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My Truck Didn't Charge My Trailer Battery!

Why Your Trailer Doesn’t Charge From Your 7 way Plug

I often get asked why the trailer coach batteries don’t seem to charge much while towing down the road. After all, the standard 7-way trailer plug has a 12V pin that is supposed to power the trailer while towing. If the trailer has LiFePO4 (lithium) batteries, the problem is even worse, as they require a slightly higher voltage to charge properly. The solution is to use a Victron Orion DC-DC converter to give the trailer batteries the proper charging voltage.

There are a couple of reasons that the 12V pin on the 7-way trailer plug doesn’t do much to charge the trailer battery...

1) In most cases, the factory wiring to the 7-way plug is too small to carry much current. Even though there is often a 20 or 30A fuse to the 7-way plug, the wire drops so much voltage that there is not enough left to charge a trailer battery

2) Many modern tow vehicles now have a “smart alternator”. In the good (bad?) old days, alternators made 14.4V and stayed there all the time. The “smart alternator” quickly charges the tow vehicle battery, then drops the voltage to 14.0 or even 13.6V to save energy.

3) Some European vehicles have a “really smart alternator” that actually reduces the voltage even lower to save fuel, then uses the alternator only to charge the battery when going down a hill or braking. The electric car people know this as regenerative braking. American trucks aren’t doing this yet (that I know of).

To illustrate the problem, I came up with a little experiment with my 2016 Ram 3500 diesel truck. I built a custom 7-way plug that gave me direct access to the 12V line. Then I built a variable load for the 12V line that allowed me to control the current and measure the voltage on the 12V line. Then I measured the voltage and current at the 7- way plug with different loads. Finally, I repeated the measurements but with the Victron Orion DC-DC boost converter installed.

Here is the equipment I used for my testing.

2) Custom 7-way plug (I also use this to power my 12V Fridge/Freezer)

3) Variable Power Resistor for fine load adjustments

4) 300W Inverter as a big 12V load to drive the fans

5) Two 120V fans as the main load and to cool the variable resistor

I used the inverter (4) and the fans (5) as the basic 6A load since the variable resistor (3) could not handle enough power. The variable resistor brought the load up to 9A, which is the Victron limit.

The first thing I did was measure the voltage and current at the 7-way plug with the engine off. The Ram ALWAYS has power applied to the 7-way plug. This means your truck starting batteries are ALWAYS connected to your trailer. So if you pull the trailer batteries down while camping, it will also drain your truck engine starting batteries. THIS IS A BAD THING. Always unplug your trailer from your truck while camping, so that the truck will actually start when you want to move on. The really good news is, the Victron has an ignition detection input that detects when your ignition key is on, so it automatically disconnects your truck from your trailer when the engine is not running.

With the engine off, the readings at the 7-way plug:

Volts Amps

12.5V 0

11.4V 6A

10.6V 9A

So you can see that even a modest load of 9A causes the voltage at the 7-way plug to drop to 10.6V. If your lead-acid battery was at 10.6V, it is considered completely dead. Lead-acid batteries require at least 14.0V to charge at a decent rate, and lithium batteries actually like at least 14.4V. So the 7-way plug is not charging anything at this point.

A better measurement is taken with the engine on, with the alternator providing charge and a voltage boost.

With the engine on, the readings at the 7-way plug:

Volts Amps

14.0V 0A

12.9V 6A

12.4V 9A

So you can see that even with the engine on and the alternator providing charge, the voltage at the 7-way plug is not enough to charge your lead-acid trailer battery. Not even close to enough voltage to charge a lithium trailer battery. Also, when the Ram “smart alternator” detects higher temperature, it lowers the voltage down from 14.0 to 13.6 or lower, making the problem worse.

So here is the solution. Victron makes the Orion series of DC-DC converters that will boost the voltage from the 7 way plug to the proper levels to charge either lead-acid batteries or lithium batteries. The beauty of the Victron Orion is that it limits the current to the trailer at 9A. This is low enough that it will not blow the fuses in your tow vehicle. When the Victron boosts the voltage, it requires the 7-way plug to supply extra current. For example, for the Victron to supply 9A to your trailer batteries, it has to pull 12A from the 7-way plug. Most 7- way plugs are fused for 20-30A, so this is not a problem.

Here is the voltage and current data at the 7-way plug and at the output of the Victron booster, with the engine running and the alternator charging...

7-way plug Victron output

Volts Amps Volts Amps

13.9V 0A 14.6V 0A

12.3V 8A 14.6V 6A

11.8V 12A 14.6V 9A

So you see at zero current, the Victron boosts the voltage to 14.6 volts. This voltage is adjustable, and I set it to be 14.6V because that is what my Battle Born lithium batteries like. When the Victron is supplying 6A to the trailer batteries, it is still at 14.6V. But you see the 7- way plug has dropped to 12.3V, which, without the Victron voltage boost, would not charge the trailer batteries at all. The Victron maximum current is 9A, when I got slightly above 9A, it started to reduce the output voltage to prevent the tow vehicle fuse from blowing. This is an excellent safety and convenience feature. The 7-way plug has to supply 12A and drops to 11.8V when it supplies the Victron at its maximum limit. This is well within the truck’s fuse and wiring limits.

Some people still complain at this limit because their tow vehicle has a 220A alternator and why can’t they put hundreds of amps into their trailer? Mainly, the truck requires a lot of that current for itself, and you are only allowed to pull so much current from the truck, usually 50-100A (see the Ram Upfitters Guide). In any case, you can pull a lot out of the 220A alternator, but you need to re-wire your truck and trailer wires to the 7 way plug. In some cases, I have done that and used up to a 50A booster for the trailer. But the purpose of this blog is to use the existing truck and trailer wiring for simplicity. The Victron Orion-TR 12/12-9 is compatible with existing wiring and allows for a very simple and low cost upgrade to boost your trailer charging capability.

At Leisure Solar, we have upfitted many RVs, Sprinter Vans and trailers. We understand the limits and capabilities of every vehicle and off-grid install we do and design your upgrade the right way. Call us to discuss your power upgrade.


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