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12 Volt RV Electrical System

Like oil in an engine, the12 volt RV Electrical System power is the life-blood of an RV. This is true even more in a motorized RV considering all the sensors and black boxes used in modern engines, both gas and diesel.

Batteries

The source of 12 volt RV electrical system power is the battery, or multiple battery bank. Automotive and RV batteries are made up of multiple cells that when connected internally produce either 6 or 12 volts.

 The starting or chassis battery on a motorized unit is designed to produce a lot of current(Amps) to turn the starter motor and start the engine. This starting current is measured in Cold Cranking Amps(CCA). The 12 volt chassis battery(s) are usually recharged by the vehicle alternator when the engine is running. This battery supplies 12 volts to not only the starter motor, but to all the electronics such as engine control units. Chassis air conditioning and heating blower motors and controls as well as the radio and dash controls are also supplied from the chassis battery. Both electric levelers and hydraulic leveling system pumps get their power from the chassis battery.

The "House" or coach battery is usually a bank of what is known as "Deep Cycle" batteries. They are usually measured in the time it takes to discharge a particular current(amps) over a certain time. This is Amp Hours. According to Wikiedia "A deep-cycle battery is a lead-acid battery designed to be regularly deeply discharged using most of its capacity. In contrast, starter batteries (e.g. most automotive batteries) are designed to deliver short, high-current bursts for cranking the engine, thus frequently discharging only a small part of their capacity. While a deep-cycle battery can be used as a starting battery, the lower "cranking current" imply that an oversized battery may be required.

A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs. cost method is to keep the average cycle at about 45% discharge.  There is a direct correlation between the depth of discharge of the battery, and the number of charge and discharge cycles it can perform."

These deep-cycle lead-acid batteries generally fall into two distinct categories; flooded and valve-regulated lead-acid, also known as Absorbed Glass Mat(AGM). The flooded type are the most common as well as being the least expensive. Flooded cell batteries with regular maintenance can be expected to last 4 to 6 years in RV use. AGM batteries are sealed and require no regular maintenance, but are much heavier and almost twice as expensive as flooded cell types. They should last much longer than flooded cell batteries.

The maintenance required for flooded cell batteries is to keep the electrolyte mixture at a level above the lead plates in the cells. This is done by adding just enough distilled water to cover the plates or reach the mark inside the battery. The tops of the batteries should be kept clean and free of grease and oils to avoid short circuiting them. The battery contacts should be inspected regularly and cleaned of any corrosion. Any time you are working around batteries, you should wear eye protection and ensure there is good ventilation.

Lithium Iron Phosphate (aka LiFePO4 or LFP) Batteries are the newest technology in battery banks. Early lithium batteries were prone to fires and even explosions. The newest versions are non-explosive. You need to make sure that it is impossible to over charge or overly drain the lithium batteries, which can permanently damage them.  You need to make sure in particular that it is impossible to over charge or overly drain the lithium batteries, which can easily permanently damage them. To do this, many lithium systems incorporate some sort of EMS (Energy Management System) that can cut off current to/from the batteries when necessary. I believe the very best information about Lithium battery banks for RV use is from the folks at Technomadia. Here is a great article on this subject: http://www.technomadia.com/2011/11/lithium-update-3-lithium-battery-cost/

House 12 Volt RV Electrical System

All the things that are basic to enjoying your RV are provided by the house 12 volt RV electrical system. The battery provides power for  interior lights; demand water pump; the furnace, kitchen, and bathroom fans; the thermostat that controls heating and air conditioning; Slide-out motors, control boards for propane appliances such as the refrigerator and water heater; hard-wired LP and Carbon Monoxide detectors, and the inverter if so equipped.

 Using a propane heater on overnight can discharge the house battery quickly if you are dry camping. This is because the blower motor draws a lot of current. The halogen or incandescent lamps in most RV's can draw a lot of current if many are left on. This is the reason LED lamps are becoming so popular as they draw about 1/5th as much current as the regular lamps. Some demand water pumps require almost 10 amps of current when they are running. Prolonged water pump usage can cause excess battery drain. An inverter if you are so equipped will draw current in proportion to the AC load placed on it. Heavy use of AC powered appliances can cause excessive battery drain.

Fuses

Each separate 12 volt circuit in an RV has a protective fuse somewhere in the line. These are to protect the device being powered and the wiring from over current due to short circuits. Unfortunately, these fuses can be almost anywhere. Our diesel motorhome has 3 12 volt fuse panels under the dash, another below the 120 volt breaker panel, and others spread around. Some are individual fuses in a line and not in a panel. Try to get a wiring diagram from your RV manufacturer to help you find these fuses.

 NOTE: Never ever put a larger fuse in place of the one that is blown. Electrical circuit fires are a major cause of total loss RV fires.

Phantom Loads

Many RVers don't know about phantom loads. These are loads placed on the 12 volt RV electrical system from appliances that are on 24 hours a day. These can be illuminated switches, clocks, gas detectors, stereo memory, motion activated lights and the circuit boards in refrigerators and thermostats. These loads can draw down your battery in a few days without regular battery charging.

Battery Charging

Converters

When your rig is plugged in to park power, many rigs have a converter that changes 120 volts AC to 12 volts for use by all the 12 volt circuits inside. Sometimes the converter also has a battery charging function to keep the house batteries charged. Many of these are of low quality and can overcharge a battery and boil the electrolyte causing the battery to fail prematurely. There are several aftermarket chargers that have three stage safe battery charging circuits inside.

Inverter Chargers

These systems contain a 3 stage battery charger, a 120 to 12 volt converter, and an inverter to convert 12 volt battery power to 120 volt household power. The charging circuit in a 3 stage charger is as follows:

Bulk Charge - This is the first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until the voltage rises to near (80-90%) of full charge level. Voltages will vary from 10.5 volts to 15 volts.

Absorption Charge - In the 2nd stage the voltage remains constant and current gradually tapers off as internal resistance increases during charging. In this stage the charger puts out maximum voltage  around 14.2 to 15.5 volts

Float Charge - The 3rd stage is after batteries reach full charge, and charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is the maintenance or trickle charge.

This 3 stage type of battery charging will allow for the longest battery life.

Generator

The rig's generator (if equipped) will provide 120 volts of AC household current to power up the battery charging circuit. Many boondockers use their generators morning and evening to keep batteries charged. Bear in mind you are using up fuel at a rate of about 1 gallon per hour of generator use on average depending on the AC load on the generator.

Solar Panels

Many folks use solar panels to charge their batteries an so allow them to camp off the grid for as long as their holding tanks will allow. There is a separate chapter on solar, but each system has a Charge Controller. It is a regulator that goes between the solar panels and the batteries. It is designed to keep the batteries charged at peak without overcharging. Meters indicating Amps from the panels and battery volts can be added as an accessory and are highly recommended. We have found two companies consistently mentioned as good sources of solar systems and knowledge. They are AM Solar http://www.amsolar.com/ and RV Solar Electric www.rvsolarelectric.com/

Wind Power

We are seeing more and more RV mounted wind generators. Obviously you need adequate consistent wind to produce the rated output of these units. Here is good information on them: http://turbineel.net/rv-wind-turbines/

When all else fails, you can start your rig and the alternator will charge the batteries, but this is a noisy and fuel consumptive way to do it.


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