Have a 2004 F-350-SD 6.0L having on going problems charging 2-group 24 DC batieries in my toy hauler they are both in good condition but I can't get enough current to them to charge them while on the road with the refrigerator operating on 12 volt at 14 amp, have changed the supply to the trailer 3 times still no luck. Last upgrade is a 90 amp electronic isolator from HellRoaring Technoligies that has vertually no loss across the switch device which is designed to close when the charging system reachs 13.4 volts, this device is connected to the left hand truck battery using 4 gauge high quality stranded wire the wire runs to a seperate 9 pin plug which is designed to take 4 ga, wire in the box. From this point a 4ga. 12V+ and 12V- run from the trailer side of the 9 pin to the batteries, total run 35 ft from truck battery. After installing this system I ran the RV batteries down to the point that they where at 8 volts and then pluged in the cable and started the truck expecting to see a massive charge going to the trailer as the manufacture was concerned that my over size wire would put to much demand on the alternator they recommend 8 gauge or 6 at the most but I already had the 4 ga. in place from my last system using a electro-mecanihcal switch after a few checks with my clamp on amprobe I detirmined that the alternator was not performing as all I could get out of it was around fifty to 60 amps going by memory so off to the dealer. The tech detirmined that the batteries where fine but the alternator was not putting out its rated 130 amps so replaced it and it and the new one put out 155 amps at 14.5 volts. Had no time to check its operation as we where leaving on a 3500 mile trip the next day so just assumed everything worked now.
Wrong.
Found that the system indicator light that indicates current flowing to the trailer would only come on after driving for a couple of hours at 60 miles a hour or more as soon as I slow down the truck wont make the 13.4 volts for the system to work and this was starting off with fully charged RV batteries as when I have 120 volts connected to the trailer the automatic charging system keeps the batteries topped up. Second day out started with low batteries and system worked worse, only way it will work is if the batteries are chargedthen the voltage will fall from 12.7 to about11.9 durring the day.

Have not got back to dealer yet but as far as I can see the problem is in the trucks charging system alternator puts out 76-78 amps most that ever goes to the trailer is 20 amps at 2000 rpm or higher. The computer system in the truck may be to smart for the system as tech told me that the left battery is the main battery and has a sensor wire to the computer so maybe it won't allow full output from the alternator as it measure that the truck batteries are in a good state of charge.
Won't be using the trailer anymore until spring but intend to order a 2008 for spring an don't want to face the same problems again.
Sorry for the long story but needed to get the problem acoss in the hope someone has a answer.

 

The alt and charging system has to "see" the load before it can kick in and charge. Apparently there is too much resistance somewhere in the system. It could be anywhere though, but a good way to troubleshoot the problem is with a voltmeter.

What you need to do is hook everything up, including the trailer with the trailer batteries discharged. Half charged would probably work, but the more they are discharged, the more problem area is going to stand out.

With the truck running and the trailer unplugged, start at the battery isolator, and see what the voltage is. I am assuming you have the alternator output hooked to the isolator you installed. A healthy system will be around 14.5 volts. Then plug the trailer with the discharged batteries. The voltage should sink at the alternator output. The more it drops, the more the alternator will output to try and drive it back up.

If it doesn't drop much, then the alt does not "see" the load, and it will just be lazy and not have much output. So the voltage will probably still be near 14.5 volts. If this is the case, then work your way downstream toward the trailer, with the trailer still connected with the discharged batteries. Go from connection to connection, including through the isolator, and every connection to the trailer plug and in the trailer. Keep track of the voltages at every point. If you have a good connection, you should see hardly any voltage difference or drop. If you suddenly happen upon a connection or part of the system with .25v drop or more, then there is going to be a problem there. You probably by now can also feel this connection with your hand, and it may be warm. If you find something like this, try to figure out what is causing the resistance. The largest resistance or drop you will find normally will be in the isolator. If you measure the voltage at the front of the truck on the 4 gauge wire, and then measure it at the rear and get a significant drop, then the 4 gauge wire is too small.

All 12 lead acid batteries are around 12 volts on their terminals. The higher you can get the voltage from the truck at the batteries themselves, the more they will charge. If you end up with 12 volts at the battery terminals with the truck charging them, then they will not charge. It would be nice to have the 14.5 volts at the trailer batteries, but that will probably not happen in the real world. 13 or 13.5 would give you a nice slow charge, hopefully while still running the refrigerator too.