BATTERIES & CHARGING
A PRIMERcontributed by:
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 and present
this information by relating electrical terms into something that may be a little easier to understand. I'm going
to try and 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.Electrical Water
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. Hrs. is the normal standard. This relates to liquids by stating a container's capacity. Depending upon the
total volume of liquid being discussed, deferent terms are used but here lets think of is in gallons.
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 dia 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
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
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
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 volts/cell.
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 charging. 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 battery.
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
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
length. 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 overflow. 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 difference.
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 needed to add water.