Electrical - 12V DC

I have two electrical systems in my truck: the 120V system to run my appliances (rarely), and my 12V system that runs my fridge control panel/fan, my water pump, my toilet fan, and occasionally some 12V lights and fans I put in to circulate warm air from my woodstove.

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There are technically two separate electrical systems in any conversion or RV; We have the original DC electrical system that our engines use, that use the chassis of the vehicle, and we have our own separate electrical system we build into our living space. The first question for your electrical system is what type of system do I want to use?

Many people choose to do a DC setup for their living space, and connect their indoor batteries to their alternator in their vehicle electrical system to help charge them while they drive. This works well for a lot of people, and there are a lot of helpful people who can teach you how to do that. (But unfortunately, not me.) I chose to do a combination, AC and DC, for my living space - and I chose to separate it entirely from the electrical system of the engine (aside from grounding the inverter and AC breaker panel). I chose to invest in solar panels and an occasional generator instead.

The DC system for your vehicle uses the chassis of your vehicle as the "common ground", which means it uses your chassis as the conductive pathway from your equipment (a motor for example), for the negative wire, back to the battery to complete the circuit. DC only uses two wires, Red for positive and Black for negative, and one way or another, the circuit always connects back to the battery terminals. In a DC system, the negative/black line is called "Ground". [It is not the same thing as your ground wire in AC - The wire that brings the current back in AC is called a neutral.] Since there's so many parts to an engine that pulls current, it wasn't feasible to run two direct connections to each piece from the battery, so there is one direct connection for the positive, and the negative wire from the equipment is attached to the chassis, which is connected to your negative battery terminal, so you can essentially connect back to the battery anywhere on the vehicle. This is why it's called a "common" ground.

For the DC appliances in my home, I chose to do a direct line to each piece of equipment instead of using the chassis as a common ground. The reasoning for this was mainly because my refrigerator requires a direct isolated connection anyway, and the amp draw for the remaining water pump and toilet fan (normal computer fan) is very low, and the voltage drop through the chassis with that small of a current wasn't a problem I wanted to deal with.

So the DC inside the living space is separate from both the AC in the truck and the DC from the engine. 

I have a straight forward system for my DC. I have four 110amp hour AGM batteries, which I wire in parallel to increase my amp hr capacity to 440amp hrs. (positive to positive, negative to negative).

I connect one of the positive terminals to a main fuse (based on my system draw I used a 40amp fuse), and then from this fuse I run a wire to a fuse box. Each appliance has two lines that should run all the way back to the batteries (actually, back to the fuse box)- a positive Red wire, and a negative "ground" Black wire. The positive line is wired to the one side of the fuse box, and the negative is wired to the other side - connecting these wires and creating a whole circuit is this fuse, which protects the line. If it starts to spark, drawing more current than what you want (depending on fuse size) the fuse will melt and it can't conduct current anymore so it essentially shuts the power off to the device. Thermal DC breakers used in car engines use this same principle, but once the high current is gone they can bend back into place and work again. Once you blow a normal fuse, it's going in the trash.

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So if you only had a DC system - all you need is to wire your batteries together (or only have one), and then connect the positive and negative terminals to a fuse box, which then runs to the appliances and back. The current will flow through the positive battery terminal to the fuse box, then to the appliance through the positive wire, back to the fuse box through the negative wire from the appliance, and then to the negative battery terminal. If all of these connections are there, then the appliance will run. If you wanted to shut the power off, you could remove the wires from the negative terminal, and this would shut the power off because it breaks that full circuit. I preferred to put battery terminal cutoffs onto my batteries, so I can just un-screw these little terminals and turn the power off immediately, or re-connect immediately. I highly recommend them, and they were simple to install.

Fancy side-post battery cutoff switches. The metal that threads through the two pieces, connected to that black screw head, is what makes the electrical connection - so it has to be screwed down almost all the way for it to connect. It'll make a small-small tick spark when it does connect - it's safe.

My battery bank before I doubled it

All about DC systems:

 There are some misconceptions about DC that it's more dangerous than AC - an idea that's true for long distance power transmission, but not so true for you and I. I've since learned that DC is quite safe in small enclosures, and can be done well. Especially if you use fuses. Here are some quick tips I've learned about DC that might help you:

Voltage drop: The voltage of your current will drop based on the length of your line to the appliance, and you compensate for this with your gauge of wire. For example, here is a picture from my water pump's manual;

So I actually had 12g wire laying around, and that's what I used for my run (about 22 ft or so from where my battery was. Ran indoors inside conduit.) 12G wire can carry up to 20amp current, which was more than enough for my pump. Make sure you use a large enough wire gauge for your appliances.

Type of wire: You can use solid or stranded wire for DC - the main difference is practicality; It's easier to maneuver and utilize a stranded line than it is to use a solid. But you could use either one.

Lower Voltage Uses More Power: When you're thinking of how many amps are coming out of your battery, then yes this is true. The appliance doesn't really use more power at a lower voltage, it just requires more amps to compensate for the lower voltage to get the same job done. In our case, you can treat it like it pulls more power. Here's my fridge as an example:

Above it says at 120 Volts, it draws 2.7 Amps an hour.

At 12 Volts, it pulls 18 Amps an hour (!)

Why does it pull so much more power running at 12V?

Ohm's Law is the trifecta relationship between voltage (V for Volts), the resistance within the materials(Ohms, or R), and the resulting current(Amps, or I). If any of these variables are changed, it changes the other. Since our fridge needs a certain amount of power to run certain components, we assume these components don't change and the resistance within our circuits stays the same. The voltage is lower (10 times lower - 120 Volts to 12 Volts) so the amperage required to compensate is almost ten times higher, at 18 Amps. This is why it pulls more power at a lower voltage.

You don't have to know this by heart - you just have to keep in mind that if something pulls a lot of power, like an air conditioner, you probably can't find it in a 12Volt version - because it would draw 50amps an hour and a battery system can't handle that.

If you have very low energy draw, then an all DC system is ideal. You could wire up your batteries into a 24V system or a 48V system (a lot of off-grid solar systems are a higher DC voltage) and then step down to 12V for your appliances and that might be even more efficient than a 12V only... There are options. I've heard people have run a DC air conditioner off of their vehicle alternator, which does produce a lot of amps but that's beyond my knowledge.

The downside to using an Inverter, is the hourly draw an Inverter takes just to be on. My inverter pulls almost an amp an hour, just to be on, to convert my 12V to 120V. I talk more about this on my 120V page, if you want to look into an inverter. 

So if you want to use higher energy appliances, they need to be at a higher voltage. Or even better, have them run on natural gas or propane, etc. as that will be way more efficient.

Battery Capacity vs Amp Hour Availability:

This is something I heard a lot about, but didn't really understand until I moved into the truck and started testing my battery bank out. People say you need 10X the amp hours in the battery to produce that equivalent current at a higher 120V (Ohms law^), and this is really important to understand about your battery bank size.

When I originally started, I had two 110Amp hour AGM batteries, and I quickly discovered that this was way undersized for what I wanted to do - even for something as small as a 5amp coffee pot.

You would think that a 220amp hour battery bank could supply a 5amp 120V appliance pretty easily, as that's a very small percentage of the overall amp hours, but in reality if we want to create 5 Amps of current at 120Volts, then we're "allocating" / temporarily utilizing 50Amps worth of power from our 12Volt batteries, while it's producing that 5 Amp.

Why does it allocate 50amps of power when I'm only actually using 5 amps?

For the inverter to work, it's forcing this slower / lower voltage system (the energy stored in the battery at 12 volts) to collectively re-organize into a higher voltage, which is the only way that 5 amps of current can stay structured enough to be used. The only way a lower voltage system could put out a higher current is by utilizing more amps to create that higher voltage, as there's a direct relationship between voltage and amps. 

            When you're using the battery:

It's kindof like a balancing act as well; when you're pulling a lot of current from your battery, the voltage will drop. This is normal. When there's no current being drawn from it, the voltage will sit at it's normal "resting" voltage.

Okay, so what size bank should I actually have?

Remember that each type of battery can only be discharged to a certain percentage without getting damaged. The depth of discharge means how much I can deplete the battery; so 80% DOD means I can bring the battery down to 20% before damaging it.

AGM / Sealed Lead Acid = 80% DOD  (depth of discharge)

Lithium = 0% DOD (this is probably different between each battery manufacturer)

Non sealed Lead-Acid: 60% DOD

So my battery bank was actually 176Amp Hours, and 50amps was 28% of the overall capacity. It should be able to supply this.

When I ran my coffee pot, it pulled so much current that my voltage dropped to 10.1(ish), which the battery can do (in fact my battery chart says it can drop to 9.6V under a heavy load), but my inverter alarm went off and I already had messed with my batteries a couple days prior and they didn't go back to a normal resting voltage for a week or more after that - so I turned it off and I decided to double my battery bank and got rid of that coffee pot. I don't have a shunt installed yet, so I'm not sure what the actual startup current was, but it seemed it was way higher than 5 amps. The batteries are since all back to normal.

A 1000watt microwave for example pulls 1000watts at full power, or will still pull 1000watts at 50% power but it cycles on and off while it's running, resulting in "50% power usage". I think a lot of appliances will pull a lot more power when they start up, and since our electrical ratings are based on 1hr measurement, they just divide this by an hour etc. OR in the case of my coffee pot it draws a huge amount of power for 10 seconds, cycles off to cool the heating coil, then turns back on, and continues to cycle. It will come out to 5amps when you divide all that power by an hour rating, but in reality it pulled way more than 5 amps.