Will a Battery Isolator relay also protect the Tow vehicle alternator?

[Rob Culver]

Originally Posted by rickst29

You don't need that battery isolator relay at all, you DO need a DC->DC charger.


- - -
So, you need a device ("DC->DC battery charger" to pull low voltage (from the engine compartment), only while the engine is actually running, and "boost it up" higher near the LFP batteries when those batteries aren't full. The basic Renogy 20A DC->DC battery charger https://www.renogy.com/12v-20a-dc-to...ttery-charger/is cheap and usually adequate, the best one (Victron Orion "Smart" 30A) https://battlebornbatteries.com/prod...dc-dc-charger/ costs more than 2x as much, but I recommend the Orion.

I'm toying with the idea of putting in a DC to DC charger like the non isolated Orion one that Rick suggested. But is there any possible way to put this charger on the TM side using the existing 12 volt input from the 7 pin connector as the input in to the charger? That seems tricky due to the need to supply 12 volts to the break away line. Has anyone succeeded in putting their DC to DC charger on the TM side rather than the tow vehicle?
Thanks in advance..

[rickst29]

Originally Posted by Rob Culver

I'm toying with the idea of putting in a DC to DC charger like the non isolated Orion one that Rick suggested. But is there any possible way to put this charger on the TM side using the existing 12 volt input from the 7 pin connector as the input in to the charger? That seems tricky due to the need to supply 12 volts to the break away line. Has anyone succeeded in putting their DC to DC charger on the TM side rather than the tow vehicle?
Thanks in advance..

That charger should be installed in the TM, and preferably close to the TM batteries (electrically speaking). The Trailer battery charge wire has a 3-way junction following the break-out of individual wires from the large bargman cable end (behind the fridge). One end leads to the 12v fuse board. In original wiring, the battery cable is also connected on that board. The other end leads to the break-away switch, allowing emergency brakes to be powered by either of those 12v sources (the TM batteries OR the bargman 12v "TBC").

But that wiring allows backflow when TBC side is lower voltage than the boosted DC->DC output voltage. To solve that new issue, You should add a 5-way relay, favoring TM battery power when battery power is present, and switching to connect the bargman TBC directly if/when battery power fails. output common goes through the break-away switch.

[Rob Culver]

Wow! Thanks for that comprehensive answer. I take it you implemented that exact solution in your TM? Do you have a specific part you can recommend for this 5 way relay? And does that mount in the area behind the refrigerator I presume? Thanks much...

[rickst29]

Originally Posted by Rob Culver

Wow! Thanks for that comprehensive answer. I take it you implemented that exact solution in your TM? Do you have a specific part you can recommend for this 5 way relay? And does that mount in the area behind the refrigerator I presume? Thanks much...

My own TM has something much more complicated and expensive (which you shouldn't try to duplicate).

A 5-pin "Bosch style" 12v relay can be bought at any car parts store, or Amazon, or eBay) but you also need a socket (converting the Relay "pins" into easily connected wires. Her'es an Amazon listing for both items:
https://www.amazon.com/Augeny-Car-Re.../dp/B0CNLBW59N

The Relay can go behind the fridge. After breaking the 3-way junction of the TBC wire, the wire which goes down, out and forwards to the breakaway switch should be connected to relay pin 30 (power "common", the fat wire with light orange insulation in the harness from Amazon).

The thin TBC lead from the bargman cable needs a new "splitter" junction. One downstream wire connects to relay pin 87. That pin is connected with pin 30 when battery power fails, and the relay coil magnet loses power. The other "downstream wire" is input 12V to the DC->DC charger.

Because the back of the refrigerator gets very hot, I would not put the DC->DC battery charger in that location. So that second "downstream wire", which is new, goes somewhere else to reach the DC->DC charger unit. (maybe under the sink, or near the WFCO, or maybe alt the way back to nearly reach the batteries). Near the batteries is best. This wire does not connect the WFCO 12v fuse board anymore, it connects only as "input" to the the DC->DC boosting charger unit.

The "+12V output" of the DC-to-DC boosting charger should go a short distance, reaching a 12V battery connection. (it's usually operating at more than 12v, of course.)

This leaves one unused and loose wire end behind the fridge. (It used to go from the WFCO 12v fuse board into the old TBC junction.) it should now be connected into another "spliiter" junction, connecting both pin 87A (the big blue wire on Amazon's socket) and pin 85 (the thin yellow-insulated "12v" wire for the coil.

The thin white wire of the Relay base (the coil "ground") be be interconnected to any other 12v grounding wire or grounding lug behind the fridge.

[Rob Culver]

Originally Posted by rickst29

My own TM has something much more complicated and expensive (which you shouldn't try to duplicate).

A 5-pin "Bosch style" 12v relay can be bought at any car parts store, or Amazon, or eBay) but you also need a socket (converting the Relay "pins" into easily connected wires. Her'es an Amazon listing for both items:
https://www.amazon.com/Augeny-Car-Re.../dp/B0CNLBW59N

The Relay can go behind the fridge. After breaking the 3-way junction of the TBC wire, the wire which goes down, out and forwards to the breakaway switch should be connected to relay pin 30 (power "common", the fat wire with light orange insulation in the harness from Amazon).

The thin TBC lead from the bargman cable needs a new "splitter" junction. One downstream wire connects to relay pin 87. That pin is connected with pin 30 when battery power fails, and the relay coil magnet loses power. The other "downstream wire" is input 12V to the DC->DC charger.

Because the back of the refrigerator gets very hot, I would not put the DC->DC battery charger in that location. So that second "downstream wire", which is new, goes somewhere else to reach the DC->DC charger unit. (maybe under the sink, or near the WFCO, or maybe alt the way back to nearly reach the batteries). Near the batteries is best. This wire does not connect the WFCO 12v fuse board anymore, it connects only as "input" to the the DC->DC boosting charger unit.

The "+12V output" of the DC-to-DC boosting charger should go a short distance, reaching a 12V battery connection. (it's usually operating at more than 12v, of course.)

This leaves one unused and loose wire end behind the fridge. (It used to go from the WFCO 12v fuse board into the old TBC junction.) it should now be connected into another "spliiter" junction, connecting both pin 87A (the big blue wire on Amazon's socket) and pin 85 (the thin yellow-insulated "12v" wire for the coil.

The thin white wire of the Relay base (the coil "ground") be be interconnected to any other 12v grounding wire or grounding lug behind the fridge.

Thanks very much Rick for those excellent instructions. Looks relatively straight forward as long as you can be sure which wire is which and can run the new wire perhaps all the way back to the battery compartment. Thanks again. Much appreciated!