Wiring Q: camper to truck: simple Q from electrical ignoramus

[olsen_karl]

I'm buying a Northstar truck camper to put in the bed of my GMC Sierra. The camper has a 7-way RV-type plug at its front end, to connect to the truck. The camper does not have taillights on it (i.e. it doesn't cover the truck's taillights), so I don't need to connect any turn signals, backup lights, etc. on the camper.

The only reason I need to connect the camper to the truck is to help maintain the battery charge, and perhaps recharge it some, while driving. The camper has its own 12V deep cycle battery inside, plus the usual RV-type power converter for 110V shore hookup.

I'd like to put a 7-way RV-type jack in the front end of the truck bed, inside the bed. Considering that all I need is a 12V hot and a ground, how should I wire this to the truck? My concern is that if I wire it directly to the truck battery, without any disconnect of any sort, I may drain the truck battery. Is there some sort of automatic isolator I can use?

I think I would prefer not to wire the camper into the existing 7-way plug at the trailer hitch on the truck, as the wiring for that is pretty thin, and running a long ways -- it seems like a better idea to run dedicating wiring for the camper up to the truck.

 

[dwh]

Yes, you need to isolate. The diode type isolators - the ones with the big heat sink that you can buy at any auto parts store - aren't good, because the voltage drops as it passes through the diode, so the aux battery always ends up a 1/2 volt or so short of being fully charged (.5v is actually pretty significant).

What I use and recommend is a simple "split-charge relay". It's just a solenoid that ties the batteries together when the key is on and isolates them when the key is off.

I've pasted this link enough, that I finally decided to bookmark it so I can find it faster. Looks like the price is down this week...

http://www.amazon.com/SOLENOID-GOLF..._2?s=automotive&ie=UTF8&qid=1311877071&sr=1-2


You wire the two big terminals to your two batteries, and one of the smaller terminals gets power from an ignition circuit (when the key is on "run" but NOT when the key is on "accessory" - that way you can park and listen to the radio in the cab of the truck but not have the batteries tied), and the other small terminal goes to negative (A.K.A. ground).

(Some solenoids like this only have 3 terminals. On those, the case of the solenoid is grounded, so they don't have a terminal for a ground wire. The one in the link has 4 terminals since the case is not grounded - i.e., it's suitable for mounting to a fiberglass boat bulkhead or a fiberglass golf cart body or anything else that might or might not be grounded.)


Use AT LEAST 10 gauge wire from the solenoid to the 7-pin, and stick a fuse or breaker in it to protect the wire in case of a short.

For a #10 wire you'd protect it with a 30a fuse or breaker. I like Bussmann ShortStop breakers. You can find them at any auto parts store. I prefer Type 2 "modified reset", which won't reset until power is removed from the circuit:

http://www.wiringproducts.com/contents/en-us/d60.html

 

[olsen_karl]

Thanks dwh - very helpful, and less expensive than I thought this might be.

I found a link to this when searching around this morning (through rv.net, IIRC), but seems a bit pricey:
http://www.powerstream.com/battery-isolator.htm

 

[bob91yj]

What size Sierra? The 3/4-1 tons are pre wired for a 5th wheel 7 pin in the bed. The wiring harness is usually wrapped up, tucked in the frame on the driver side, right at the split line between the cab and the truck. A word of caution, the factory 12v hot wire is hot with ignition off, and will drain your truck batteries, ask me how I found that out.

Some 1/2 tons have it as well.

 

[olsen_karl]

What size Sierra? The 3/4-1 tons are pre wired for a 5th wheel 7 pin in the bed. The wiring harness is usually wrapped up, tucked in the frame on the driver side, right at the split line between the cab and the truck. A word of caution, the factory 12v hot wire is hot with ignition off, and will drain your truck batteries, ask me how I found that out.

Some 1/2 tons have it as well.

It's a 2000 Sierra 2500, new body style, 4x4 extended cab longbed; the so-called light-duty frame and semi-floating 14-bolt, but with the heavy-duty package (brakes, cooling, etc.), 8-lug wheels. I bought it about 3 months ago and have been fixing up the little broken things, and periodic maintenance items. I decoded all the RPO codes in the glove box, and checked for that 5th wheel harness already -- the RPO codes don't include the camper wiring package, and the truck doesn't seem to have the harness coiled up on the frame rail.

The strange thing is, this truck has a bunch of holes drilled across the center of the bed, which I assume are mounts for a 5th wheel hitch. (No big center hole for a ball though -- just two rows of widely spaced holes, centered horizontally over the rear axle.) Maybe the former owner just routed the 5th wheel wiring up from the trailer hitch at the back of the truck.

 

[dwh]

Thanks dwh - very helpful, and less expensive than I thought this might be.

I found a link to this when searching around this morning (through rv.net, IIRC), but seems a bit pricey:
http://www.powerstream.com/battery-isolator.htm

The Powerstream isolator is a split-charge relay such as I described. The difference is that they add a small "black box" which controls when the solenoid gets energized.

I don't think that black box is really necessary, and in fact I don't much like it. It disconnects the batteries when the voltage drops below a certain point, and reconnects when the engine battery voltage gets up to a certain point.

I prefer to disconnect the battery whenever the key is off. I don't want to depend on a black box which could fail, and I don't see a need to have the batteries tied when the key is off, except perhaps if you are charging the aux and need to charge the main as well.

You can solve that by simply adding a switch powered from the engine battery to energize the solenoid manually when needed (very rarely).

 

[Basement Yeti]

I had a smart solenoid isolator, and like dwh, I did not like it at all and switched it for the constant duty solenoid he recommended.

The 'smart' part of mine actually failed and was keeping the batteries connected 24/7. It caused the two batteries to connect and try and equalize with each other which ran my chassis battery below the point it could start my engine.

The constant duty solenoid works a treat.

 

[Bogo]

I'd forget the 7-way connector as it can't handle the amps if you connect up the starting battery to a nearly dead camper battery. Instead I'd do a standard isolated dual battery setup with an Anderson connector for connecting up the camper battery when the camper is in the bed of the pickup. Use 2 AWG or 4 AWG wiring to connect both the ground and positive poles. Have a fuse or circuit breaker on each end of the positive wire connecting the batteries. In line in that wire, possibly under the hood, have the isolator hardware.

A lower cost way to get the wires is scavenging them from jumper cables. Buy or borrow a crimper to crimp on the lug ends. Don't solder. Use heat shrink tubing with weather sealing glue to keep moisture out of the wires.

The negative or ground wire will go from battery negative, to truck side Anderson connector negative pole. A second one will go from camper side Anderson connector negative pole to battery negative. No fuses are needed in it because it should always be at the chassis ground potential. The reason to have it is for a solid return path for the potentially high ground currents that can happen when connecting two batteries with different charge states.

The positive wiring is more complex and depends on the isolator selected. I will explain for a plain solenoid. Most won't be much different. There will be a short jumper from the battery to the fuse block. A second wire will go from the fuse block's other terminal to the isolator truck battery positive terminal. A third wire will go from the isolator house battery positive terminal to the truck side Anderson connector positive terminal. a 4th wire will go from the camper side Anderson connector positive terminal to the fuse block by the camper's battery (house battery). From the other pole of the fuse block goes a short jumper wire to the battery positive terminal. If you are using circuit breakers, substitute "circuit breaker" for "fuse block" in the above paragraph.

As for how the isolator hooks up, who knows. That depends on which one you get. For a simple solenoid type isolator, there will be a wire that goes to the engine run circuit so it turns on when the engine is running. Unfortunately the engine is running when you are trying to start it. I prefer a bit of logic to prevent that. In fact there are commonly available ones that will only turn it on when the engine alternator is outputting and the voltage is over a set voltage.

Look at the battery separators here: http://order.waytekwire.com/products2/M50/75/200/0/1/Battery Separators/Battery Management Products/ The interesting thing with the separators is if you get a bidirectional one and put a solar setup on the camper. When it has the camper battery fully charged, the bidirectional separator will hook up the starting battery and charge it up too.

Put wire looms around all wires so the wire looms take up the abrasion, not the wire jackets. With big power wires I individually loom them.

 

[Bogo]

On solenoid type battery separators and isolators. All of them will eventually weld or carbonize their contacts. This usually happens because to light of duty one was used or it has plain warn out. What do you do, replace it with a new one. The alternative is to pay allot more and get an all solid state one. They too can get overloaded and may even fail on. Diode based isolators, while totally passive, put a diode drop between the alternator and the battery so the battery never gets fully charged.

My favorite idea is to forgo all isolators and just put in a second alternator and have it only charge the second battery bank. The main alternator just charges the starting battery, runs the engine and other normal truck loads like it did originally. Second alternator mounting hardware is available for your truck as both OEM, and from secondary suppliers. I don't like this method for your situation because the second battery isn't always hooked up. Running an alternator without a load is seriously sub optimal. If you moved the second battery to the truck, and then ran the wiring from it to the camper shell, it would work. This means the camper shell wouldn't have power without the truck present. You might not want that. If you have an alternator failure in a remote place, well, you have two. Swap the wires on the back of them and go...

 

[olsen_karl]

Thanks everyone - I'm leaning towards going the route suggested by dwh, with that solenoid.

Bogo - this is a relatively budget project, so I'm leaving the stock single alternator setup. I plan on charging the camper battery at home with a standalone charger plugged into 110V -- this is what I did with the prior camper, which was a pop-up trailer, and it worked fine.

I'll think about changing the wiring to use 2 or 4 gauge jumper cables with weather-sealed connectors. That's a nice idea, and wouldn't be hard to do.

I haven't picked up the camper yet. I'm retrieving it next weekend, but am in the planning stage for getting it onto (and connected to) the truck. The seller says the battery may be OK, or may be dead; he hasn't used it in awhile. If it's dead, I'm considering going to two 6V Costco golf cart batteries in series, as the camper has enough space for the larger battery setup.

 

[dwh]

I'd forget the 7-way connector as it can't handle the amps if you connect up the starting battery to a nearly dead camper battery.

What amps?

http://www.smartgauge.co.uk/nosurge2.html

 

[Bogo]

What amps?

http://www.smartgauge.co.uk/nosurge2.html

Run the numbers when the dead battery is at 8 to 10 volts which is entirely possible if a furnace has run the battery down over night.

 

[dwh]

Run the numbers when the dead battery is at 8 to 10 volts which is entirely possible if a furnace has run the battery down over night.

Yea, that link I posted HAS the numbers. That's why I posted it.

At 8-10 volts the resistance of the low battery would be so high you'd be lucky to see any amperage flow at all - much less any big surge.



Quoting that article:

"Further, the internal resistance of a deep cycle battery increases as the charge status falls. So a flat battery will have a much higher internal resistance than a fully charged battery (this is why a very flat battery takes such a long time before it starts to accept a charge - an effect we have all seen)."



I can ask has anyone here ever hooked up a normal constant voltage charger (typical automotive charger) to a dead battery? How much current flows through that battery? Nada. It can take a long while before the battery voltage comes up (internal resistance drops) to where it will begin allowing even 1 amp of current to flow through it.

With any charging system, the amperage flow of the circuit depends on the supply voltage vs. the resistance. The higher the supply voltage, the more current (amps) will overcome the resistance of the battery and flow through it.

I have a 100ah deep cycle FLA aux battery, and a 10a Shauer constant voltage charger in my camper. I run that battery dead all the time and then recharge it off the Shauer and a small generator.

It takes a couple of hours for the amp flow to even get up to around 6a, and that's about as high as it ever goes.

Not because the charger can't supply more - it can do 10a - but because the 12.6v that charger supplies is simply not a high enough voltage to overcome the resistance of the deep cycle battery *enough* to get more amps to flow.

(Which is why I plan to eventually swap it out for a 15a constant *current* charger that will raise its supply voltage however high it needs to be until it reaches a full 15a flow rate.)

Same thing happens battery->battery.

There is no big surge because the voltage differential isn't high enough to overcome the resistance of the low battery by very much - so not many amps (if any) flow though the circuit.



(This same effect is why many so-called "smart chargers" won't charge a totally dead Optima. The internal resistance of an Optima is lower than a normal battery, so when it goes dead, it goes down to a lower voltage - at which point it actually has a *higher* resistance than a normal battery. When the smart charger sees that no current is flowing, it shuts down.

That's why you have to "trick" some chargers to charge a dead Optima - they have to see *some* current flowing or they shut down.

My old dumb Shauer will also shut down - but it just trips an internal auto-reset breaker and tries again after the breaker cools off and resets. It'll keep doing that, giving a few watts to the dead battery each time, until the dead battery finally comes up enough that a noticeable current begins to flow, and the breaker then stops tripping and the battery starts to actually get charged.)




And...

"We're not quite finished yet. The proponents of this false rumour usually state "one full battery and one flat battery" so that is the example I based the above on. They are obviously unaware that this state of charge does not cause the greatest current to flow.

The worst scenario actually occurs with a fully charged engine start battery and the auxiliary battery bank at around 40% charge state. The reason is that, although the terminal voltage falls as the battery approaches totally flat, the internal resistance rises. At 40% state of charge the internal resistance is considerably lower. The result in the above example would be that the initial current surge would be around 40 amps, and even that would only last a minute or so (thus discharging the engine battery by 40A * 1/60th hour = 0.7 amp hours) at the most until the surface charge on the auxiliary bank raised it's terminal voltage to that of the engine start battery. Once this had happened, the current would be negligible."

 

[Bogo]

Take a look at a specification sheet for a modern deep cycle battery. If you care to keep the blinders on, don't even seek out the internal resistance curves for AGM or Gel type batteries or for that matter any battery made by Concorde. The internal resistance in a modern design deep cycle lead acid battery only goes up a few percent from 12V to 10V. It is only when the cell voltage has dropped way down to 3 or 4 Volts that the internal resistance gets greater than 2X of the fully charged amount. Of course temperature changes this, and the warmer it is, the less the internal resistance rises at low charge voltages. I used to have internal resistance curves for Trojan deep cycle batteries, but I can't seam to find them. May have had it in print form and thus tossed. Odyssey has resistance curves for their AGM lines on the web, but not deep cycle lines. If you want to find the internal resistance numbers, look for batteries that meet military specifications. They aren't published for most consumer batteries.

BTW, there are a few errors and omissions with the SmartGauge writeup. Are you only going to be using it at 0F? CCA is when it is bloody cold out, ie Cold Cranking Amps. Lead acid batteries output higher currents when warm, like even at room temperature. Roughly 40% to 45% of capacity is lost from 70F to 0F. So their 1500CCA is more like 2500 warm cranking amps when it is 70F out. That means the resistance of their engine cranking battery is lower than they calculated. "Dead Band" voltage only governs when charging occurs. At voltage differences as little as 0.01V current will still flow, and it will flow at the rate governed by the sum of the inline resistances time voltage difference. This is why paralleling of different types of batteries or batteries of different use ages is bad. The lower voltage/weaker battery will drain the higher voltage/stronger one. The saving grace for this is it only applies when the batteries are at rest. A small draw or charge current negates this issue. Rest voltages are surface voltages and only influence initial current rates. Current rates will rapidly change to ones based on actual charge state voltages. At rest also assumes the discharged battery has been allowed to rest. Most likely the user won't cut absolutely all loads, then wait and hour and instead will just start the engine to charge the battery back up. What happens then? The voltage difference will be much higher than the 0.6V they say. They also don't properly take into account paralleled house batteries. For internal resistance, paralleling works the same as for resistors. resistance total = 1/(1/R1+1/R2+1/R3...) So, two paralleled 0.02 Ohm batteries ends up with 0.01 Ohm resistance, and three is 0.0067 Ohms. So for their calculation substituting 2 0.02 ohm batteries into the dead battery pack means a total ohms of 0.012 for 50 Amps current. That is just for battery to battery transfer. We haven't added in what will come from the alternator after the engine starts or for the fact that the 0.3V shouldn't have been subtracted out.

Another thing is the flat contacts in the RV-7 connector are at most 40 Amp rated, and I'm not sure they are that high. 30 Amps may be more like it. The round pin contacts on a semi trailer are only 40 Amps and they are more substantial.

 

[olsen_karl]

For anyone else looking into this, I found a schematic that covers this:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/18595593.cfm

This will help my wiring project, and gives me some ideas for wire routing -- the pics in the rv.net link are of the exact same engine I have in my GMC.

Thanks again for all the input.