Upgrading 12v charging to camper

[noteven]

I have a diode isolator under the hood on my camper truck, with battery cables run through a marine circuit breaker to an Anderson quick connector. On my previous truck camper the battery compartment was near the front on the driver side. I disconnected the 12v wire in the camper connecting 7 way cord and run the cables directly to the camper batteries. This setup supplied sufficient amps  to operate the fridge on DC when driving and keep the batteries charged.

New camper is a Cirrus 820. The fridge will not stay operating on LP when driving, and the DC mode puts a strain on the OEM DC arrangement. It has the batteries in the right lower rear, as far away from the alternator as possible. 😀 The converter is in the step to the bed overhead.

Should I run new cables from the isolator all the way to the batteries, or can I connect at the converter?

I’m thinking cable to the batteries...

Edited September 8, 2019 by noteven

[Kirk W]

Is your new camper on the same truck as the previous one? I ask because the amp rating of the truck alternator can plan a part in this issue if it isn't hight enough capacity. Second question is if you normally remove the camper from truck? If you don't then I'd just run a new power supply to the camper using a welding cable as a lead. I would also install an isolation relay to open when the truck isn't running and so prevent the truck running the truck's battery down. A good heavy relay will have far less voltage drop across it than does a blocking diode. 

[noteven]

Kirk - truck 1 has a 136amp alternator (Dodge diesel) and the diode system on it.

Truck 2 has 150 amp alternator (I think) F350 6.2 gasohol and no isolator equipment - yet.

I like your idea to use a mechanical  relay.

Yes the camper is easily unloaded so I do that as necessary.

[oldjohnt]

Good questions, here's my take:

First of all, powering a 3 way fridge using DC power requires hefty current, so the trucks alternator has to have sufficient capacity to power and maintain the truck plus run the fridge.

Even though the trucks alternator can supply a degree of charging amps into the campers battery bank (assuming you have such), due to cable voltage drop and the alternators regulation scheme its never going to supply the quantity, especially the smart quality of charge a Smart 3/4 Stage Converter/Charger, although, sure if alls up to par it can power the fridge and maintain the trucks engine battery as you would like when driving. Your campers converter/charger can then take care once you're plugged to shore power

The BIGGEST PROBLEM is the line voltage drop which necessitates large cables and low resistance connections and isolation devices. Obviously you cant do this using the normal 7 pole plug so your idea of heavy cables, high current connectors and an isolator can get the job done.   I have used both solid state isolators (basically diodes) as well as pure mechanical relay type. The thing is a typical diode has a 0.6 voltage drop  across it while a high current rated quality relay isolator has far less UP UNTIL ITS CONTACTS DETERIORATE (may or may not ever do so ??) that is. The mechanical relay isolators require 100% duty cycle components IE dont just think you can stick a Ford starter relay on there. As you're aware the isolator allows the trucks charging system to deliver amps back to the camper but NOT let the camper use up your trucks starting battery. A mechanical relay type isolator closes when the trucks engine is running but opens otherwise while a solid state diode isolator remains in the circuit and acts as a one way current flow check valve.

PS Just saw your update. A 150 Amp alternator stands a good chance of powering the camper fridge while driving plus maintain the engine battery.  If you use big enough heavy (use fine stranded such as welding cable for improved flexibility and more vibration tolerance) cables,,,,,,,,,,heavy duty high quality connectors,,,,,,,,,,and a quality isolator you should be good to go.

Your decision is to use a solid state isolator or a mechanical relay. While there's the 0.6 voltage drop issue in a solid state, once installed they require no maintenance or worry with switching, so simple to wire and install, and can last indefinitely (seldom go bad), while a mechanical device requires switching circuitry and they "can" with age develop contact resistance, although if quality and high current rated can last a longgggggggg time. Sooooooooooo choose a good quality high enough current capacity unit regardless if you go mechanical or solid state WE CANT MAKE THAT CHOICE FOR YOU do your homework and make an informed decision. I never really had trouble with either type but do know the mechanicals can "possibly" with age and time develop contact resistance and may have more voltage drop then a diode, especially if they are poor quality or are rated too low for your use GET A GOOD ONE with more capacity then actually required is my advice and you should be just fine !!!!!!!!!

 

 Best wishes, take care

John T          

Edited September 8, 2019 by oldjohnt

[noteven]

Kirk & John - truck 1 has a 136amp alternator (Dodge diesel) and the diode system on it. It’s cables to the camper are at least as heavy as the battery cables. With the engine running I measure 14.7 volts out at the isolator, and 14.6 at the camper batteries.

I was thinking I would use the same gauge wiring etc on Truck 2. Previous campers fridge with through roof top vent would stay lit on propane while driving. New camper will not. It has sidewall upper vent. New camper has 170watts of Zamp solar equipment. It all works fine for my little bit of energy needs when boondocking. I don’t like pummeling the batteries with low voltage while driving. It also forgets to switch to propane when you park not on shore power when the fridge is left in auto mode. I think it senses some last bit of surface charging from the truck and says “I can stay on DC”. If you are on shore power and disconnect it auto switches to propane. This I discovered one -20F evening Walmart docking in Helena when I had brain flatulence and forgot to switch fridge to propane and my heating system shut down at 10.5 volts like it is supposed to. Tracked some snow in on the door mat that had melted at bedtime. It were re fwozen at 0 4 hundred the next morning 😯. Lit the fires in the Cummins and everthin come back to life. In Alberta we call this “the freeze and learn method” 😀.

I will probly wire to the batteries. This camper has removable insulated belly panels that gives access to all the tanks and etc in the basement so I can make a proper job of it.

Thanks

Edited September 9, 2019 by noteven

[oldjohnt]

11 hours ago, noteven said:

With the engine running I measure 14.7 volts out at the isolator, and 14.6 at the camper batteries.

If the trucks engine battery charging system is raising the camper batteries to 14.6, nearly same as at the truck, that's great so your cables must be big enough !!!!!!!!!!!!!! I'm a bit surprised voltage is that high at the camper ??? have a good voltmeter??  but it depends on the loads and resistance. I have no idea of the specs physics and electronics in modern solid state isolators (must be better then what I used in years past) but with results that good Id at least consider use of the latest technology solid state isolators in the new unit.  

11 hours ago, noteven said:

New camper has 170watts of Zamp solar equipment. It all works fine for my little bit of energy needs when boondocking.

If the solar is left on when driving that's yet another 170 watts to run the fridge on DC plus maintain the batteries GOOD TO GO !!

11 hours ago, noteven said:

I will probly wire to the batteries. This camper has removable insulated belly panels that gives access to all the tanks and etc in the basement so I can make a proper job of it

Don't forget catastrophic over current protection. (you mentioned marine circuit breaker which sounds fine). With such huge batteries and big cables short circuit current could be hundreds and hundreds of amps YIKES.  As previously discussed use adequate connectors which you seem to have covered. 

If the auto switching isn't doing what you like, I guess you're stuck with manual changeover from DC to LP. 

Congratulations on the use of big cables and isolation, you're doing this RIGHT ……………………..Thanks for the feedback and update also. 

John T

    

Edited September 9, 2019 by oldjohnt

[Kirk W]

13 hours ago, noteven said:

It also forgets to switch to propane when you park not on shore power when the fridge is left in auto mode.

I don't think that this is caused by batteries as the auto mode should use 120V as the preferred energy source. It has been a long time since I last worked with a 3 way refridgerator, but I am pretty sure that part is the same as with 2 way and what I had in my 3 way for our popup back in the 80's. In that one it would shift to shore power from either source if available. I think that you have 2 issues.