Isolation requirements for battery charging from three sources?

Thread Starter


Joined Apr 16, 2010
I have a travel trailer with a 12V house battery bank. When the trailer is plugged into shore power, the battery is charged by the "converter" built into the trailer. When the trailer is connected to the truck, the house battery is charged from the truck alternator. Now, I am adding a solar array that will be connected to an MPPT charge controller that will charge the house battery.

Do I need to consider diode isolating the output of the MPPT charge controller from the house battery to prevent damage to the charger and/or the solar panels? Likewise, do I need to isolate the trailer converter? And what about the output from the truck?

Since the truck was prewired with the tow harness, it seems likely that protection was built into the harness. Likewise, the travel trailer converter is probably protected. And maybe the MPPT charge controller has protection built in?

Has anyone here actually dealt with this issue? If so, please reply with your findings and actions. Thanks.


Joined Nov 30, 2010
I believe your truck alternator has diodes in series with its output. Check
I expect the converter has diodes to create its output. Check
I believe solar panels don't have that protection diode. Please check!

Don't worry. Somebody else will come along in a minute because they have actually installed solar panels.:p


Joined Mar 14, 2008
Are you concerned about the travel trailer discharging your truck battery?
If so, you need some method of disconnecting the trailer from the truck when the truck engine is off.
A relay controlled from the ignition switch accessory output could do that (the accessory output is off when starting thus you won't have starter current trying to go from the trailer batteries to the starter).
I wouldn't assume there's any protection built into the prewired tow harness except perhaps a fuse.

If the charger is connected continuously to the batteries, then it should need no further protection.

I assume the MPPT output is designed to be continuously connected to the batteries, so it also should need no further protection.

Thread Starter


Joined Apr 16, 2010
Okay, I am back on this project. Attached is a diagram of the connection scheme described in post 1. Now, my question is where do I need fuses? And are there any new thoughts on the need for blocking diodes? Thanks.Travel Trailer Charging.JPG