I posted something similar in the trailer section but I thought I would ask in here. I have a Haulmark toy hauler trailer that I have installed 2-93amp/hr deep cycle batteries in. I have upgraded the wire from the 7pin harness to 8 guage to the batteries in the trailer. What determines how many amps my truck will give to the batteries for charging while running and if I installed 2 more batteries would that overload the ?30amp battery charging circuit. What would happen if all 4 batteries were very discharged, will they demand more amps?

Also, what is the purpose of a fuse and a relay in the battery charge circuit and what is the guage of the wire that goes to my 7 pin trailer wiring harness in the truck.

I assume that since my truck is a Lariat it has the highest amp factory alternator. It also has the trailer tow package.

I sure wish DC power was as easy to understand as AC, I can wire a house but this still confuses me.

 

First you need to get some DC basics down.
In a battery charge circuit, the difference between the battery voltage and the charge circuit regulator control determines the charge current value.
That being said, for example a battery that is discharged or has less than a full charge will accept current from the charge circuit and taper off as it's voltage rises up to the regulation setting of the alternator regulator, then 'float' at that value.
Said another way, in terms of resistance, the battery is a very low impedence device such that when in a discharges state, it's internal resistance goes down hence it presents a high load to the charging circuit and causing large current flow until it's internal resistance rises. The resistace (impedence) is in terms of tenths of an Ohm resistance as a load.
Going on to your truck application, the alternator has a certain safe limit for output. For example if the alternator rating is 100 amps or (1500 watts) it means it will charge at that rate for the "reasonably expected time" the truck's battery should take to come up and taper off the charge rate but not continiously due to possible overheating of the alternator.
What this means is you cannot just add batteries and in a discharged state, and expect the alternator to deliver all the charge current they will take without possible consequences. The charge circuit was not designed to do this.
Some ways to solve the application problem:
The extra battery charging could be done thru a resistance to keep the charge current within the alternator's rating. This take longer in terms of time. Don't discharge all the batteries at once but charge them singlely.
Small generator, seperate alternator wth the same considerations, upgrade truck alternator to one like is used on and emergency vehichles.

Of course the charge lead to the batteries must be heavey enough not to overheat and cause to much volrage drop or it become a series charge limiting resistance.
There's no way to get around electrical law for short cuts, cost and application. Power = voltage X current and all the other laws that never change and must be adhered to for sizing, fuseing etc.
It's not any different for AC circuits because all the same applies except for the alternating componant that has it's effects.
Good luck.

 

Bluegrass, thanks for all your help! It makes a little more sense now.

When I said I understand AC it does not really mean I understand electricty, I just know how to make AC work, properly withing limits and building code.
14g = 15 amps, 12g = 20 amps, 10g = 30 amps, 8g = 45 amps. Amps x volts = watts. My guest 2611A battery charger that I have installed in my trailer uses 2.5 amps, therfore I could hook up 6 of them (hypothetically) to a 15 amp breaker on 14g wire. It does however have 2-12V 5 amp outputs. My trailer has a 30 amp/120V panel. I hook up 2-1500 watt heaters which are 12.5 amps each that makes 5 amps left over or 6-100 watt light bulbs totaling 3600 watts. AC is no sweat. I guess the problem I am having is that the amp draw from a 60%charged battery draws more power then a 95% charged battery. Whenever I do something AC it is a constant amount (except for startup on some items). DC battery charging it appears is not a constant and I cannot figure the "max" from a discharged battery.

If I had 4-60% discharged batteries in the trailer and the truck is running would it blow the 30 amp fuse or just charge the batteries slower? Is there some formula to use for battery charging? I believe the charge circuit to the 7 pin harness will not allow anymore then 30 amps so overheating the alternator shouldn't be a problem. I don't plan on killing the batteries that often as I do have shore power 80% of the time. I don't plan on using the truck as the main charging unit for the batteries. The batteries in the trailer are hooked up parallel so charging one at a time would be difficult.

Am I correct in this assumption. I hook up a continuous 1500 watt inverter in the trailer (no truck connection) I have 2-93 amp/hr batteries totalling 186 amp/hrs. 1500 watts would use 125 amp/hrs (watts/volts) so depending on the shutdown voltage of the inverter (10V) my 2 batteries would last about 1hr 20 min making 1500 watts of power. I am going to try that this weekend just to test my theories.

Once again bluegrass, thanks for all your help and anybody else that could shed some light would be great.

 

You could use knife switches to cut some batteries out of the curcuit till the others charge. That would limit max charge current. I ran a #10 2 wire power cord from my truck battery along the frame to the hitch in the rear. Then used a twist lock plug there and wired direct to trailer battery. I didn't even use my truck charge circuit. I used a 40 amp circuit breaker near the truck battery.

Worked fine for years. I keep a dummy receptical on the hot plug to keep from shorting.