Charging Trailer Batteries From the Tow Vehicle Alternator

Dave and Helen Damouth www.damouth.org

23 August, 2001

A frequent question in RV circles concerns the possibility of using the tow vehicle's engine and electrical system to recharge trailer batteries.  (A similar issue exists in motorhomes, with significant differences because of the much shorter path between alternator and house batteries).

A typical tow vehicle, for example my 1996 Dodge RAM 2500 HD, has a 130 amp alternator.  Normal electrical use in the truck is much less, leaving plenty of excess capacity for charging trailer batteries.  But in fact, I found that I got only about 5 amps of battery charging when my trailer batteries were low, and less than that when the batteries were charged to above about 70% of capacity.

The poor charging is due to the voltage drop in the long run of skinny wire from the truck alternator back to the trailer battery (probably about 25 feet).

One solution to the voltage drop problem is to install heavier wire.    Another solution is to install dual alternators, or a single alternator with dual outputs - the second output is used with a separate voltage regulator to efficiently charge the trailer batteries.  I'll discuss both options:

A  Heavier Wiring:  I chose this solution, and used 64 feet of #2 welding cable, to run from the truck engine's alternator back to the positive terminal of the  trailer batteries, and then from the negative terminal of the batteries back to a solid engine ground in the truck.  Some of the details below are specific to my Dodge, but the general approach should work with any vehicle.

On the Dodge, as delivered from the factory, there is an electrical distribution box on top of the left wheel well. This box contains some heavy connection terminals and a few big fuses and relays for various truck circuits. The charging cable from the alternator to the battery goes through this box. It was a convenient place to connect my new heavy trailer charging cable, and also had room to shoehorn in another fuse for the new cable.

Inside this box, I ran a couple of inches of heavy wire from the terminal where the alternator wire is bolted, connected to one end of a new 80-amp fuse. (This fuse  is a little black cube with tabs for attachment bolts on each side - available in any auto parts store.)

From the other end of the fuse, I ran welding cable down to a battery relay, which I bolted to the bottom of the battery support bracket, against the wheel well.

From the output terminal of the relay, I ran welding cable down and along the frame of the truck to the back, where it comes up from the frame just under the left side of the tailgate, and extends a 18" or so beyond the bumper.

The negative cable from the battery is bolted to the engine block, down low near the front. I ran welding cable from this same bolt back along the frame, following the same path as the positive cable.

Where the two cables come out at the back, they are terminated in standard welding cable connectors. (But see note below). When not in use, these two cables lay neatly in a crack between the bumper and the body, in the center under the tail gate.

In the trailer, heavy cable runs from each terminal of the batteries to my main battery   power distribution box, which I mounted on the front wall of the trailer, inside a kitchen cabinet (as in most travel trailers, the batteries are outside, on the frame against the front of the trailer - my distribution box is directly behind the batteries, but inside, out of the weather). This distribution box holds a battery disconnect switch, some big terminals where all trailer power connects, a few fuses for circuits that don't go through the fuse panel further back in the trailer, and the big current-measuring shunt, which is in the negative side of the battery circuit.

I ran welding cable from the positive and negative main battery connections in this distribution box, down through the floor and forward to where they connect to the truck cables, terminated in the matching half of the welding cable connectors.

The battery relay is available in auto parts stores, and also in some RV supply stores. I powered it from the ignition circuit, and ran this power line through a toggle switch I installed on the dashboard inside the truck, so power only goes to the coil of the relay when both the ignition is on and this new toggle switch is on. That way, when the engine is off, the relay opens and there is no power on the heavy cable, and I don't risk shorting it out when connecting and disconnecting. And when the trailer batteries are fully charged, I can flip the toggle switch and stop the charging process. Actually they still get a small charging current through the wire in the original wiring harness, which I did not disconnect. There is still a danger of shorting out the trailer batteries, if the positive welding connector from the trailer were touched to the frame of the trailer. I'd prefer a connector where all terminals were recessed down inside a plastic shroud so they couldn't accidentally be shorted - but I haven't found such a thing that will handle high current, is reasonably weatherproof, and connects positively enough so I'm sure it won't rattle loose on the road. My second choice would probably have been an ordinary household 220 volt connector - typically used for clothes dryers and kitchen stoves. (See note below for an even better connector choice.)

The #2 welding cable seems to be about right. I'm getting about 25 amps of charging when the batteries are only modestly discharged - probably quite a bit more if the batteries are really low - I haven't checked.  Heavier cable is available - but of course costs more. For my rig, I needed 32 feet of cable, for each (positive and negative) cable.

I had the welding supply company put the terminals and connectors on the cable (after I had carefully measured the needed lengths). These are standard parts which they had in stock.

Where the cables run along the top of the truck frame, I used a few nylon cable ties to hold them in place

I spent about $140 for the cables, terminals, and connectors, and perhaps another $15 for odds and ends - fuse, relay, etc.

Overall, I'm quite pleased. I get plenty of charging, and the cables are fairly convenient to connect, disconnect, and store. I only connect them when I really need the extra charging. When I'm traveling between campgrounds with hookups, my batteries start out with full charge and I don't hook up the heavy cables at all.

Note added 7/4/05: Today, a reader pointed out a much better connector for use between truck and trailer. Meltric makes an LC style connector, designed specifically for mobile battery charging, which from the catalog description seems to be perfect for this purpose. Both positive and negative connections are integrated in a single housing, and the contacts on both male and female ends are enclosed, so that they are unlikely to be accidentally shorted. It has low insertion force and a lever to compress the contacts and hold the male and female half together. It's available to fit several sizes of large welding cable. See http://meltric.dirxion.com/WebProject.asp?BookCode=mel09flx&SectionIndex=0&PageIndex=125#. (If this link fails in the future, go to http://www.meltric.com/html/literature.html Click to open the .pdf version of their catalog and enter "LC" (without the quoters) in the search field. This will lead you to a c atalog page labeled "LC-Battery Connectors". It's also worth noting that a male/female pair of these connectors has a list price of $300 or more, depending on the specific model chosen!

B:  Dual Alternators: Another option is to install a second alternator, or replace the existing alternator with a dual-output alternator. This is particularly practical for Ford owners.  Ford heavy-duty pickup truck chassis' sold for ambulance or police use have dual alternators, and these parts can readily be fitted to any  Ford pickup or motorhome chassis.  This is the most elegant, most effective, and most expensive solution.

One alternator output is controlled by the standard truck regulator and charges the truck battery as usual. The other output is controlled by an external "smart" regulator of your choice, and charges your house batteries. This regulator uses a separate voltage sense wire which is connected directly at the house batteries, automatically compensating for any voltage drops in the high-current charging wiring. The only disadvantage of this solution is that it's expensive.   Except for the Ford, it  will require custom-made mounting brackets to fit the new alternator in addition to or in place of the stock one. On top of that, a suitable alternator and regulator  (again, except for Ford) will probably set you back around $1000. My West Marine catalog seems to have gone into hiding, so I can't dig out any specific costs and brands on this.

With this approach, it will still be necessary to upgrade the wiring from the new alternator back to the trailer batteries, although the cable need not be as heavy as in solution A.

In addition, I strongly urge everyone to install a digital voltage/current monitor for the batteries, so you will know what's going on. Then you can see exactly what is happening - how effectively your generator or converter/charger is actually recharging, when the charging process is complete, whether the two outputs really add, etc. I have the Link 2000 which will do all the above for both house and engine battery, and also is a convenient remote controller for the Heart charger/inverter.

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