Building a Heavy Duty Charging Circuit for Camper--how big?

[twodownzero]

So I discovered my camper isn't charging when I'm driving and diagnosed why: my charge wire fuse was burned from God knows when, and my solar panel connector was corroded to the point of not working at all. I'm ordering parts now.

In the meantime, I want to make a new charging circuit for my camper. Why? Two reasons. The stock ampacity is******(20 amp fuse, likely far less than that in the real world) and the worst part is that it's hot all the time, so my camper would draw from my truck batteries and kill them. So as to avoid that issue, I'd like to upgrade.

Looking online, many have built 100+ amp charging circuits with 2 AWG wire. This strikes me as overkill but I'm open minded enough to reconsider that thought if someone can convince me. I do understand that voltage drop is an issue with only 12-14 volts and so I can be persuaded, however, it is difficult to splice wires as they increase in size so I don't want to go nuts.

I plan to alleviate the "hot all the time" problem by using a continuous duty solenoid and by using a relay to close that solenoid so as to protect whatever circuit I steal the key-on 12 volts from. The solenoid will be rated for much more than I plan to run wire for, so the real question is determining what sort of ampacity this circuit ought to have. I will fuse the wire at the appropriate ampacity which will protect the solenoid and the wire running down the frame to the camper.

So how big of a wire do I run? The camper's charge wire is 10 AWG, I believe. I am not going to install a separate plug for the camper so the 7 wire plug in the bed is what will be being used to charge the battery.

I'm inclined to use a 40 amp fuse and run 8 awg wire. This seems small but is probably enough for what I'm expecting to do. My refrigerator will NOT run on propane when driving; it blows out. So I need to have enough current capability to get some positive charge into the battery and run the fridge.

Is a 40 amp circuit enough? The factory circuit in my truck is only 20 amps and so this seems like a significant upgrade, but by the time I run the wire all the way down the frame, jumping from 8 to 4 or even 2 awg isn't a huge expense and I could go bigger.

If I go larger than 8 awg, though, I face another issue. My truck has two 7 pin connectors--the factory one at the bumper and a secondary one I added in the bed. I plan to cut the charge wire that goes to the secondary one in the bed and hook up the new charging circuit only to that wire, so that I don't cut any factory wires and so that my charging circuit is only used for the truck camper. If I use a wire larger than 8 awg, I think I will have a hard time splicing that into the 8-10 awg wire that is existing. What kind of junction block is available to make this transition cleanly? I'm not against running a larger wire to reduce voltage drop, but if I can't make a clean connection at the rear where I hook it up, all may be lost that is gained by the larger conductors.

I know I've written a lot so here's the Reader's Digest:

1. I want to run an ignition only charge circuit for camper
2. I want to use big wire and fuse it appropriately but I don't want to go complete overkill.
3. I want to know how to hook up whatever big wire I end up using at the camper end because splicing a 2 awg wire to an 8 awg one is not easy.

Thanks all!

[dcsparky]

To deal with the issue of fully charging my golf cart batteries, I installed a DC to DC charger with a 2awg welding cable from truck batteries. Installed a quick disconnect like the ones used on fork lift.

The unit is the CTEK DS250 dual with the smart charger. This combination allows me quickly charge low house batteries when the truck batteries are fully charged by adjusting the output voltage based on the state of the batteries. It will back off the charge automatically as the batteries approach a full charge.

When I first installed the charger the house batteries where at around 60% charge and my truck alternator was putting out 60A to house batteries. The output is automatically adjusted as the house batteries % increase. The other benefit is that the voltage output is adjusted based on the state of charge of house batteries independently of the truck batteries.

Its also a battery Isolator with a built in 20A solar charge if you have solar panels. And the solar charger will also charge your truck batteries when the house batteries are charged.

My converter only charges at 30 to 35 Amps so charging with a generator will actually take longer. I rarely take generator along. By the time we arrive at the campsite batteries are at full charge.

[stevenal]

smkettner wrote:
I recommend a relay similar to this:
Blue Sea automatic relay
Shop around or find a lower priced brand.

Lower price brand.

[twodownzero]

I forgot to update this thread. I did in fact run this wire. I ended up using a maxi fuse (50 amp) because the breakers were expensive and the fuses that are feeding my stereo in my trucks have been there for a decade or more and are still going strong. If I blow the maxi fuse somehow, it's easy to get a replacement as well.

I went with 8 AWG wire. Any bigger than that and the price went up pretty fast and the 8 can handle enough to do the job.

I soldered on all of the lugs and the splice, although I agree that the solder point between the 8 AWG and 10 AWG at the camper connector was a pain. fortunately I have a pretty good butane torch and was able to get it hot enough.

I used a golf cart continuous duty solenoid. It looks a lot like a ford solenoid although it has a separate ground terminal rather than grounding through the mounts.

I believe the wire I used to trigger the solenoid to close is the cigarette lighter circuit that comes on with the key.

Between that and fixing my small solar panel I look forward to NOT! dead batteries when I arrive at my next destination to camp.

[time2roll]

Here is some #8 wire to compare

I recommend a relay similar to this:
Blue Sea automatic relay
Shop around or find a lower priced brand.

To step down to #10 I recommend a regular butt splice over solder.
Step-down heat-shrink butt-splice 8 to 10-12 (local supplier is probably best)

[twodownzero]

My truck has a 140 amp alternator and my camper only has one battery and no room for more batteries. I think I'm going to take the advice and go with 8 awg and a 50 amp breaker. I can easily solder the 8 awg wire to the existing 10 awg that goes to the outlet in the bed and the ability to even get a 10 amp charge rate while going down the road should do just nicely.

I will also look into getting my propane to stop blowing out when going down the road but even then, a 50 amp circuit running to the camper ought to work out nicely.

[AnEv942]

Our Dometic RM3663 12v heater element is 235 watt, or 16-19 amps depending on voltage. Some are larger, many smaller.

On a TC refer I would verify what DC element your trying to power.
30 amp draw, assuming your only charging at 12.5v, would be like a 375 watt element? A normal charge rate though @13+v would need like 400 watt heater to get a 30 amp draw?
Obviously there are other needs (battery charge etc) but, usually nice to start with real numbers...

IF your going to add an auxiliary charge line my first recommendation is reconsidering adding a dedicated additional plug. That is where your limiting yourself from the get-go. Plus you need the equal size ground.
The second relay to energize solenoid not needed-fuse on key-on source sufficient. Could though its yours, just an additional draw, albeit small. The ability to disengage solenoid/aux charge line, (switch on energizing feed) is a good idea.

[HMS_Beagle]

bb_94401 wrote:


Vi = 14.5 at your alternator; d = 40 feet (20' out and 20' back); 8 AWG R = 0.0067056; I = 30A

Vo = 13.7 VDC (the typical float voltage @ 50F) little to no charging occurring

To achieve a 1% voltage drop and get Vo = 14.4 to get near a typical adsorption phase charging voltage, limits the current to 5A in 8 AWG wire.

Little wonder that very little charging goes on with 10 AWG wire, given the length of run when you combine the truck and TC wiring.

A flooded LA battery will charge at 13.7, just not as fast as 14.4. And if it is drawing the 30 amps causing the voltage drop, then it is charging at - you guessed it, 30A. That is why I think absent other problems, #8 is OK for a normally sized battery bank (200 AH). If you have 400 AH and want it charged quickly, then #4 or #6 would be better.

However if you are already drawing 30A for your fridge, then you are starting at 13.7 and dropping further to the extent that the battery is charging (and a battery at 50% charge will draw a lot, even at 13.7V). That is why I say fix the fridge problem so you don't have to run it on 12V. That makes everything more complicated.

[MikeJinCO]

PianoTuner has good thoughts so read his carefully. Some very random thoughts, I bought a liftgate plug, it is one big chunk of hardware. I ended up using a Anderson Connector. I'm using #4 with a Carquest "85" amp continuous duty relay that really only carries 65 amps-Cole Hersee is the brand, I will carry a spare, but then going to South America. On my '99 Ford I couldn't get a fuse tap to work with the mini fuses so had to put in a relay as the only circuit tapped into was a main motor control circuit and I didn't want to add more load to it. On our GMC the wiper motor circuit near the drivers door is a good one to tap. The other day with the inverter running the microwave(about 115 amps) I cranked up the motor and the alternator was sending over 80 amps to the batteries-last time for that experiment. With my 115/130 amp(not sure)alternator, 4 gp24 deep cycle batteries at 40% and the motor at 1500 rpm it was sending back about 45 amps. Bought most of the stuff on EBAY. Blue Sea Systems has good info and electrical equipment for the expensive yacht trade,

[pianotuna]

Hi,

Another issue is the duty cycle of the alternator. It would be unfortunate to overload it to the point where it failed. That is why I added switches to control the solenoid(s) manually.

[bb_94401]

It will take time to install your solenoid / isolator (0.6 VDC voltage drop) /relay, fuses and then run the wire in split loom for protection. If you have a single battery, without much Ah capacity 10 AWG or 8 AWG that some recommend makes sense. But for bigger battery banks, 8- 10 AWG is inadequate.

Why is there always time to redo and fix the problem with larger wire and dedicated connectors a second time, than do it right the first time?

While it is possible to run some of your 12 VDC devices in the TC as you travel, the real issue is that the length of wire to the batteries from the alternator and back will cause a voltage drop as you try to push current through it. Measure your runs and do the math with your desired charging current to determine the size of wire you need and the amount of voltage drop you will tolerate.

Vo = Vi - ((d/10) * I * R)

Vi= Voltage in at power source (alternator)
d = length of wire (source to battery and back)
I = current in A
R = resistance in Ohms per 10 feet of wire
Vo = voltage out at TC batteries

10 AWG R = 0.0103632
8 AWG R = 0.0067056
6 AWG R = 0.004572
4 AWG R = 0.0024384
2 AWG R = 0.001524

Vi = 14.5 at your alternator; d = 40 feet (20' out and 20' back); 8 AWG R = 0.0067056; I = 30A

Vo = 13.7 VDC (the typical float voltage @ 50F) little to no charging occurring

To achieve a 1% voltage drop and get Vo = 14.4 to get near a typical adsorption phase charging voltage, limits the current to 5A in 8 AWG wire.

Little wonder that very little charging goes on with 10 AWG wire, given the length of run when you combine the truck and TC wiring.



Supplies:

An online place for insulators, power posts, large fuses/holders and terminal can be found at VTE Warehouse

Most battery only stores, are a good source for terminal lugs, Anderson power pole connectors and flexible wire cable (made by stranding 30 AWG or 34 AWG wire, not the cheap Type II SAE battery cables that use 22 AWG for stranding the cables). They also will swage the lugs onto the cable and shrink wrap it.

Marine supply companies also are a good source for bank select switches, disconnect switches, fuses and other 12 VDC system components (3 stage charging external regulators for 2nd alternator systems.

Solar power companies are also a good source for components.

[ab257]

Is there anywhere with a DIY wiring diagram and comprehensive parts list with vendor for a separate charging circuit? Including isolator, cable to connector in truck bed, connector, cable to camper batteries, fuse, etc? It would be great to have a standard starting point.

[time2roll]

#8 is fine, I would go up to 50 amp fuse.

Use a bidirectional relay to control power. When voltage on either side indicates charging the relay closes. This will send solar power up to keep the start battery charged and when the alternator is running send power back to the TC house battery.

Try cleaning the fridge burner. Also add some screening or home furnace filter media to reduce the wind effect.

[HMS_Beagle]

One of your major problems is trying to run the refer (20-30 amps) while driving. Whatever else you do you should also fix that. Try adjusting the metal shield around the burner to eliminate gaps. On my last camper I had to tape up a few of the vent holes near the burner in the outside cover. This requires some experimentation, there is probably an aggressive eddy entering the cover somewhere.

Very heavy wire from the truck will make only a small difference in recharge times, because lead acid batteries will only absorb very large currents for a short period. Something like #8 (and connectors to match) will do most of what can be done. If you go to #2 you may get more current for the first 1/2 hour or so.

[bukzin]

Great ideas.

Keep them coming!