2021 P400. Has anyone measured the output of the positive pin, voltage and current, at the trailer connector. Trying to determine if I need to use a dc to dc charger to charge RV lithium battery bank while in motion.

 

I have an Orion DC/DC charger for a lithium bank in my trailer. First time I plugged it in it blew the fuse that was on the supply line for the 7 wire plug. As I recall it was quite undersized, perhaps 10amp? The spec for that connector for charging the battery required for trailer brakes varies but in general 25-30amp is normal.

I am not sure what Landrover uses for the wire gauge but in my annoyance I put a 25amp fuse in (tempted by 30a) and haven’t had any issues since.

Why do I bring this all up? It’s because what I noticed was that the Defender lets the voltage drop a lot, sometimes down under 13v at the Orion, probably related to fuel efficiency and idling the alternator. When this happens the Orion just pulls harder to get the proper (14.2v in my case) output which is what will blow your fuse. My Orion is rated at 18a fwiw.

 

If trailer plug has same output as lighter socket which i believe it has, I've got news for you, voltage is fluctuating from 12.4 to 14.5 easily. I have voltage readout plugged in, I see voltage readout all the time.
However I believe when it properly detects trailer it goes steady at 14.4V

 

It's not voltage but amperage you need to worry about. Trailer harness wiring cannot support extended periods of higher amperage current at 12v. On my camping trailer setup I use the 12v hot pin from the trailer harness to trip a relay that engages the DC charger connected with a 6ga wire and Anderson connector.

 

The Lithium battery bank on our Kimberley requires a separate charging connection. The small single wire from the 7 pin connector just won’t cut it. What was recommended by Kimberley and what we have installed is listed below. It works great. Of note, we relocated the 4pin, 7pin, and additional Andersen connection noted below, up from the factory locations for our convenience.

Charging wire runs from the battery below the passenger seat (North America) to the rear cargo area (breaker installation) and back to the rear bumper. As we do not have the extra rear seats in our 110x, ours runs to a resettable breaker installed in the rear storage area below the removable foam storage trays. I'd prefer somewhere more convenient but c'est la vie; it does it's job and is reasonably convenient/accessible.

Bits and Bobs In general: 6 Gauge multi strand copper wire + 50amp Andersen Connector + 50amp resettable breaker + lugs, lock washers, etc for connections. Links below.

Our DC to DC convertor is installed trailer side (part of trailer systems design) as pictured below and handles the charging management for the trailer battery bank from the defender.

Hope this helps
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6 gauge Wire
Andersen SB50 gray Connector.

50 AMP Resettable breaker

50 AMP Resettable breaker

5/16” Lugs for breaker connection
Assorted Andersen flush mount, plugs/bungs, cable end covers, etc

 

The wiring charts and information here is a good reference. Remember that the standard in that 7 wire plug was designed to charge a lead acid breakaway battery (required for all electrically braked trailers). This is why those plugs use 12 or 10 gauge wires which are rated (see charts above) for a 25-30amp draw.

This is another reason why a DC/DC charger is needed because a lithium bank will pull far more current than these wires can support. The Victron Orion box limits to 18amp maximum which is why I use it. This application is perfect for it.

While you could use an Anderson connector and push significant power this is not a general purpose design as most tow vehicles will not have this support. Conversely a design that stays within the specifications of the 7 wire plug will work with any vehicle. It won’t place an unexpectedly high load on the vehicle charging system causing it to fail prematurely or need a special high amp auxiliary alternator added. One of the common problems I see on forums is that while it is possible to go over the top and build a large and expensive solution, often the users would be better served with a streamlined setup.