Tuesday, April 12, 2011

Life In our RV – Part 8 – Electrical System

Another really long involved one… I hope it is correct but if  any electrical engineers out there (Harvey) find any errors, please let me know.

There are really three separate but interconnected electrical systems in the RV.

Chassis Battery

The 12 volt chassis battery is the same as the battery in your car. It is mainly used to start the engine and power all the electrical components as in a car. It is charged by the alternator when the engine is running. This type of battery is good for providing short bursts of high current such as is needed to run the starter motor. They are not good for powering things for a long time and like to be kept fully charged and can get harmed or ruined if you run them down too far without charging them.

House Batteries

The 12 volt house batteries are what are called deep cycle batteries. They don’t like to provide high current but are very good at providing low to medium current for a long time. They can also be discharged to almost 80% of their total capacity without harm, although most people never go below 50%.

A batteries capacity is measured in amp hours. With a 1 amp hour battery you could power something that required 1 amp at 12 volts for one hour before the battery is completely discharged or 2 amps for 1/2 hour, etc.

Batteries like to be drained slowly. The higher the current you draw the less total amp hours you get out of the battery. The rating on most batteries is determined with a load of 25 amps. For example the batteries I have are rated at 225 amp hours so I could run the 25 amp load for 9 hours but if I tried a 50 amp load it would not run for 4.5 hours but much less.

I actually don’t have 12 volt batteries but 2 6 volt golf cart batteries connected in series to get 12 volts. The 6 volt batteries are physically about the same size as the 12 volt ones but have much thicker plates to more easily withstand the deep discharges.

The house batteries power all the lights in the house part of the RV. They also power the control board in the fridge, the fan and control board in the furnace and they power the inverter I bought to provide 120 V AC (as in your house) when we are not connected to electrical hookups.

The batteries are in the open under the steps.


120 Volts AC

This system powers all the normal things you have in your house. The wall outlets, the microwave, the fridge, the air conditioners, water heater, etc.

When I get to a campsite I pull a very heavy power cord out of a compartment to plug into the campsite electrical system to power all my appliances. This is called “shore power”, taken from the boating world.

The RV is designed to be plugged into a 50 amp outlet. Since volts times amps equals watts I can power 50 amps * 120 volts = 6000 watts of appliances. So I could easily run a 1500 watt space heater, an 1800 watt hair dryer and the 1000 watt microwave without blowing the breaker.

A lot of the public and some of the older private campgrounds only provide 30 amp outlets. I have an adaptor for my 50 amp cable but we have to be more careful with the things we try to run at the same time.

Surge Protector

A the end of the shore power cable I installed a heavy duty surge and voltage protector.

Campground electrical system are sometimes a dangerous thing to connect your RV to. There could be hundreds of RV attached and who knows how well they are all wired. Also the wire runs to your outlet from the distribution panel could be very long or have undersized wire which causes the voltage provided to be a lot less that the expected 120 volts. The problem this causes is that a 1500 watt heater draws 12.5 amps at 120 volts. At 90 volts it will try to draw 1500 / 90 =  16 amps which will at least blow a breaker and could overheat the wires and possibly cause a fire.

When I first plug in, the protector checks that the outlet is wired and grounded properly. If not it doesn’t let the power through. Then it watches for voltages low or high out of the expected range and suppresses voltage spikes from such things as lightning.  If anything does happen it disconnects the RV.

You can buy these either with normal plugs on each end but then you have to leave your $400 surge protector hanging from the power outlet outside the RV. I opted for the permanently wired one that is somewhat safer inside the storage bay.



If the campground doesn’t have electrical hookups, the RV has a 6500 watt generator in one of the compartments. The generator runs off the engine fuel tank but is hooked up so that it will always leave 1/4 tank.

To use it I just plug the 50 amp shore power cable into an outlet wired to the generator. Some RV have automatic switchover boxes that connect to RV to the generator when it is started.

The problem with using the generator is that it is VERY LOUD and uses a lot of gas. We almost never need 6500 watts of power. I wish it was smaller and quieter.


Smart Meter

The RV comes with a monitor panel to display battery charge level but it just basically 4 LEDs saying 0,1/3, 2/3,full which is pretty useless.

One of the first things I added to the electrical system was a Trimetric 2020 Battery Monitor. It provides a digital readout of the battery voltage and current number of amps being drawn from the batteries but the most useful things it does is record the number of amp hours than have been used since the battery was charged. When we are camped using battery power alone this is the easiest and best measure of how much battery capacity I have left.



As an alternative to the generator, when we have no hookups,  I installed a 2000 watt inverter that takes 12 DC power from the batteries and converts it to 120 volt AC.

The inverter can provide power to all the wall outlets and to the fridge. If it senses shore power it does nothing but if the shore power goes away it will start drawing from the house batteries.

Now the number of batteries I currently have could never provide 2000 watts of power. 2000 watts at 12 volts = 2000/12 = 166 amps. That would very quickly kill my batteries. I plan to add more batteries in the future once I can figure out a place to put them that can support the weight and has the proper venting.

This means when we are using the inverter we cannot use any high wattage appliance such as the microwave, toaster oven, coffee maker, etc.

I have a separate outlet, not connected to the inverter, for our electric space heater. As we tend to have it on for long times I don’t want the inverter to ever try and power it if we have a shore power failure (which has happened).

Having the inverter also allows us to power our laptops, the fridge and the stereo when we are driving.

A note on inverter types. In your house the 120 volt AC is delivered as a smooth sine wave (remember your high school math). Some cheaper inverters try to approximate this with a square wave. A lot of high end electronics such as some TVs will refuse to run on the square wave type.

Installing the inverter was a real pain. The 120 volt circuit breakers are at the very back on the drivers side. The inverter should be installed as close to the batteries as possible so that the 12 volt wiring is as short as possible (since it carries more amps). The batteries are located under the door mid way down the passenger side. Since the invert can draw up to 30 amps at 120 volts you need to use heavy 10 gauge cable. You must also use stranded cable everywhere as it stands up better to the flexing from the movement when driving.

To split off the items to be powered by the inverter I had to remove those wires from the main circuit breaker panel and add a second panel that is powered by the inverter.

So the sequence was:

  1. remove circuits from the main panel
  2. add a 30 amp breaker to the main panel
  3. snake 30 feet of 10 gauge cable from the 30 amp breaker to the inverter
  4. add the second breaker panel and wire up to the outlets
  5. snake 30 feet of 10 gauge wire back from the inverter to feed the second panel.
  6. connect 6 feet of 2 gauge cable to the batteries

One problem with this big an inverter is that just having it turned on and not powering any loads it still draws a bit more than 1 amp from the batteries so I just usually turn it off when we don’t need it.

It does have what is called load sense mode where the inverter will go into a low power state and it will wake up every so often to see if you have turned something on and will then will supply power but it doesn’t seem to work very well with very low power things like clocks and our router.

Battery Charging

First a short course on the type of batteries called “flooded cell” that I have.

With no load, a fully charged battery will show a voltage of 12.7 volts. At 50% charge it will show 12.1 volts and 10% it will show 11.5 volts.

However the best way to tell the charge state of a battery is to measure the specific gravity of the electrolyte in each cell with a hydrometer. This is like a turkey baster with a float and a calibrated scale inside. You open us the cell and suck up some of the liquid. The level at which the float goes up indicates the specific gravity of the scale. The manufacturer has charts of specific gravity to charge state.

Batteries like to be charged by what is called a 3 stage smart charger.

First the charger enters bulk charging mode. It raises the voltage well above the full level. For my batteries the recommended voltage is 14.8 volts. It also pushes a high but not too high current into the battery. This is usually 15% to 20% of the amp hour rating so in my case the charger supplies about 40 amps.

As the battery voltage starts to rise as it charges, the charger switches to what is called the adsorption stage. Here it keeps supplying the high voltage but starts reducing the amount of current so as not to overcharge them.

Once the batteries stop accepting current, the charger enters the float stage and reduces the voltage supplied to a bit over the full limit (in my case 13.2 volts) and supplies small amounts of current only when the battery accepts it.

The charger also has a temperature sensor connected to a battery post. All of the required voltages vary with the battery temperature. For example a cold battery need to show a higher than 12.7 volt reading to indicate that it is charged.

The charger I got is actually a combination inverter/charger called a Xantrex ProSine 2.0. With the combination, both use the same 120V and 12V wires. This unit is very programmable so that you can set the proper charging parameters as recommended by your particular battery manufacturer.


The remote display panel is used to program the charger, displays any alarms such as low battery voltage, and gives a real time display of the amount of battery power it is using and the status of the 120V AC input.


Solar Panels

When we stay somewhere without electrical hookups you will still have to charge the batteries somehow if you stay long enough. Normally this would be done by running the noisy generator to power the battery charger. This is rather wasteful as the charger only draws a few amps at 120 volts from the 50 amp generator.

I wanted a nicer way to do this so we installed some solar panels on the roof of the RV.


I have 2 150 watt panels installed to give me about 25 amps of charging current. I can easily add more panels as the need arises. The system can handle up to 45 amps of charge current.

The panels are connected to a charge controller designed for solar panels. It is also a smart charger and take the 17 volts from the panels and converts it to the proper voltage to charge the batteries.


I guess it gets hot as the huge black part is al heat sink.

I did all the wiring from the batteries to the controller and from the controller to just below the roof. I let the professional installers punch the hole in the roof and mount the panels because I wanted it done properly without any leaks.

The mounting brackets also come with arms so in the winter when the sun is low in the sky you can tilt the panels up for more direct exposure.

Because I like gadgets I got the optional display panel which tells you battery status and what power you are getting from the panels.


Unfortunately the two times we have camped without hookups so far, the sites were so shaded that the panels we useless and I had to use the generator. 

Battery controller

There is also a box that does sometimes connects the house batteries to the chassis battery.

For one, if you happen to run down the chassis battery you can press a button that connects the house batteries and you can try and use them to start the engine.

Running down the house batteries does not affect the chassis battery.

The controller also allows the engine alternator to charge the batteries although it is not a smart charger and is not all that good for the batteries especially if they are somewhat discharged. I watch once as I started the engine, the alternator tried to push 100 amps into the batteries.

I have heard of people that have disabled this function and just rely on their solar panels when they are driving. Not sure yet…

Battery Maintenance

Every month you should open the cap on each cell and check that the electrolyte level is high enough. The manufacturer tells you what to look for. If it is low you should add distilled water to bring it up to the required level.

If any of the cells where low you should run the battery through a charge cycle to mix up the electrolyte before you proceed.

Then you should measure the specific gravity of each cell. If any differ from the others you should get your charger to do an equalize charge on the batteries. This put a high voltage (15.5 volts) but a moderate constant current into the batteries to try to force the low cells to charge. Every hour you should stop the charger, let the batteries rest a bit and check  if the specific gravities have become equal.

What the RV Originally Had

When I first got the RV it had no inverter and no smart charger.

The batteries were 2 12 volt Group 27 deep cycle batteries rated at a total of 200 amp hours.  These are more like marine batteries that you would use to start your engine or run a trolling motor. Not nearly as good as the golf cart batteries (but a lot cheaper).

It also came with what is essentially a 12 volt power supply to run the 12 volt lights when we are plugged in. It would also charge the batteries but did a terrible job of it and would never supply enough voltage to fully charge them.

It essentially ruined the batteries. Even after I added the smart charger they would never fully charge. I could only get about 30 amp hours out of them before they were dead so I bought the new ones.


In order to conserve power I converted some of the incandescent 12 volt bulbs to LED bulbs. As these bulbs cost about $15 a shot I only converted the ones we tend to leave on a long time.

Two of the old bulbs would draw about 5 amps from the batteries. Two LED bulb draw 0.2 watts.  I essentially never worry about those lights being on. They are a bit dimmer though.

Running on Batteries

When we are without hookups and running on batteries every amp counts.

  1. We only use the incandescent bulbs when necessary and for as short a time as possible.
  2. If we don’t need the inverter we shut it off.
  3. I have switches and power bars to disconnect anything that we don’t need but that draws power even when turned off.
  4. We swap out Jennie’s 30 inch TV for the 17 inch one from the bedroom. It uses about 1/4 the power and even has a low power mode.

With some lights, both computers, Jennie’s PVR and TV on we draw about 10-12 amps. If she watches TV for about 4 hours we use about 50 – 60 amp hours in a night.

With the laptops, I haven’t yet figured out if it is better to run them on their batteries and then charge the batteries or to just leave them plugged in all the time so as not to drain the laptop batteries.

Enough for now.

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