed since Relay last turned off &
Chassis Battery voltage low and needs charging &
Coach Battery has a charging voltage &
Coach Battery is less than 15.5V
2) Relay is turned off if:
a) Ignition goes from on to off state
b) Relay has been on for 1 hour
(prevent overcharging and allow to view separate voltages)
c) Anytime Ignition and Generator are both on.
d) Anytime either Battery goes above 15.5 volts for 30 seconds.
So it will change the battery for 1 hour, then disconnect the solenoid to check the voltage and if it passes it will leave it disconnected.  10 minutes later it will check the voltage again, and if its  12.5v (80%) or more, will leave it disconnected.  So it’s possible to arrive at a destination after several hours an only have a 80% charged battery.
FYI the BIM and Trombetta solenoid are located in the engine bay, drivers side, behind the radiator expansion tank.  You can find the solenoid by getting someone to flick the emergency start switch and follow the sound.  You can verify the part numbers installed on your vehicle.
i came across some blogs with battery charging issues that had faulty solenoids, the Trombetta being quoted as a failure point.  It’s easy to test, just hold down the energency start switch and check the voltage on the house battery increases as the engine battery charges it.  (assuming the engine battery is 12.7v or more)

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