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Hi my name is Robert....
Normally you would have a battery isolator or solenoid that allows the coach batteries to charge. On some set ups the solenoid or isolator will open up at a set voltage and allow the house battery to charge, when the voltage gets to a set point on the house battery it switches back to the chassis battery...
When this voltage goes above 13.3 volts for approximately 1 minute, as happens when the engine is running normally (normal alternator output voltage of a cold engine is approximately 14.4 volts), it will close the isolator relay providing charging current to the coach battery. This delay allows a cold engine an opportunity to start and warm up before having the heavy load of a discharged coach battery placed on it. If the voltage should fall below 12.1 volts for more than about 1 minute, the relay will drop out to feed all the alternators available output to the chassis battery to keep the engine running. This might happen when the alternator is not able to supply sufficient current to all of the loads and charge the coach battery at the same time. When the chassis voltage goes above 13.3 volts again, the relay will again close in about 1 minute to retry to charge the coach battery.
Not all RV's are able to charge the house battery going down the road, IF you are at 12.3 volts and it is not dropping off then I would keep an eye on it, IF the voltage keeps dropping and gets below 11.5 volts then there could be a problem. To find the solenoid or isolator follow the cable from the alternator to the isolator, normally you will have input from the altenator in the center, then on each side output to the chassis and house battery.
Hope this helps some, IF you have more questions or concerns please feel free to contact me.... Robert
Many times they will use the boost solenoid for the charging, so if that is not working then it could be the entire problem. When you hit the boost switch it will tie the house and chassis battery together so you can start the engine, many times you can hear the solenoid click, if you can not find it by sound follow the battery cables and you will see a cable going to both the chassis and house battery, it could be near the battery or inside a battery control box. Check for voltage across the solenoid when you hit the boost switch, it should go up, if not it is bad. If you see a battery control box with fuses, breakers and the solenoids inside please get me the make and model of that box.
Getting a wiring diagram can be very difficult, what is the Make and model number of the RV?
Sounds good and get me the make and model of the RV, will see if I have any prints...
Hi was just checking in to see if my answer helped????? IF you have more questions or concerns please feel free to contact me.... Robert