How much alternator do I really need?

christopherglenn,

12V voltage/amp drop is governed by the size (gauge) of the wire and the distance traveled. Yes, a #10 gauge wire will have a sizeable voltage/amp drop occuring in the distance between the front of a tow vehicle where the alternator is back to the front of the trailer being towed. That's why you should not use less than #6 ga wire from the alternator back to the batteries being 20'/24' charging wire size for up to 3 storage batteries fast and efficiently in or at the trailer. Depends mostly upon what the owner wants or expects from their RV batteries.

Your 5% to 10% alternator output is a little low and way low if the alternator is on a diesel engine as they idle at a higher RPM than most gas engines. using your 10% figure for example for amp output from the alternator, it's simple math the tells you that a 100 amp alternator puts out 10 amps at idle while a 200 amp alternator puts out 20 amps at the same idle RPM's. Yes, alternator amp size does make a big difference! Example of a known: My 260 amp dual alternators put out 47 amps at idle on my GM diesel truck and thats 47 amps of charge with nearly zero line loss with my #2 ga copper both positive and negative wire from my alternators back to the trailer batteries.

As the voltage (pressure) drops in DC the the line loss increases by 2X going from 12V to 6V. Check the charts. An example from the charts is the 2% voltage line loss of 12 volts on #14 ga copper is 16 times the line loss on the same length using #2 ga copper wire. Source: 101 wire loss chart

Same is true for solar or alternator produced current. A simple example is: What good is it to have a 1000 gallon per minute capacity at 50 psi waterpump when you only have a 1/2" inside diameter hose choking it's delivery to a tiny fraction of water actually delivered and even less and less as the 1/2" diameter hose length increases? Same with wire and electrical current. Hundreds of charts on the Internet.

Again, both + and - large capacity wires are best to deliver the amps to the trailer batteries and that's important as using the vehicle's steel chassis for the - (ground side) is like using a small gauge wire for the negative side as steel is not nearly as good a conductor of electricity as the copper wire is. Even worse with high amperage current. Notice they never use steel wires between power poles for current carrying lines! It sure would be cheaper to do and they would if they could.

There is a voltage drop in the charging wire from the trucks batteries / alternator all the way to the trailers batteries. The lower the trailers batteries are, the more current (amps) can be pushed from the alternator to the trailer. Real world 5-30 amps is all that goes to the trailers batteries. If you have lots of lights on the trailer, you can suck up another 10-20 amps there as well. Most any alternator over ~100 amps will be fine when you are running down the road - at idle they put out 5-10% of their rating in most cases. The truck can use most of that with the lights and heater / ac on.

You could install some solar panels for less money and would prove far more useful.
Otherwise base oem stock alternator is fine.

#10 wire isn't nearly big enough to get enough amps back to na RV battery or battery bank. That's why I use #2 ga fine strand welder wire or sometime they call it "cable". Go striaght back and fuse it with an 80 amp marine slow blow fuse. Cheap setup and assures safety. The fuses come in 60, 80 and 100 amp and are a couple bucks each but I've never blown one yet in 10 years. It allows real charging and fast! The fuse holder screw clamps the fuse legs between contacts due to the high current and is approx $18 at West Marine here but it depends where you buy it. East and west coast the price is usually $22-$24 so I've been told. This allows you to charge with 60+ amps which makes short work of recharging. Just imagine how many big solar panels you'd need to even get 20 amps of charging. It would be cost prohibitive and you need a controller also that's capable and God forbid if there's clouds or tree shade.

110 to 120 amps is plenty. I wonder why someone would want a 265 amp alternator - optional on some Ford trucks. The wiring going to the trailer is only going to carry 12- 15 amps in stock configuration, and probably has a 20 amp fuse.

What I would recommend is upgrading the stock wiring to something like a #10 wire protected by a 30 amp auto reset circuit breaker. Then run a wire directly from the alternator (where the voltage is the highest) to the circuit breaker, to a fog light relay (rated at about 30 amps) to the power plug for the fifth wheel. Then you will be able to move up to 25 amps to the RV battery. Probably will only charge at 15 - 18 amp range due to length though.

Much of the battery charging should come from being plugged in at a campground, or from running a generator if you are dry camping. It would take many hours of driving with 10 - 12 amps going into the battery to fully charge it.

I would recommend that you get a couple of 100 - 150 watt solar panels. They will keep the battery full regardless of if the truck is charging it or not. Parked or not. Even while dry camping you can keep the battery pretty much full. They are not that expensive anymore. SunElec.com Then look for a 30 amp PWM controller.

From Home Depot, you can make mounts from 6" long aluminum angle and then run #10 grey wire from the roof down to the battery and controller through the refrigerator vent and behind it. Use rubber roof sealant to put below each mount, and above each screw, it will stay water tight.

Good luck,

Fred.

For the average RV'er 150 amp alternator is enough. However, I have dual alternators I'd ordered on my GM diesel truck with 260 amps total because I have 9 batteries to keep up and recharge.

The 9 are: 2 high capacity 12V starter batteries for the truck's diesel engine. 2 Group 31 deepcycle 12V batteries in the truck bed between the rear wheel inner and the front of the bed on the driver's side used for the 5th wheel if needed and always used when the Lance TC in on the truck giving 3 group 31 batteries for the 1500 watt inverter in the TC. Our Carriage Carrilite 5th wheel has 5 more group 31 deepcycle 12V batteries in it that power our 2500 watt inverter. One each + and - #2 gauge fine strand welder cable 8' long each go from the marine barrel switch + I mounted at the top of the 5th wheel lower bulkhead wall to turn current off or on for the + cable to connect the truck alternators + the 2 bed batteries.

Therefore, I have 7 group 31 deepcycle batteries capability for the 2500 watt inverter. Lots more amps when the truck diesel engine is running and all 9 batteries recharge real fast when the engine is running. That's why I have dual alternators and it works great and has for 10 years now. None of the batteries have ever been drawn down below 50% before being recharged and they last a min of 6 years and the 2 I replaced last fall were in there since Sept 2004 and still decent but way past time to be replaced.

We don't have to be misers on our 110 AC use and are not when we're not connected to shore power. Use the toaster, the countertop toaster oven, coffee maker, George Forman sandwich grill, tv's, all the lights we want, and the wife frys bacon and fish outside in the electric frypan so it doesn't stink up the 5th wheel, etc, and never worry about killing or deeply discharging the batteries. Also have a Honda EU2000 generator on the back of the 5th wheel and the Lance TC has a 3400 watt built in generator but they make noise and the inverter doesn't. Sure handy if or when needed!

The high out-put alternator is for service vehicles...tow trucks, ambulances, etc.
As posted, the 'standard' alternator will be just fine for normal RV towing and everyday applications.

150 is plenty. The RV only has a 30 amp circuit from the truck.

The stock one will serve you fine. There's no inherent large draw in a fiver. Even if it had a residential fridge your truck would keep up.