Trailer Battery Charging

[charleywatson]

My Tundra alternator doesn't charge the trailer battery while underway. Not a problem when going to a campsite with hookups, but between dry campsites it would be nice to utilize the truck's charging capacity to recharge the trailer. I'm thinking about running a heavy gage wire between the positive terminals of the truck and trailer batteries. I think there is adequate negative continuity through the hitch and trailer frame to not need a negative charging wire.

Is this a good idea, or is there some pitfall I am missing?

[rambrush]

What connector r u using? 7 wire round or 4 pin flat?
7 wire round is the way to go all can go through that. Do you have the factory tow package?

You can do it the way you talked about + to + but it had better be fused and wire loomed.

[akwight]

My truck will charge the battery of the trailer while in transit. I have the 7 pin connector with the trailer brakes. Oh yeah, my trailer has a converter in it...don't know if your trailer does, this is what charges the battery while in transit, the converter will also charge the battery when hookedup to 110V. Really slick setup.

[cupidstoy]

i believe on the 2000 model even the factory tow package does not carry the charging signal and the 7 pin connector wasn't part of the tow package back then. you need a fused wire that can carry about 30 amps. be sure to fuse as close to the connection point as possible. i would also run a ground wire from your trailer for future use as it is a better ground then through the hitch.

also, don't forget to fuse the other end and make sure the wire is switched by the key when not in the "on" position. you don't want the batteries (truck and trailer) tied together when not charging and you have to fuse both ends since there are power sources at both ends.

[geojim]

Originally Posted by cupidstoy

i believe on the 2000 model even the factory tow package does not carry the charging signal and the 7 pin connector wasn't part of the tow package back then. you need a fused wire that can carry about 30 amps. be sure to fuse as close to the connection point as possible. i would also run a ground wire from your trailer for future use as it is a better ground then through the hitch.

also, don't forget to fuse the other end and make sure the wire is switched by the key when not in the "on" position. you don't want the batteries (truck and trailer) tied together when not charging and you have to fuse both ends since there are power sources at both ends.

That is correct. The factory package was limited to the light connections (four-flat connector) on 00 to 03 Tundras.

Start by looking at typical wiring diagrams like this one http://tekonsha.com/instructions/BC%20Wiring.pdf, of couse, this only shows the basic connections for the brake contoller only, which requires another dedicated 12v fused run from the battery (seperate from the 12v trailer batt charge cuircuit)

You could get a 7way like this: 4 Pole to 7 Pole adapter which connects directly to the facory connector.

Instead of making up your own harness for the for the ground wire, 12v supply, electric brake actuator wire, optional reverse lighting on trailer, you should go to an RV shop and buy the bundled wire, which is sold by the foot. That will save you the hassle of chasing after the correct guage & colors, and wrapping it in wire loom.

Run this up zip tied to the frame all the way to the battery, leave a few feet extra so you have enough to go back into the cab to connect the blue wire (from brake controller) later. Connect the ground wire (white) right to battery or at ground strap (what I did).

The point made about connecting the 12v hot wire to the battery with a fuse, and in such a way to be only energized only when igintion is on, is a good tip. This is exactly what I did. I just got a 30amp relay, connected to my accessory circuit to activate the relay, an auto-reset 20a breaker at the connection to the pos batt lead, so this only makes the connection to the trailer battery hot only when engine is running. If you leave the whole rig hooked up, say overnight, this prevents the trailer batt from dragging your truck batt down.

I know I am missing alot of details here. And don't quote me on the colors , but this will give you some basics anyway.

Took a while to get it all straight. But works great now.

Hope this helps!

[RockyMtnRay]

Originally Posted by akwight

My truck will charge the battery of the trailer while in transit. I have the 7 pin connector with the trailer brakes. Oh yeah, my trailer has a converter in it...don't know if your trailer does, this is what charges the battery while in transit, the converter will also charge the battery when hookedup to 110V. Really slick setup.

All travel trailers have a converter. But much as I hate to burst your bubble, converters (including yours) do nothing more than convert 110V AC to 12V DC for consumption by the trailer's internal lighting, motors, etc. The converter has nothing to do with charging the trailer's battery during transit when the power coming into the trailer is already 12V (actually around 12.9 to 13.5 volts)

Most (but not all) converters have a separate battery charging section. That separate battery charge section includes a charge state monitor to prevent the converter from overcharging the battery. Some converters have a sophisticated 3 phase (bulk, absorption, float) battery charge section that truly prevents overcharging; others are very primitive and are just on/off basic chargers.

[charleywatson]

Yes, I have the 7-pin connector and factory tow package. The reason I questioned whether I'm getting underway charging is that the voltage at the Tundra alternator is at least 1 volt higher than at the trailer battery with the engine running. Although now rhat I think about it, current could be flowing to the trailer and the voltage drop is due to resistance in the wiring. Hmmm... I should check the amperage flow.

I noticed that the auxillary wire (center pin connector) seems to be unused. Perhaps I could run that in parallel with the charging wire to reduce resistance and increase charging current.

Thanks for the replies. Very helpfull!

[charleywatson]

OOps, I missed reading the intervening posts prior to posting my last. Great info and alot to chew on. I was thinking the 7-pin connection came from the factory, but it must have been added by the RV dealer that sold me the trailer. Thanks again!! Back to the drawing board.

[cupidstoy]

i may be wrong on this, but i think on my truck the center pin is the ground pin. i'm also suspicious that you are not getting a good connection somewhere along the line if you have a full volt drop between the truck and trailer. i would check all the junctions and clean the pins on your connector really good to get any corrosion or surface dirt off. use a good dielectric grease on them (small tube is a couple of bucks at the electronic store). a 14 guage copper wire shouldn't have that much resistance in only 50 feet or so. adding another conductor isn't the solution.

[geojim]

Originally Posted by cupidstoy

i may be wrong on this, but i think on my truck the center pin is the ground pin. i'm also suspicious that you are not getting a good connection somewhere along the line if you have a full volt drop between the truck and trailer. i would check all the junctions and clean the pins on your connector really good to get any corrosion or surface dirt off. use a good dielectric grease on them (small tube is a couple of bucks at the electronic store). a 14 guage copper wire shouldn't have that much resistance in only 50 feet or so. adding another conductor isn't the solution.

I agree w/ the above with the exception of the common misconception that the center terminal pin on a 7 way is a ground. It is actually for reverse lighting or other aux circuit. The #1 pin (it may or may not be labeled), which has a white wire running to it, is actually the ground.

[geojim]

Maybe you should check the voltage right at the connector black wire terminal, see this wiring diagram. Try with the engine running and off, at the truck batt, the connector, and the trailer battery, then compare the results. If you get no voltage at the connector, maybe that circuit was never completly wired, or the fuse is blown.

[RockyMtnRay]

Heartily agree with everything that geojim has provided in this thread. And with the exception of the center pin being a ground (as geojim notes, the center pin is for aux circuits such as backup lights), I definitely concur with cupidstoy's post...especially about carefully checking/fixing the connectors.

My suspicion is that since the battery charge wire on a '00 Tundra had to be added by a 3rd party (likely an RV dealer or hitch shop), it was done hurriedly. At that very likely means that crimp on connectors were used. Crimp connectors are pretty poor for high current circuits to begin with (the recharge circuit can carry as much as 20 amps) and they become even worse over time due to corrosion.

The only really correct way to connect either the battery recharge or brake control circuit to the bargman plug at the back of the truck is to use soldered connections. Yep, properly soldered connections inside either good shrink wrapping or, better, fusion tape that will exclude all water and prevent corrosion.

[j4x4ar3]

Originally Posted by charleywatson

My Tundra alternator doesn't charge the trailer battery while underway. Not a problem when going to a campsite with hookups, but between dry campsites it would be nice to utilize the truck's charging capacity to recharge the trailer. I'm thinking about running a heavy gage wire between the positive terminals of the truck and trailer batteries. I think there is adequate negative continuity through the hitch and trailer frame to not need a negative charging wire.

Is this a good idea, or is there some pitfall I am missing?

Although all the information in this thread is good if you're dry camping and say use 1/2 your battery capacity... how long do you plan to be driving between dry camps to expect to put any sort of real charge on the battery? IF you can get a full 10-12amp charge put on the battery which isn't likely since the trailer circuits are usually 20amps to account for brakes and lights as well then it would still take several hours of continuous engine running to charge the trailer battery. Best you should be able to expect from using the truck as a charging means is to keep the battery topped off. Many RV'ers will pick up a small Honda EU1000i and charge their batteries at the campsite as charging a 1/2 dead or more battery from your truck is impractical.

[cupidstoy]

you bring up a good point, but i partially disagree with some of the numbers in your post.

first, we are talking about a dedicated charging circuit with 30 amp capactiy coming from the tow vehicle alternator. if you have a tow package from the factory (which he doesn't) the alternator is capable of putting out 130 amps when cold (considerably less when hot according to most stereo install guys) but still a pretty decent amount of current. i believe the stock, non-heavy duty tundra alternators can still put out a solid 70 - 90 amps. while it's true that some of that capacity goes to run the tow vehicle accessories and rv lights, etc, there is still plenty left to charge a single battery or bank of batteries.

electricity flows to the path of least resistance. a low battery acts as a big capacitor, soaking up electrons to reverse the discharge reaction on the plates. once all the "easy" loads (12-13 volt potential differnce) are met (lights, electronics, etc) the remaining current goes into the lowest batteries first (biggest voltage difference) and then as voltages begin to approach the charging voltage, current flow lessens gradually until top charge is reached and the batteries in the system "float". the alternator in the tow vehicle is responsible for sensing the system voltage and increasing or decreasing it's current output accordingly.

this point illustrates the need for protective elements in the charging circuit. for example, a complete dead battery at one end of the circuit or a battery with a bad cell (internally shorted), could potentially draw a great deal of excess amps over the conductors ability to carry current (30 amps max). thus, the need to fuse the conductor at both ends to break the circuit under this dangerous condition. more advanced control devices are available such as diode protected circuits or voltage dependent current controllers, but i don't think those are being considered here.

assuming you have a single battery in your rv/trailer, at most it has about 130 amp hour capacity. so, unless you have a LOT of accessories running off your alternator while running down the road, you could put a full 20 - 30 amps per hour into your low battery as you drive. in 5 hours, you would have a pretty well charged battery (in the range of 12.3 to 12.5 volts with 12.7 volts being fully charged when measured at rest and 10.5 volts being considered effectively dead). the last several tenths of a volt are the ones that are difficult to get as the current flow lessens proportionally to the voltage different. this is actually desireable for a lead acid battery as rockymtnray points out, modern sophisticated charges are actually designed to charge in three modes: bulk, top and float. i believe this is basically what the denso alternator in the tundra does very well - put out current propotional to the voltage present in the system although i don't know that it does so in three well defined stages.

so, unless you have an excess of accessories parasitically stealing all your alternator output, you should be able to easily charge a single battery in an RV in between campsites. additional batteries in parallel in a battery bank system would take more time of course.

i routinely do this very thing with my dual battery setup and have no problem top charging my second 130 amp-hour battery during short trips. if i do a voltage check and it isn't up to par, i just trickle charge it the last few tenths of a volt up to full charge and i'm good to go for another 24-30 hours on the next cycle. my typical load on the deep cycle battery is a cooler, heated seats, inverter and some lights all of which are used at different times and in different combinations.

my set up is a little different in that i have used very heavy gauge wire between the two batteries (1/0 gauge) so i am not limited by the 30 amp max in the trailer scenario. all that means, is i can pump as much current into a dead battery as it will take making my first few minutes with a fully dead battery more efficient at charging (like with a car starter charging setup delivering high amps for a few minutes) but as the voltage rapidly increases, the current levels off very quickly and it settles down into a normal load. this is the only way i know of to increase his charging efficiency on the trailer battery without buying a generator. both methods being expensive. copper conductors have really gone up in price. i paid over a hundred dollars for two sixteen foot runs of 2 gauge cable for the inverter and you would need something like that going all the way to the trailer battery to get better charging rates. even larger diameter cable would be needed on a long run like that.

just make sure you have an alternator capable of putting out plenty of extra current and supplement with a trickle charger at powered camp sites or a gas or diesel generator at dry sites.

sorry for the long post. i just went through all this and wanted to share what i've learned. i'm really happy with my setup. ;-)

[tomhole]

Next time I hook up (maybe tomorrow), I'll measure the amps going into the battery from the truck. The consensus on the RV.net fora is that it is not much more than 10A. Running a 285hp generator seems inefficient to me. The input from the rv folks is to buy a 1000W inverter, hook it directly to your truck battery while the truck is running and plug a quality 25A three stage battery charget into that. I am only parroting what others have said. Can't argue the point either way. A genny works, too.

Tom