12v for the Minimalist Weekender

[Tukanu]

I’m going for a super simple 12v system for my build, and I have a few questions:

1. Is 12 ga adequate from the battery to the box?
2. Is a 30 amp breaker too large?
3. Will 16 ga work for all the appliances, including the vent fan?
4. What size fuse for the LED lights?
5. What size fuse for the vent fan?
6. What size fuse for the ports?
7. Do I need to ground the (-) to the trailer frame? If so, what size wire?

The battery and charger will be housed in a vintage cooler mounted to the tongue.
The longest wire run will be out to the galley.
Thanks for your input.

[tony.latham]

1. Is 12 ga adequate from the battery to the box?
12 ga will work fine for the main feeder to the fuse box.

2. Is a 30 amp breaker too large?
Yes. 15 amps for 12 ga.

3. Will 16 ga work for all the appliances, including the vent fan?
Yes, and protect all 16 ga wire with a 7.5 amp fuse.

4. What size fuse for the LED lights?
You are protecting the wires, not the appliances (or lights). 16 ga, 7.5 amps. (Go look in your home's breaker box. Each outlet circuit has a twenty-amp breaker no matter what is plugged into that circuit)

5. What size fuse for the vent fan?
Ditto - 16 gauge wire, 7.5 fuse.

6. What size fuse for the ports?
Ditto

7. Do I need to ground the (-) to the trailer frame? If so, what size wire?
If you are charging from the tow vehicle it's a good idea but if this system is isolated, I don't see the need. 12 ga would be fine.



(And no, that sheathed cable is not solid Romex. It's 12 ga stranded marine cable.)

When you buy your wire, make sure it is stranded copper and not copper-coated aluminum. Also, read the fine print and make sure the gauge is AWG. Some wire coming from Asia is misleading on that.

Tony

[Tukanu]

Thanks for the quick response Tony. I have been reading post for the past two days and my head is spinning with all the conflicting suggestions.
Not clear on your grounding answer...no need to ground the neg terminal in my simple plan? I will not be charging off the tow vehicle.
Thanks,
David

[tony.latham]

I will not be charging off the tow vehicle.

Then I can't see any need to ground the frame.

Tony

[tony.latham]

One other thing. When you get your fan and probably the lights, black is positive and white is ground. That's the RV world being goofy. Go figure.

It's the same industry that loves OSB floors, particleboard drawers and three-year warranties.

Tony

[Tukanu]

One other thing. When you get your fan and probably the lights, black is positive and white is ground. That's the RV world being goofy. Go figure.

It's the same industry that loves OSB floors, particleboard drawers and three-year warranties.

Tony

Thanks....now my head is spinning again.

[RJ Howell]

Plus 1 to Tony's response. He's a pretty smart dude and has been in the 'Rodeo' before. Nothing like first hand experience!

My only add to any of this is to do a power/solar audit. What are you going to use, what battery should be there and how to keep it charged.

This is what I did and can be adjusted to your requirements.



This spreadsheet was built on a Mac. Happy to share if if wish. Yet you can see how it works.
Now if truly a minimalist, you may find a lithium battery bank that will suit nicely. 35ah is small and easy to do.

Just MHO and a small add in hopes it helps.

[Tukanu]

This is good stuff, but a bit over my head.
Based on my rudimentary schematic:
two reading lights a couple hours per day
cabin dome 2 hours
galley dome 1 hour
fan on low......6 hours per day
possible plug in a fan for 3 hours
charge a tablet or cell phone 2 hours
ghost

That would be the max. I plan on 2 nights capacity.
Will the 35AH be enough?

Thanks for your input.

[tony.latham]

Will the 35AH be enough?

I think so. But you're playing with about 17 useable amps ÷ 2. It would work without a doubt if you threw solar in the mix.

Don't panic. Solar is simple.

I put a 100 watt flexible on the roof of my new build and a 30 watt on my tongue box. I'd suggest a 30 watter on your tongue box and have your system wired to accept a set of folders if that doesn't catch enough sun (and it won't unless you park with it facing the south).



On my previous build, I had 20 watts on the box and two home-grown 30 watters hinged together with a couple of old junker tripod tubes for legs. The inexpensive SunGuard controller was in the box. I rarely had to put the folders out.



The one thing I am not seeing on your "graphical schematic" is a means for monitoring your battery voltage. At the very least, add a voltmeter or you won't know how things are going.



(That neon USB might drive me nuts.)



Tony

[Tukanu]

Thanks. I looked at that one, but wasn't sure what it is telling me. When the volts drops to certain level, does it mean there isn't much juice left? What is the critical level?
I could go to 55 AH without much more cost.

[MtnDon]

....When you get your fan and probably the lights, black is positive and white is ground......

Actually, to me, that makes total sense, comparing house wiring where black is the hot wire and white is called the neutral. (There is also a ground that is bare or green when insulated.) Looking at it that way, the AC wiring black wire corresponds to the DC system positive, also thought of as a hot wire. The AC system white then corresponds to the DC system white, which I call the negative wire. Some folks call it a ground wire, but it really the negative as it is connected to the negative battery terminal.

[troubleScottie]

I think you are ignoring what is being said.

There are 2 ways to determine your run time - math or just do it.

The math is simple.

First how much power do you have. A lead acid battery should only be discharged to 1/2 its capacity. A new fully charged Lead Acid 35AH battery has only 17.5 AH before it should be recharged.

Each item consumes power, typically measured in Amp. So the power used is just the number of Amps * time used in hours = xAH.

So you need to multiple the Amps each device uses times the number of hours you expect it to run. Adding all these up determines you power usage. You can compute it on a daily or trip basis. The total of all the devices cannot exceed the 17.5 AH that your battery puts out OR you have to recharge. Recharging at any time during your time extends your power.

Each device has a manufacturer's estimate of power consumption. It may/may not be accurate. The best way is to measure the power usage use a meter. Measure the usage over a long period of time and compute the average. Some devices turn on and off as they run so use less power.

LED do draw power, but it is very small and often is ignored. However that is not always the case. Powerful LED work lamps can draw 4-5A and generate a lot of light. Reading light might be high eg 2.2W ( on amazon: LIGHTEU RV Reading Light 12V 2.2W Bedside with USB Port(5V/2A),Touch Switch Dimmable Warm 2 Pack(Chrome 2X A2-USB) ) So about 8hours total time. If you are running both, 4 hours.

A major draw is the fan. A Fantastic Fan draws around 3amps while running assuming it is not thermostatically controlled. So it alone can only run for 5.8hr before discharging your battery.

So to your original ask, 35AH running a 3A/36W fan 6 hours means your battery is out of charge after one night. Adding in the other power draws just speeds up the power consumption.


The other thing you do need is some way of determining the charge remaining on your battery.

As mentioned before, the simplest but not the most accurate is a voltmeter. Generally lead acid batteries lose voltage as they lose/use power. There are documents that discuss this. Note: the same is NOT true for Lithium batteries.

Using a shunt and state of charge (SOC) device allows for very accurate measure of charge remaining. Generally, they are more expensive. And take a little more setup work. Lots of youtube videos on this.

[RJ Howell]

This is a simplified voltage SOC chart. Do realize these voltages are the battery at rest, meaning left alone for 24hrs.



Now this is the meter 'I' use. https://www.amazon.com/MICTUNING-6-5-100V-Digital-Voltmeter-Multimeter/dp/B01JOUZELG/ref=sr_1_6?crid=3DX1WM6C9NAP7&keywords=12v+meter+display&qid=1570278958&sprefix=12v+meter%2Caps%2C199&sr=8-6 I have one on the solar panel showing incoming and one on the battery showing outgoing. What I like about these is they use a external shunt (not a big fan of the intervals yet..) and reads cumulative Whr's.



So here I show 12.46v (battery voltage) / .22a being used (or 220ma) / 2.7 watts of power / 379Wh of energy used since last reset. Sorry but didn't take a picture of the one on the panel soI could show you the comparison. Main number I look at is the Wh #. In this case if higher than 379, my batteries haven't reduced at all.
Also need to say that in this picture the panel is not hooked up, otherwise that voltage reading would be in the mid/high 13's..(incoming voltage or panel voltage)

As another said: 'You' need to figure 'your' use and work towards a battery that will work for the amount of time you require. Adding solar just adds to duration. The chart I showed you is a combination of 4 folks (myself included) testing different appliances and coming up with a comparable number between all of us. It's as close as we could get and a decent basis to work from.

For you, you may not even have Ghost draw, especially if what you plan to use has no stand-by mode. I have a lot of Ghost draw due to the appliances I have. That's where doing the audit is important, it tells you 'your' usage.

[Tukanu]

This makes perfect sense. Instinct tells me you cant run a fan all night on a battery. It would appear those power vents are only useful with AC power. Again, my instinct was to skip the fan in this minimal setup because of the cost. I think I will just pre-wire, and go for a non-power type vent.

[tony.latham]

A Fantastic Fan draws around 3amps while running assuming it is not thermostatically controlled.

Here's what my Fantastic Fan draws on the three speed settings according to my meter (which is the same $20 dollar one that RJ uses).

1- .86 / 1.25
2- .11 / 1.7
3- 1.3 / 2.5

Now... I modified the fan with a 4-ohm resistor to slow it down. The first number is the amp draw with the modification and the second without so that 3 amps is close.

But here's the kicker. I think you said you were installing a Maxxair and running it at its lowest settiing. Here's the dope with those: (Thanks to Todd, aka AKM_Guy)

Fan 1 0.13
Fan 2 0.19
Fan 3 0.28
Fan 4 0.41
Fan 5 0.57
Fan 6 0.79
Fan 7 0.91
Fan 8 1.28
Fan 9 1.66
Fan 10 2.37

As I said, I still think it's doable.

When the voltage drops to certain level, does it mean there isn't much juice left?

If you drop the voltage below 12, you're damaging the battery as RJ said. This voltage needs to be read after at least a half-hour (better an hour) with no draw on it. (If it's indicating 12.6 volts and you turn the fan on, it may read something like 11.8. Ignore that. It needs to be read after the battery is at rest.)

Actually, to me, that makes total sense, comparing house wiring where black is the hot wire and white is called the neutral.

Don: You've got me on that one. Maybe it was just me looking for an excuse to bash the RV industry.

How's your headache?

Tony