RV Battery

RV Battery

Types of Batteries

There are different battery types for very different purposes. In this article we will be
referring to the “chassis” battery and the “coach” battery. A “chassis” battery starts the engine and runs
the automotive systems in either the motor home or the tow vehicle. A “coach” battery powers the lights,
furnace, water pump and other 12 volt devices in the coach.

Starting Battery – this battery is designed to supply a large amount of current when needed to start the engine
on a motor home or tow vehicle, and then be immediately recharged by the vehicles alternator. Deep
discharging of this type will drastically shorten the life of the battery.

Marine Starting Battery – This battery is a compromise between the starting battery and the true deep cycle battery.
The plates are heavier and more firmly mounted to endure the pounding of a marine application.

Deep Cycle Battery – this battery is built with heavy plates and other internal modifications to enable it to
deliver a reletively small amount of current over an extended period of time. Care must be taken when
recharging at high amperage settings and limitations exist to prevent plate warping.

Batteries and Battery Charging

The battery (or batteries) supply your RV with 12 volt DC electrical power to operate the
lights, water pump, furnace and other appliances. They should be viewed as a storage tank for electricity and
as such they have certain limitations.

The capacity of the battery relates to the amount of electrical power that the battery can store and that capacity
will determine how long your lights will burn and how long your furnace will operate, before the battery will need
to be recharged. You can increase the storage capacity of your battery by installing a larger size or by installing
multiple batteries, depending on your camping requirements.

If you normally travel from RV park to RV park, where hookups are available, then one battery will be sufficient.
However, if you “dry camp” at remote locations for more than a day or two, you will need more storage and some way
to refill that storeage capacity.

Deep cycle batteries (or any battery, for that matter), should be recharged as soon as possible for longer service
life. A deeply discharged battery should be slow charged over a long period of time, say at 2 amps for 72 hours or
more to avoid excessive heat in the battery.

If you leave your battery on a charger, either the one built into the converter or a separate charger, for long
periods of time, make sure charger voltage is between 13.25 and 13.75 volts. Anything above 14V will overcharge the
battery and “boil” off a lot of the battery water.

State of charge chart for 12 volt batteries:

  • 12.70 volts 100%
  • 12.50 volts 90%
  • 12.42 volts 80%
  • 12.32 volts 70%
  • 12.20 volts 60%
  • 12.06 volts 50%
  • 11.90 volts 40%
  • 11.75 volts 30%
  • 11.58 volts 20%
  • 11.31 volts 10%
  • 10.50 volts 0%

Batteries should be checked after at least 3 hrs. rest.

Batteries Isolators and Charging Relays

Battey isolators are devices used to allow the RV battery to be charged by the vehicle
alternator while the engine is running. When the ignition is turned off, the RV battery and the starting
battery are effectively disconnected or “isolated” from each other. Relays are an alternative method of
achieving the same thing.

Isolators use electronic circuitry to divide the alternator output between the two batteries. Diodes (one
way electrical components) are used to prevent the RV battery from draining the starting battery when

Relays are electro-mechanical devices that basically do the same thing as a battery isolator. Relays are
simple switching devices that use a tigger current from the ignition system to connect the RV battery and the
starting battery together when the engine is running. When the ignition switch is turned off, the trigger current
is also turned off and the switch opens (disconnecting the two batteries). This allows the RV battery to be charged
from the vehicles alternator when the engine is running, but prevents the RV electrical system from depleting the
staring battery while camping. A continuous duty relay should be used as opposed to the typical starting relay.
These are available at most RV supply outlets. Wiring

RV Plug Wiring Guide


120 VAC to 12 VDC Converter The converters primary job is to convert 110VAC to 12VDC for use when plugged in to shore
power. The battery charging part is a secondary function and the charger is not very “smart”. Meaning that
it’s meant for a maintenance charge and does not monitor the requirements or state of charge of the battery
to any great extent.

Note: the newer converters are doing a little better in this regard. Using a quality external charger that charges
fast and the tapers off when the battery comes near fully charged helps to prolong the life of the battery. In the
old days, before converters, trailers had two separate lighting systems. 12 volt lights that were run from the
trailer battery, and 120 volt lights that were used when plugged into shore power.

Newer trailers eliminate the need for these two systems and convert the 120 volt incoming AC power to 12 volt DC
power to run the 12 volt lighting system. The converter also has a built in charge circuit to charge the trailer
battery when on shore power. Inverters are used when boon-docking to provide a limited supply of 110 volt AC power. They
take the electrical energy developed through chemical reations in the battery bank and convert it to useable
110 volt AC current. The amount of current is limited by the battery storage capacity.

Solar power is energy supplied from sunshine. This energy is converted by a solar panel into
electrical current.

This electrical current is stored in a battery bank to be used as required at a future time. This voltage is
controlled by a regulator to provide useful energy to charge the battery bank.


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