Hoping you all can help me work through this...I have my house/camper/aux battery under my pickup hood in the spare bay wired to my starter battery through an isolator. Works awesome. I then run 12 gauge back under the truck and hook into my 12v camper system. The battery is not connected to the camper's internal converter at all since my truck is in charge of... charging, but all my internal camper 12v- is connected to the negative of the battery directly with a wire spinner. All my 12v+ internals are wire spun together also. It works, but... What I'm seeing is massive voltage drop when using my water pump/furnace. Furnace sail switch won't even trip. Voltmeter shows down to ~9v when the water pump is finalizing it's pressurization. Yoikes. I have a couple theories...1. 30yr old wiring/massive resistance internally in the camper? Sure looks okay to me, but I'm not seeing the guts.2. Too many grounds?? I have the ground/- wire from the battery mixed in with camper frame ground wires... is that bad? I can't see why really, but there is that. Can't I just get a negative bus bar fuse block mounted in the camper and connect the aux battery from under the hood right to that and then right into my 12v circuits? Would I then even need a camper ground at all?3. 12 gauge too small for the 16' run from under the hood to the back of the camper? I've pumped a lot of amps through that kinda distance before so I don't think that's it, but maybe.Thoughts? Those are the only three things I can think of. Gonna start testing stuff. I NEED my furnace sail switch to throw or the main burner doesn't ignite and we've been camping to 20 degrees at night already... in July haha.

PS - Brand new deep cycle Napa battery...

I think 12 gauge is too small. There are a lot of charts that show what size wire to use for the amps.

jimh425 wrote:I think 12 gauge is too small. There are a lot of charts that show what size wire to use for the amps.I was suspect of this being the problem because my max amp draw is the 7.5a water pump... furnace is only 2.9a... and 16 feet isn't that long, but I'll check into it. All my fuses are 15a and none of them has ever blown. That's why I think it's mis-wire issue, but I'll go to 8-10 gauge anyway probably... as long as it's flexible since I'm running along the truck frame. Thanks for the reply!

You need #8 at minimum for good service. With low voltages, you need large sized cables to serve without major voltage drop. You also need that on both sides, going back to the battery, don't use the truck frame. http://www.powerstream.com/Wire_Size.htm

8 amps going 16' on #12 is over 3% voltage drop. Honestly not as bad as I thought it would be but still pretty high.I would think you'd want at least #8 for that run.

Camper Battery Under Truck Hood | Good Sam Community

teambeeson
Explorer
Jul 20, 2016
PS - Brand new deep cycle Napa battery...
jimh406
Explorer III
Jul 20, 2016
I think 12 gauge is too small. There are a lot of charts that show what size wire to use for the amps.
teambeeson
Explorer
Jul 20, 2016
jimh425 wrote:
I think 12 gauge is too small. There are a lot of charts that show what size wire to use for the amps.

I was suspect of this being the problem because my max amp draw is the 7.5a water pump... furnace is only 2.9a... and 16 feet isn't that long, but I'll check into it. All my fuses are 15a and none of them has ever blown. That's why I think it's mis-wire issue, but I'll go to 8-10 gauge anyway probably... as long as it's flexible since I'm running along the truck frame. Thanks for the reply!
JoeChiOhki
Explorer II
Jul 21, 2016
You need #8 at minimum for good service. With low voltages, you need large sized cables to serve without major voltage drop. You also need that on both sides, going back to the battery, don't use the truck frame.

http://www.powerstream.com/Wire_Size.htm
KD4UPL
Explorer
Jul 21, 2016
8 amps going 16' on #12 is over 3% voltage drop. Honestly not as bad as I thought it would be but still pretty high.
I would think you'd want at least #8 for that run.
Artum_Snowbird
Explorer
Jul 21, 2016
Most likely the cause of your problems is a weak connection somewhere. Normally #12 would handle the load with no problem, but if you have a weak connection it's like putting on a hidden load that will get worse and worse as it weakens and causes the amps in the circuit to climb and the voltage to drop.

The best way to check this is putting in an ammeter, and turning stuff on to see if the meter reading really matches the load. Chances are, it's where the loads come together, and it's often the negative return wire because that carries all the load back to the battery.

It's an easy test to use your long jumper cables, and bring a good solid connection on both the negative and positive lines back to where you connect in the camper. You might find that one or the other really helps and then that will lead to the problem wire.
WNYBob
Explorer
Jul 21, 2016
I have a similar system, with my 2nd battery connected to an inverter. I bought a 10' kit from Samlex, that came with #2.
wcjeep
Explorer
Jul 21, 2016
Technically your 12ga wire can handle the amperage. You may have further voltage drop from okd splices and maybe partially crushed wiring. Do you have a Flir to monitor the wiring? Can you add a second camper battery inside the camper?

What happens if you temporarily move the camper battery to the camper? Loss of distance should clear up your problems.
teambeeson
Explorer
Jul 21, 2016
I brought a battery into the camper, hooked into the 12v directly and still see voltage drop to as low as ~9v at the end of a water pump cycle so I think that rules out wire gauge/distance since I eliminated that variable.

On to testing for bad/crushed/corroded wiring one device at a time. This is actually kind of fun because I'm learning a lot more about what's wired to what and how. Always good to know your camper systems better I suppose. Thanks all for the brainstorming! Great community here!
bodacious
Explorer
Jul 22, 2016
Your converter is also a battery charger. If your battery is being charged ONLY when the truck is running(charging), your battery may not be getting fully charged. You might check the true state of charge of the battery or maybe charge it fully with a portable charger.