Tow vehicle connection for trailer battery

[KeithB]

A few months ago I purchased a 2018, 2108DS trailer. Shortly after I purchased a 2018 Ford f150 with tow package. How can I tell if the vehicle is wired to supply a charge to the trailer battery while towing? I assume the truck is wired for it as part of the tow package. Would the trailer be prewired as well? I thought I would check in with the forum before further investigation, i.e. looking up manuals, taking to trailer shop, etc.

[Morich]

I would assume Ford and the trailer are both wired "standard" but that is something to verify as some are not!

But if standard, there may be a diagram on the lid of the trailer connection to show what should be, so check there as a start, perhaps?

Then If you have the "standard seven pin connection, these drawings should help sort the things needed, like the full time power pin.(12 volt power on these drawings)

Or you might have a six pin as the last picture shows?

[backtrack15]

This could be completely wrong, but I think you could just test the trailer battery voltage with a multimeter. Without any connection to shore power, solar, or the truck the voltage is likely 12.7 or lower. While connected to the truck (engine running, maybe cycle it into drive briefly), after a minute or two I'd guess you'd see a higher (and increasing) voltage. My 2017 does charge trailer batteries. I tested it once with a clamp-on meter and it was charging at 4-amps.

[Marine359]

Originally Posted by KeithB

A few months ago I purchased a 2018, 2108DS trailer. Shortly after I purchased a 2018 Ford f150 with tow package. How can I tell if the vehicle is wired to supply a charge to the trailer battery while towing? I assume the truck is wired for it as part of the tow package. Would the trailer be prewired as well? I thought I would check in with the forum before further investigation, i.e. looking up manuals, taking to trailer shop, etc.

Your standard 7 pin setup on both the trailer and the truck will provide some 12v charging power to your battery. The amount depends on the size of the vehicle alternator, and the length of the wire run. I’m gettin about 1.5-3 amps from tow vehicle to trailer battery. Not enough to charge up a depleted battery, but enough to give you more amp hours stored at the end of your 6 hour drive than before you left. That is unless you have a 12v compressor fridge which you’re not likely to have for the year of your MM. It’s always best to pre-chill your fridge on shore power the day before departure. That way, your food will stay cold, you’ll use less 12v power while driving (with fridge on propane) and your battery will be fully charged when you arrive.

If you want more charge current from your tow vehicle, many people recommend installing a heavier gauge wire from the alternator with a separate connector to a dc/dc charger mounted near the battery. However, if you pursue that, you’ll need to check to see if your alternator can carry the extra load.

[Morich]

Originally Posted by backtrack15

This could be completely wrong, but I think you could just test the trailer battery voltage with a multimeter. Without any connection to shore power, solar, or the truck the voltage is likely 12.7 or lower. While connected to the truck (engine running, maybe cycle it into drive briefly), after a minute or two I'd guess you'd see a higher (and increasing) voltage. My 2017 does charge trailer batteries. I tested it once with a clamp-on meter and it was charging at 4-amps.

Yes, fully agree with this---if there is wiring on the trailer to the battery.
We sometimes see folks thinking of battery charging as a big mystery when it is pretty simple if we think of it as we might water.

Why does battery power flow? It is the difference in potential and we think of that potential as voltage levels and often we use ground as the reference point of voltage. At 12 volts we thing of it as being further from ground the at 10 volts, so we can look at it if we were dealing with water.

If we have two same size, same shape cylinders with one having 12 inches of water and the other 10 inches and we connect them together, we see water flowing from the higher level to the lower level. It flows pretty fast at first when the difference is bigger but slows as they near the same level.

When we connect a partly discharged battery in the trailer ( say 10 volts?) to the truck where we may have 12-14 volts coming from the alternator to the trailer connector, we get pretty fast flow for a bit as the difference is 2 to 4 volts above the trailer battery.
But we can get away with doing this through the small wires on the trailer connection because that larger flow only lasts as long as it takes for the "surface charge" of the trailer battery to build up. The surface charge builds up and the difference quickly goes to much lower current levels that suit the small wiring and then it is how fast the chemical reaction takes place in the trailer battery that controls how much current will flow from truck to trailer.

That, just like RV owners, can get folks with bass boats in trouble. They take the trailer from the lake for an hour drive back home, check the voltage and see 14 volts as they have just shut down the charging and then find next week the batteries are way down after the surface charge and chemical reaction has moved all the way into the other cells, not just the one we tested!

If we find the voltage at the truck connector and find the matching point on the trailer connection gets to the trailer battery, all we need to do is connect them together!