Charging a Travel Trailer Battery With the Tow Vehicle While Driving

[E. Graham]

Does the 12-Volt battery in a travel trailer get charged while the trailer is being towed?? I have read comments such as "...battery was down so we drove around for a while and it came back up again", and "...started the engine and let it run for a while to charge the battery".? On the other hand, I wonder why, if that's really possible, people buy generators and/or solar panels.

Our tow vehicle is a Toyota 4Runner V6, our trailer is a 17' Casita fiberglass "egg", and the electrical connection is a 7-way plug.

We have no idea whether we ever charged the trailer battery while we were on our 9-month road trip, because we always had full hookup --- we didn't use the battery.? Now we're planning to dry camp with our new Casita 17' and will be relying on our 12-volt system.? Most of the time, we'll be going away for only the weekend, with a fully charged battery.? We don't anticipate having to recharge en route.? However, we are taking a 3-week trip in late summer and would prefer not to have to take along a generator or invest in solar panels.? If we CAN charge with the engine, we'd like to.

Thank you, in advance, for your help!
Elisabeth and John Graham, proud parents-to-be of Le Mini-Lapin

 

[John From Detroit]

Does the 12-Volt battery in a travel trailer get charged while the trailer is being towed?

The answer is: It depends.  It depends on a couple of things,  But mostly the questions are this.

1: Was the trailer wired so that it can be recharged by the tow vehicle (most are, but not all)
2: Was the tow vehicle wired to charge the trailer battery when towing (This is a BIG question)

Either isolation diodes or a 2nd alternator is needed, all 3 methods have been used, the first two are the most common and the 3rd is kind of rare.  But that is the big question, was your tow vehicle wired to charge the trailer battery.

Page 2: Unless you live north of me (I'm in Detroit) Consider solar power to help keep the trailer battery topped up

 

[E. Graham]

Thank you, John.

I don't know the answer to the questions you posed.? However, we're having a hitch shop install our hitch and the weld on the brackets for the torsion bars.? I believe they will also be able to confirm (or not!) that we are set up to charge the trailer battery while driving.? And if we're not, perhaps they will be able to make the necessary changes.

We are considering solar:? someone else, in a different post, suggested a second battery combined with a solar collector.? We're not yet convinced that's the best option, however, because we will be travelling in some pretty rainy and cloudy places.

Thank you for your help!
Elisabeth

 

[John From Detroit]

Ah, that's you... Well, if the car is not rigged to charge the batteries... The hitch shop can likely fix that for you (For a feeeeeeeeeeeee of course, that's a big fee)  Well, not too big

 

[Lou Schneider]

Either isolation diodes or a 2nd alternator is needed

Wow, John - you sure like to spend money!? A second alternator may be appropriate for a 40 ft. motorhome with a couple of 8-D batteries and lots of electrical goodies.? But not for a 4Runner and Casita trailer.

Elizabeth -

Your 4Runner should have a "hot wire" running from the vehicle's battery to the 7 pin socket, then from the 7 pin plug to the trailer's battery.? This will charge the trailer battery at a moderate rate while you drive.? If for some reason the hot wire isn't there, it's easily added.

The hot wire will charge your trailer battery at about a 10 amp rate.? In other words, 6-8 hours of driving should just about fill a "dead" battery.? ?A couple of hours of driving should replace what you use during a single night - if you're parked for longer periods you may need another charging source.

Adding a second battery is a great idea, if the Casita has room for it.? If nothing else, it will double the amount of electricity you can use before you need to recharge.

If you don't drive enough to keep the batteries charged, you can always pull into a full hookup site for a night.? Let their electricity completely recharge your batteries while you sleep.

Isolator diodes are used in motorhomes because it's not convenient to disconnect the house batteries from the vehicle battery when you stop for the night.? ?They're not as necessary for a tow vehicle and trailer combo because the trailer is usually unhitched when you set up camp.? If you stop overnight with the trailer hitched up,? just pull the 7 pin connector when you set up camp, then plug it back in when you're ready to leave.

 

[John From Detroit]

I screwed up that post, Meant to say Isolator diodes, Solonoid or 2nd Alternator,  and the 2nd alternator method, although actually the best option (if you have the room) is very rare and I've only seen it on large vehicles such as Trucks or Motor Homes.

Most use either isolator diodes or solonoids  Personally.  I like diodes but then I'm high tech and they are more "elegant" than a solonoid which is kind of brute force

Side note: Any multiple battery system which uses one alternator all batteries must be capable of taking a charge. Holding it is an option, but all of them must take a charge... One shorted cell and all go dead

 

[Gary RV_Wizard]

Personally, I'd just run the extra charge wire as Lou describes. And be sure to unplug the trailer connector overnight.  A battery isolator/diode is overkill for this very simple requirement.

 

[Marsha/CA]

Hi Elizabeth,

We used to have a slide in camper that went on the bed of our truck.  When we drove, the battery on the camper would be recharged as we drove.  We had one battery.  I know the fellas here say to unplug your casitas from your truck while parked to avoid running down your truck battery; but for a couple of dollars, you can have a "one-way" pole put on your truck battery that will allow the truck to charge the casitas battery; but not allow the casitas to draw energy from the truck battery.  That's why it's called a "one-way" pole, it only supplies charging one direction.

FWIW, we could go for almost 4 days with the truck camper and not have to recharge.  Of course those were the days of no TV and no laptop computers.... ;D

Marsha~

 

[E. Graham]

I've been away for a few days, so haven't had the opportunity to say "thank you!" to the responses you've all been providing to my question.

Thank you!

Elisabeth

 

[rickst29]

It's an old Thread, but this one still comes up in Google - and the answers are mostly wrong.

The real problem.
The Toyota 4Runner, like nearly all modern Pickup Trucks and SUVs, will adjust alternator Voltage (and power) according to a "Sense Wire" which connects to a junction near the Starting Battery. Right after you start the vehicle, the "Sense" shows that the battery needs charging - and the current in the fixed alternator windings is adjusted to create high output (whatever Amps the battery and circuits will accept, at up to 14.4 Volts).

But, after a couple of minutes (depending on other loads, such as lights and stereos), the "Sense" Controller will decide that the battery no longer needs "bulk" charge - and the alternator will be adjusted to create only 13.5-13.8 Volts.
- - - - -
If the voltage under the hood is only 13.8 Volts, then it doesn't matter how "big" your trailer wires are. Even if "Voltage Drop" was zero, the trailer batteries will NOT accept much charging current (i.e., input power) at that voltage. "Boost", "Bulk", and "Absorb" charging for Lead-Acid batteries uses Voltages in the range of 14.2V to 14.6V.

The size of wires begins to matter only when current is actually flowing. And the single-battery Casita needs only about 10A (when the Fridge Heater isn't running) so #10 would be fine.

In order to charge trailer batteries from the TV, either (1) the TV "Sense" (and/or Controller) needs to be modified, so that the Alternator stays at high power; or (2) a "Voltage Boost" device must be installed (between the Bargeman cable input and the trailer batteries".

For option #1: it's very easy to put a diode along the Sense wire, dropping the "Sense" voltage by about 0.5 Volts. But, if left in place all the time, it will cook your Starting Battery (AND your Trailer batteries too, if they become "full".) So, you actually need to measure your Trailer battery by hand, and only connect through the Diode "when it needs it".

For option #2, devices from Redarc and Ctek provide intelligent "boost" and charge control - but the devices are somewhat expensive. You could build a DIY equivalent from a fixed-voltage booster and a Solar Controller, but you would probably need to add a voltage-smoothing array of capacitors (or a battery at the "boosted" voltage) to protect the "booster" from the PWM behavior of the Solar Controller.