"How to charge the RV Battery & "
How to maintain the 12 volt RV Battery ...
Batteries and Charging ... a primer
For some, electricity and the workings of things electrical is a
mystery. The purpose of this primer is to help you understand some simple principles that relate to lead acid
batteries used in RV's. By the end of this article, I hope you will understand a little about electricity and how
to charge your RV batteries. I will try to present this information by relating electrical terms into something
that may be a little easier to understand.
I'm going to try to keep this article simple and not get too much depth because complex details can
confuse and that will defeat the purpose of this article.
Voltage, current and capacity can easily be represented by things associated with water, it's flow
and storage in a container. Here is how terms used for both relate.
Voltage = Pressure
Wire gauge = Hose/tube diameter
Amperage = Flow rate
Amp. Hrs. = Capacity (gallons)
Battery capacity can be stated in several terms but for deep cycle batteries (usually associated with RV
usage), amp/hours is the normal standard. This term relates to liquids by stating a container's capacity in
Voltage or pressure is the term used to describe the force to move either electrons or a fluid. Pressure for
fluids can be thought of as height differences in addition to that supplied via pumps.
Wire gauge or hose/tubing diameter determines flow through the medium and is affected by voltage/pressure and
length. The larger the diameter has less resistance and the easier the flow but as distance increases, so does
resistance and this reduces flow. The terms used to describe instantaneous flow is amperage or amps and flow
rate i.e. gals/hr
To describe power we need to use two terms together. For electricity we use Voltage and Amperage. For liquids
it's Pressure and Flow Rate. In electrical terms, power is defined by Watts, Voltage x Amperage.
We can read a lot of information on batteries and charging them and we always find discrepancies between the
articles. These discrepancies can be due to inaccurate information, by focusing on different aspects of what
can be a complex issue, or if the information comes from a commercial source, the information may be slanted to
favor the manufacturer's product.
My knowledge comes from being a Mech. Eng. that work in the electronics business for many years and at one
point I investigated lead acid batteries for use in a class of medical life saving instruments. I've read a
number of technical manuals from lead acid battery manufacturers and done a lot of testing of LA batteries
suitable for my intended application.
In reading these manuals several things were common to all. Lead acid batteries prefer to be charge by a
regulated constant voltage power source and the voltage should range between 2.2 and 2.4 volts per cell @ 70°
F. The length of time a battery is being charged is the major factor in setting the voltage level. (Another
factor is temperature. Temperature compensation is more detail than I want to include in this article)
The voltage setting increases as the charge time decreases. If you have ever measured the voltage level of your
car battery when the motor is running you are likely to see something on the order of 14.25 volts. This is very
close to 2.4 volts/cell and charging time is relatively short and cyclic in nature. For LA batteries that are
used in standby applications and are always being charged, the voltage is much closer to 13.2 volts, 2.2
For usage in RV's the charge time can vary considerably, from over night to a month or more. What voltage
should our RV batteries be charged at. There are many factors, all with a specific answer which can get complex
depending upon how many factors you take into consideration. There are single stage chargers and multi-stage
chargers and the answer is different for each. I have not read anything stating that multi-stage chargers are
necessary for adequately charging and maintaining a LA battery. They are for the convenience and speed of
I'm going to limit my discussion to single stage chargers that are standard in most RV's. For our single stage
charger we want the voltage high enough to charge quick as possible but not so high it will damage the battery
if left charging for extended periods. I've been an advocate of the middle ground, 2.3 V/cell. For a 12V
battery this is 13.8V. I've seen it mentioned that several charger mfgrs. also recommend this voltage setting.
Set much higher than this and if the charger is left on for periods better described in days/weeks, it will
over charge the battery will "boil" it dry. There is no actual boiling as with water but the chemical reaction
looks like it is and this term is commonly used to describe what we observe during overcharging. The lower we
set the voltage the longer it will take to fully charge and if set too low we will never fully charge the
Now here is where the water analogy comes in. I'm sure all have read statements talking about charging and when
nearly charged a good charger will cut back on the current. While the current flow does drop, it is not because
of something the charger does directly. This happens when a constant voltage charger is used and is set to an
acceptable voltage. Lets think of this like siphoning water between to containers. We could do this using a
hose or even between several containers with a fixed tube and valve mounted at the bottom between them. The
magic word here is "equilibrium". Nature likes to be in balance, when it isn't, forces try a move things
towards equilibrium. We've all siphoned a liquid between containers and one thing that is always constant is
the liquid flows from the container with the higher level to the lower one. That height difference represents a
pressure difference between to two containers. And like the old saying, "water seeks it's own level", i.e.
reaching equilibrium. If we've observe the flow between the two containers we likely have observed the flow is
greatest at the beginning of the transfer and it decreases as the level in both containers become closer and
STOPS when the levels are equal. It can be said the levels are at equilibrium. This is what is happening while
charging a LA battery from a constant voltage source.
With this knowledge and understanding that a batteries voltage varies with it's state of charge, we can now see
that if the battery were connected to another source of power, there would be an current flow between these two
items and the current would flow from the one with the higher voltage (pressure) to the lessor one. This second
power source could be another battery or a charger. The rate of current flow will depend upon the voltage
(pressure) difference and the resistance in the wiring which is controlled by wire gauge (tube/hose dia) and
Knowing these things we can see the flow is highest when the voltage difference is the greatest and decreases
and the voltage difference decreases.
Going back to the water analogy, if the containers are open at the top and the top of the receiving container
is as high or higher than the supplying container, we can never overflow the receiving container. If the
receiving container's top is lower than the supply container, it is possible the receiving container could
Now back to our battery. If our supplying source is a charger then the capacity can be thought of as infinite
and the only thing to prevent overcharging our battery is if the battery can reach the voltage of the source
(reaching equilibrium). If my memory is correct, for a nom. 12V battery the max voltage it can rise to is
13.9-13.93. Now we can see if our charger is set for 13.8V the battery can rise to this level without over
charging. For charging, the only thing that controls current flow is the delta V or voltage/pressure
We can now take this knowledge and apply it to other situations we might run into with our RV. Some of us have
more than one battery to supply power to our coach. I'm sure all of have read comments about if you use more
than one battery they should be of the same age and mfgr. While this is the best situation it doesn't mean that
batteries of different age, mfgr, or even capacity can't be used with success. If the later is the case you are
likely going to sacrifice some life and or max useful capacity. The worst thing that will happen is one battery
will fail early and if that failure is a conductive state, it will draw down the capacity of the remaining
battery, again trying to reach equilibrium. Some believe that using batteries that are not matched will bring
early and sudden death to the other battery. I am not of that belief.
Going back again to the water analogy, if you have two containers, one a 1 gal can and the other a 10 gal can
and the level, elevation, of the water was the same for both and they were connected together and you started
to drain them, what would you observe? Would the 1 gal can be emptied before the 10 gal can or would the level
and capacity drop together. It would be the later and it is the same if you are talking about batteries as
well. While the remaining capacities in each container would be different, the capacities expresses as a
percentage of their max. would be the same.
To finish this off I'll talk a little about converters/chargers used in RV's. Most RV's use a single stage
charger and it will have a regulated output. But like in everything there are difference in quality and in our
chargers the quality of the regulation is one item to contend with.
In older converter/chargers there is a separate winding on the transformer for the charge circuit and a active
regulator to control voltage. With the typ ferro-resonant Megnatek converter the output voltage of the charger
is adjustable. The current output from these units for charging is quite small being about 3-5 Amps. Newer
units have solid state designs and the full output is available to run the coach or charge batteries. This
charge current runs in the range of 30-45 Amps. By their nature these devises have regulated outputs but I'm
not sure if there is any "user" adjustment but would believe these are set at the factory to a setting very
close to 13.8V.
This has been a long presentation but hope it has been useful to you. Here are a few closing comments. LA
batteries don't like to be left in a discharged condition. Measure your charger's output voltage when the
battery is almost fully charged. This is especially true for the older designs. When the battery is low, it
will pull down the chargers output voltage and you will get a bad reading on the charger setting. This is not
quite as bad for the newer designs as their current output is much higher and the likelihood of pulling the
voltage down is reduced. Battery designs can and do vary by mfgr, basic design and intended purposes but a
fully charged 12V battery will read very close to 12.7V if it has been at rest (no current flow in either
direction) for 6 hrs. If you take a reading sooner than this the voltage reading will be somewhat higher. If
the charger voltage is properly set you can float charge your batteries for long periods of time without
needing to add water.
Winterizing & Storing your RV
The RV Winterizing video by Mark Polk is a step by step RV
Mark demonstrates the easiest and most effective way to winterize your
RV water system in this easy to understand video format.