This is written for the person who wonders what use solar panels are when they do not power the AC outlets.
Numerous more qualified people could have written this. But, they didn't.
Your RV has two separate electrical systems, AC and DC.
The AC system powers the Air Conditioner, Microwave, Converter, and all wall plugs - those plugs that look exactly like the ones in homes. The sources of power for the AC system are Shore Power and the generator.
The DC system powers all fans, lights, furnace, refrigerator, "cigarette lighter" outlets not on the dash, and 12 volt TVs. Note: While the cooling section of the refrigerator will run on AC when AC is present, the control boards and ignitor are always powered by the chassis batteries. The sources of power for the DC system are the chassis batteries.
The chassis batteries are charged from these sources shore power, generator, engine alternator, and solar. Power from all these sources feed the chassis batteries through the converter located in the power distribution box. The converter changes/converts AC power to DC.
The 120V system explained by someone else.
Open the panel of your power distribution box. It's safe and I am only asking you to look, not touch. Note the black circuit breakers. They are just like the ones found in your home. They serve the same purpose. If an AC circuit is overloaded it will trip the breaker interrupting power. There should be a legend indicating what each breaker is for. Air conditioner, microwave, converter, wall plugs (which have the GFCI type of breaker), and finally the main breaker. If any one of the AC appliances is not working, checking the circuit breaker should be your first response.
Now notice the 12 or so flat-bladed fuses in various colors. These fuses are for all the DC circuits in your RV. There should be a legend showing what each fuse controls and the amps the circuit is rated for. For instance, the lights may be 20A, while the refrigerator is 10A. If a device on the DC side is not working, locate the corresponding fuse, pull it out and look through the plastic case. If the wire is broken/blown you have located the problem. Now the question is will replacing the fuse restore functionally or blow the new fuse? I can not make a general answer without more information.
Note: Newer power boxes have an LED light next to the fuse that if on indicates the fuse is blown.
The 12V System explained by someone else.
I have just referenced a technical term you probably did not learn in school. Amps abbreviated A. What the heck is an amp or properly an ampere?
The technical definition of an amp does not mean much to most of us. An amp is a unit of electric current equal to a flow of one coulomb per second. Yeah, right! Think of it this way, Amps are a measure of power. What uses a lot of amps/watts? Hairdryers, (about 1,500 watts or 12A on high) microwaves, about 1,200 watts or 12A. What does not use many amps? Your iPad, LED lights, fans, and so on. So you see that low amp appliances can be readily powered from your chassis batteries.
What's with the term watts and the hocus pocus math above? Watts is another confusing term. For our purposes, let's say that both watts and amp are a measure of power. It's further confusing that DC amps and AC amps are never equivalent. You can see this when you convert watts to amps by the formula Watts/Voltage = Amps. 1200 watts / 120 volts = 10A. 1200 watts / 12 volts = 100A! Huge difference. This is why you can not use a high amp or watt device or your batteries would be exhausted in minutes. Remember when you looked into the power distribution box. The highest amp fuse was 20A, so there is no way you get 100A from your battery. If you put a wrench across the battery terminals you could get the entire 225A and weld the wrench to the terminals, start a fire, and an explosion because 225A is a lot of juice. [Submarine batteries store 10,000AH]
Suppose a battery has a capacity of 100AH. (Amp-hours) That means when fully charged, the battery can deliver one amp for 100 hours, two amps for 50 hours, and so on. In more practical terms the furnace uses about 7A. So if it ran for two hours during the night, it would consume 14AH leaving 86AH in the battery. The battery would be at 86% capacity.
If you had a volt-ohm meter, a VOM, after running the furnace you could test the battery and see that the voltage would no longer be at the maximum fully charged voltage of 12.9volts. So as amps are consumed, voltage drops. Hint: the four lights on the panel indicate an approximate, at best, condition of the battery. It does this by measuring the voltage of the battery. A tenth of a volt determines which light glows. A tenth of a volt will determine if the yellow or red light glows.
Now let me muddy the water even more. All batteries have a certain number of discharge cycles before they become unreliable. The deeper you discharge lead-acid batteries, including AGM's, the shorter the life of the battery. I try to never let my batteries go below 75% of their capacity.
We use our rig about 6 months a year that means 180 charge cycles each year. I get about 4 to 5 years out of the batteries until they get temperamental - such as taking a long time to charge.
Let's see what could be typical overnight use. We will keep the furnace using 14A, add the use of a few LED lights for three hours, call it 3A, the refrigerator 8A, the TV 8A and now we will have used 33A, leaving 67AH in the battery. That would be below MY threshold of 75% of capacity. (Argues for the use of a Wave 3 heater to keep warm to avoid using the amp hungry furnace fun and limiting TV watching)
Let's say you have the traditional 6V Trojan batteries. They are rated at 225AH, so woohoo that means you have 450AH so you could stay up all night watching sat TV with no worries. Except, when you connect them in series to get 12V means you only have 225AH because of some silly law that says that the way the world works. We will get to all that, but for now, just accept it.
Let's return to the example above that used a total of 33AH overnight and see how that would affect a real-world 225AH battery. 225-33 leaves 192AH or discharge percentage of only 15%. Quite tolerable. If you can restore the battery back to full the next day, you are golden. If not, another 33AH use will bring you down to 159AH or 70% capacity. That's below MY threshold. Do it a third night and your battery will be dead. Yes, it still has volts and amps, but not enough that things like your refrigerator will work.
Anyone can boondock for one night. Solar becomes convenient to recharge your batteries for longer periods off the grid.
I hope that all this made suffering through the strange terms worthwhile.
Charging the Chassis Batteries
The chassis batteries are charged from these sources shore power, the engine alternator, generator, and solar panels. All RV's have the first two. Let's talk for a moment about charging with the alternator. It happens as you drive, without you taking any action. The standard alternator starts out delivering about 30+ amps, but within a few minutes settles to about 18amps. (Why does it drop? I don't know) [Measurements made on my battery monitor]
18amps is a most respectable charge rate. It's still going to take a few hours to fully charge the battery. So unless you are driving long days, it will not get the job done. sure you can idle the engine to charge the batteries, but that is not kind to the engine or your wallet.
Inverters
I prefer everything to be 12V, but that is not possible. Gadgets with screens require all sorts of different voltages. To keep them charged you either need 120V or a device called an inverter. It changes/inverts 12V to 120V. This device should not be confused with the converter whose purpose is exactly the opposite, 120V to 12V.
You can buy small inverters rated at 400Watts. And they might work for your purposes. I have tried them and was disappointed. I settled on a 2000Watt inverter direct-wired to the chassis batteries and I have had no problems. It charges our five Apple products, a MiFi, camera batteries, runs a hairdryer on low, etc. Later I will talk about short DC wire runs long ac wire runs and wire sizes. But that is too technical for now.
VOM's Think of a volt/ohm meter like a thermometer. A thermometer usually shows either normal or higher readings to indicate illness. A VOM will show the condition of the battery in volts, but the values shown will normally be less than fully charged. [A fully charged battery will read 12.9volts, a battery being charged will show around 14.3volts, but that is a temporary condition only when the charger is operating]
I think you need a VOM. You can buy an adequate one at Harbor Freight for $10 or one that has more functions that you will probably not use for $40+
A VOM has three settings. AC volts, DC volts, and several Ohm settings. [Let's not worry about the ohm settings right now]
VOM Primary use: Finding out if AC or DC voltage is present and if so how much.
Practice - Turn on the VOM. Set the VOM to AC volts. Stick one probe into each side of an AC outlet. It will read about 120volts or it will read 0. If it reads 0 in your RV, you are not connected to shore power, the generator is not running or a breaker is tripped.
Now set the VOM to DC volts. Pull the DC fuse labeled for Lights in the power distribution box. Put a probe on each of the fuse terminals. It should read a little over 12 volts. [If not, you have a problem]
One word about the Ohm settings. I primarily use it in an RV to determine if a circuit is complete - has continuity. Set the VOM to the highest Ohm setting. Use any wire handy like an extension cord. [That is not connected to anything] Put one probe on one of the exposed flat blades. Put the other probe into either of the slots on the other end of the extension cord watching the VOM display as you do it. The display will either briefly go from zero to some number and back to zero. If so, it shows you have matched the wires in the cord and that you have a closed circuit. If the display does not change, move one of the probes to the other side and it will show a closed circuit. So the ohms reading, in this case, is infinity indicating a closed circuit. It also shows which of the two wires is matched to the one at the other end. This introduces the concept of polarity. I will stop here for now.
This could also be used to test a fuse to see if it is blown - open. Not all fuses have the see-through feature.
CPAP Machines
We now both need a CPAP and they suck amps, even without using the heater. Both machines together eat 15% of our batteries every night.
I think all machines sell a cable that you can plug into a "cigarette lighter" outlet, as they are internally powered by 12V. [The "brick" that comes with the machine changes the voltage from 120V to 12V, so you can leave that one at home]
Lazy Daze installs these 12V outlets when the rig is built. Our outlet was not in a place that could be used for the CPAPs, so I took the trouble to wire in "cigarette lighter" outlets just for the machines. BTW, all wiring can be accessed from the raceway under the bin floors.
Trouble Shooting No AC or DC Power
Don Malpas Jun, 2021