Battery question

[Field Trip]

As we roll down the road does the truck keep the coach batteries charged (assuming the coach is plugged into the truck)?

[Stripit]

Depends on how the trailer wiring is done, but on my unit yes it did.

[JAKL]

Yes and no. Your truck will supply ~2A of charge current to your unit. If your unit electrical system is off, it will trickle charge your battery bank. If you are rolling with your residential refridgerator on, you are discharging more than you are charging.

[Cummins12V98]

I guess I should check my battery voltage next time at the end of the day. I work my batteries when tearing down and setting up with NO shore power. I run my Residential frig at times 8-10 hours and the Frig is 0 and 38 degrees as it normally is when I arrive at camp. I am thinking mine is keeping up just fine.

[Chad Heiser]

I guess I should check my battery voltage next time at the end of the day. I work my batteries when tearing down and setting up with NO shore power. I run my Residential frig at times 8-10 hours and the Frig is 0 and 38 degrees as it normally is when I arrive at camp. I am thinking mine is keeping up just fine.

What you describe is a function of the size and storage capacity of your battery bank and really has nothing to do with whether your truck is charging the batteries while you are going down the road. Assuming you are breaking camp where you have been sitting plugged into shore power for a couple of days or more, your battery bank should be fully charged. You unplug and use 12 volt power to run the slides, jacks etc while hooking up to the truck. Your inverter then also powers your fridge while unplugged and running down the road. You then get to your next site and reverse the process until you eventually plug in and allow your batteries to recharge.

All of this will occur whether or not your truck is sending a charge voltage to the batteries as you drive down the road. It will continue to happen until you plug in again or discharge your batteries to the point they will no longer keep up with the 12 volt loads. With a decent sized battery bank this could be several days or with a small battery bank this could be several hours.

The only way to know whether your vehicle is doing anything significant in the way of charging your batteries is to take a state of charge reading when you unplug from shore power, then again before you start your truck engine with the trailer umbilical connected and then again when you get to your destination and turn the truck off. The difference in these readings will tell you if the charge coming from your truck alternator is doing anything to maintain or recharge your battery bank.

The simple physics of the set up say that it may provide a basic charge to keep a fully charged battery bank from discharging as a result of parasitic loads (stay topped off), but it probably isn't doing much more than that. Your alternator on your truck only has so many amps to give out. Most of those will go to your truck batteries and the power your truck requires to run down the road. The little bit left over are being pushed from the front of your truck to the rear through (probably) a 12 gauge wire, then through the trailer umbilical cord (again probably a 12 gauge wire) and then through trailer wiring until it finally reaches the battery bank (most likely more 12 gauge wire). Your looking at close to 40 feet of that small gauge wire with multiple connections to cross before a charge ever reaches the batteries. The voltage drop and line loss in that situation will be significant and will result in a very small charge actually making it to the trailer batteries.

This is a very long explanation to basically say that yes, the truck will provide a charge to the trailer (providing all the correct wiring and fuses are in place), but that charge won't do a whole lot to keep up with heavy usage on the trailer side while under way. Having a fully charged, decent sized battery bank when you start will allow you to run a residential fridge, etc while under way whether or not the truck is doing anything to help.

A much better charging source for your batteries while running down the road (during daylight) would be a decent sized solar panel (or panels) with a solar controller and appropriately sized wire. This, however, is also not necessary if you have a decent sized, fully charged battery bank to start the trip.

[Cummins12V98]

I am only running two 6V batteries. I would think the truck is delivering plenty of juice to keep the batteries charged enough to keep the frig at the specified temps after 8-10 hours on the road. I personally do not consider 12ga wire to be "SMALL".

[anijet]

The resistance of 40' of #12 copper wire is only .07 ohms, not much. Late model trucks usually have 140 amp (or larger) alternators. Truck battery, after a normal start, should be mostly charged within the first 20 minutes or so. Even a well optioned truck shouldn't use more than 60 amps running the AC, lights, entertainment, and whatever else is available (with a charged battery). Plenty left for charging the RV battery. I have departed a campsite with a less than 60% battery and it was better than 90% when arriving after a 7 hour drive.

[Chad Heiser]

At 120 volt, 12 AWG wire is a good size and will handle a decent load. At 12 volt, 12 AWG wire is not good for large loads at all.

To run 13.3 volts (approximate output of an alternator) 40 feet at 25 amps with up to 3% loss acceptable would require 0000 AWG wire. If you want less than 3% loss, then even 0000 AWG isn't beg enough.

With 12 AWG wire running 13.3 volts 40 feet with 1% acceptable loss, the most amps you are going to push is 0.25. So yes, over time you will be able to send a charge to the trailer, but it will not be very much. It will be enough to put a small charge into an idle bank with no draws on it or keep a fully charged bank with basic parasitic draws topped off. It is not something you want to rely on to keep up with a heavy draw while driving.

[wingnut60]

My experience with several travel trailers and 5 or 6 tow trucks, bears out what Chad has said---12 volts going long distances thru a small wire doesn't do well--
Best you can hope for is to keep well-charged batteries from going down.
You can always use your battery monitor panel to see what the differences are when hooked up/engine running high idle and not hooked up...