Whats the biggest battery you can get in a festiva?

[Dragonhealer]

^ Yup! +1

[ryanprins13]

No, that not quite how that works... And you wouldn't want the starter battery to be doing anything but start the car hence the master switch. The alt does all the charging. If you were to use your starter battery to augment the aux battery, that's all it will do... It won't "charge" the aux battery. There will likely be a small voltage differential between the two and that will slowly push into the aux battery, but that's as far as it will go.

Ok, i guess i misunderstand how batteries in parallel work. If thats the case, that 2 batteries in parallel dont equilize in voltage quickly then i will probably just buy some 14gauge flat 4 wire and go from the starting battery back to a small deep cycle battery in the trailer. That wire should be able to handle 70-80 amps. Ill get a decent starting battery, but not a $350 one.

Sent from my SM-G920W8 using Tapatalk

[Dragonhealer]

Ryan we are getting into trouble here. You should look through the basics of electrical theory, and I'm not an electrical engineer, but we are talking about a 12V DC system and not discussing the "given that" parts that are constant.
I don't know how to cover this breifley, but I'll try.
How much power can a given wire handle? Depends on several factors, but we are talking POWER, electrically this is expressed as Watts (volts x amps = watts). At 12 volts a 50 amp load is 600 watts, at 120 volts a 5 amp load is 600 watts, same same? Not really, as resistance matters a lot more at the lower voltage. In a household circuit, 14ga single conductor 100ft long will handle 600 watts forever, but in a car you would find 12ga to 8ga multistrand wire to handle the same load depending on lengh of the wire and engineered margin of safety (resistance buildup at connections, lengh of wire, etc), the lower voltage has much more voltage drop through resistance, including wire lengh, than the higher voltage circuit.
If the deep cycle battery is 15 to 20 feet away from the alternator / engine battery, and won't ever be used to crank the engine, I would not use smaller than 8ga assuming a 55 amp alternator, 6ga if a larger alternator is fitted.
Additionally, in charging / cranking circuits, every connector should be properly soldered after crimping if you expect the circuit to last.
Forgive me if this isn't very clear, the back pain is extreme and I am medicated, still can't sleep

[ryanprins13]

Ryan we are getting into trouble here. You should look through the basics of electrical theory, and I'm not an electrical engineer, but we are talking about a 12V DC system and not discussing the "given that" parts that are constant.
I don't know how to cover this breifley, but I'll try.
How much power can a given wire handle? Depends on several factors, but we are talking POWER, electrically this is expressed as Watts (volts x amps = watts). At 12 volts a 50 amp load is 600 watts, at 120 volts a 5 amp load is 600 watts, same same? Not really, as resistance matters a lot more at the lower voltage. In a household circuit, 14ga single conductor 100ft long will handle 600 watts forever, but in a car you would find 12ga to 8ga multistrand wire to handle the same load depending on lengh of the wire and engineered margin of safety (resistance buildup at connections, lengh of wire, etc), the lower voltage has much more voltage drop through resistance, including wire lengh, than the higher voltage circuit.
If the deep cycle battery is 15 to 20 feet away from the alternator / engine battery, and won't ever be used to crank the engine, I would not use smaller than 8ga assuming a 55 amp alternator, 6ga if a larger alternator is fitted.
Additionally, in charging / cranking circuits, every connector should be properly soldered after crimping if you expect the circuit to last.
Forgive me if this isn't very clear, the back pain is extreme and I am medicated, still can't sleep

Ok, i will let you know what i know and tell me if im still wrong. The trailer battery will not be charged unless the car is running and the alternator does all the charging, the starting battery doesnt come into play. 14.4 volts at 50amps is 720 watts. 0.288ohms which doesnt mean anything to me. So, flat 4 wire with the insulation it has should be able to handle 20 amps per wire up to 50ft im told. Times 4 wires is 80 amps. I can get 4 wire 14 gauge in super thick insulation than can handle 25 amps per wire.
However like you said, were talking dc, not high voltage, we get huge line loss and i dont want heat. So, largest line loss i was willing to go with my inverter wires was 5%. Went with 1 awg wire, 9ft, worst case senario is 150 amps at 12volts which would be 2.8% loss which is 5 watts of heat, so not bad.
Now this would be 12.5 amps per wire at 25 feet minimum. Thats 10.9% loss which is 20 watts of heat per wire for a total of 80 watts of heat over 25ft of wire. A huge power loss, but not too much heat and the battery should charge quickly. Voltage drops from 14.4 to 12.8 volts.
8awg is exactly the same as that, 10.9% loss and 80 watts of heat.
6awg is a 6.8% drop, still huge, my 14.4 volts is now 13.5v.
Anything bigger than that is just rediculasly massive wire, expensive, not worth doing and im back to using one huge battery because it would be cheaper. 4awg wire is $120 to go 25ft and then i need a connecter at the car/trailer plus connectors at each battery. 2 batteries for $250, plus master switch or a big relay, total of like $470.
One battery for $330 plus flat 4 wiring and connectors is like $380. The biggest draw i would have at the trailer at any given time should be 34 amps at once, and that would be for under a minute, so flat 4 shoukd be fine for that.

Is there any way i can reduce the amperage of what goes through these wires?

Sent from my SM-G920W8 using Tapatalk

[ryanprins13]

Check your local welding supply
I got 00 welding leads for 1.12 a foot .
That's what I ran In my race car it's really flexible because it uses lots on very small strands rather then large ones like most battery cables.

Thats crazy, ill have to check around. I paid i think $5/ft for my 1awg welding wire to my inverter. Problem with any f that big stuff is the only nice quick disconnect plugs i see for wire that big is $50.

Sent from my SM-G920W8 using Tapatalk

[ryanprins13]

What about having a small battery in the trailer, only running 120v from my inverter to the trailer and having a 2 or 4 amp trickle charger on it? It would be highly inefficient but save on wiring.

Also what do regular 5th wheel holiday trailers use to charge their batteries? They have a single 14 or 12awg wire coming from the tow vehicle, thats it.
Some sort of current regulator at the batteries?

Sent from my SM-G920W8 using Tapatalk

[loudrango]

xs power batteries all the way

[Advancedynamix]

What load will the trailer battery see? Alternators don't see maximum amperage from charging a battery, unless there is a problem with the battery. The alternator is rated at 80A/14VDC in order to keep up with the electrical load that the vehicle itself is placing on the electrical system. Will you ever have an 80a load placed on the electrical system of the trailer (while the car is running?) Batteries typically charge between 2 to 16 amps. If you place an appropriate fuse, and Diode in line with your trailer circuit, that wiring won't ever see more than 20A.
The Diode protects the system from back feeding in the case of your car battery voltage being lower than your trailer battery voltage. Without this, you could put excess load on the circuit if you tried to start the car with a dead battery and the trailer hooked up. The Diode is a 1 way valve for electricity.
The fuse is there for obvious circuit protection.

[firebush357]

Do you have an Advance Auto Parts up there? If so you can order Batteries online and pick them up in the store using the discount code TRT30 for 30% off (Have to spend at least $50) & (Save up to $50 per order) If you are going to order 2 or more batteries just make multiple transactions. I saved $99 on my two Batteries for my truck.

[Dragonhealer]

What about having a small battery in the trailer, only running 120v from my inverter to the trailer and having a 2 or 4 amp trickle charger on it? It would be highly inefficient but save on wiring.

Also what do regular 5th wheel holiday trailers use to charge their batteries? They have a single 14 or 12awg wire coming from the tow vehicle, thats it.
Some sort of current regulator at the batteries?

Sent from my SM-G920W8 using Tapatalk

Don't know about the travel trailers, perhaps someone with experience can chime in.
I think you will be better off if the deep cycle is charged by the alternator, just my opinion.