Power for Dry Camping
Technical Issues and Projects
Read This First: Disclaimer: I've written what I
personally did, and my opinions. Don't assume what I did was safe, and
don't assume it will work for you. Do more research, and make your own
choices. I am not responsible for your outcomes! :-)
Types of Chargers
You can find cheap single-stage bulk chargers rated
from 5 Amps to 40 Amps or so in the automotive department in Wal-Mart.
There are not what you want for battery charging in an RV. What you
need is a "smart" charger with control circuitry that will fully charge your
batteries as quickly and as safely as possible.
Batteries don't charge in a linear fashion.
When they are deeply discharged they accept a charge more quickly. Then,
as they approach full charge (say 85% or more) they gradually slow. Then
after becoming fully charged they require a small charge to maintain at 100%.
For this reason you need a 3 stage (or 4 stage) charger:
Stage 1 is called Bulk Charge.
When a battery is 50% to 90% charged it will take a charge very
quickly. A smart charger senses the battery voltage and supplies maximum
current at an increasing voltage level to attain a nearly full charge.
Stage 2 is called Absorption (or Acceptance).
At this stage voltage is held at a preset maximum level (typically 14.5 volts)
while current slowly tapers off until the battery is 100% charged.
High-end chargers may also include a temperature sensor that measures battery
temperature. Optimum charge voltage varies based on temperature from as
low as 13.8 volts to around 15.5 volts.
Stage 3 is called Float. After a
battery is fully charged it requires a lower voltage to maintain its charge
while waiting to be used. This is typically 13.5 volts for regular
lead/acid flooded cell batteries, and 13.2 volts for AGM batteries. As
with stage 2, the optimum voltage varies with temperature.
Another 4th stage that some chargers are capable of
is known as Equalizing. This involves taking the batteries up to a
higher voltage level, about 15.5 volts (or 1 volt higher than Stage 2 if you're
using temperature control), for about 2 hours. This ensures
all battery cells are equally charged. A more complete discussion of equalizing is included in some
of the links at the bottom of the page. Note that AGM batteries should NOT be equalized in
Q. Don't I already have a battery charger in my RV?
Yes, it's likely that you do. Most RV's have a
converter. The converter turns 120vac into 12vdc to power your
lights and 12 volt appliances. It also charges your battery, but depending
on the setup it may charge the batteries slowly with only 10 or 12 amps.
In older RV's, and even some new ones, it may not be a "smart charger" as
discussed above. It may have a fixed output of 13.7 volts -- lower than it
should be for fast charging, and higher than it should be for float.
Some chargers sold in the last few years have an
optional plug-in module that enables them to function as "smart chargers".
Two popular converters with this feature are Progressive Dynamics and Iota. If you
have one of these in your RV, you might just need to buy the module. If
you have an older inverter, you could replace it with one of these advanced
models, or add a dedicated battery charger.
Q. How large does my charger need to be?
For dedicated battery charging (separate from your
converter) a 40 or 50 Amp charger is adequate for an RV battery
system with Four golf-cart size 6 volt batteries. What about a larger
battery bank? And how does battery type effect charging needs?
Most flooded cell lead-acid batteries should be bulk
charged at a C/10 or C/8 rate. "C" stands for "capacity". The charge
rate is Capacity divided by the index number. So a 440 Amp Hour bank of
batteries can be safely charged at 440/10 to 440/8 or 44 - 55 amps.
Charging them a little faster may be OK, but when you get above C/5 you'll be
heating them up too much, and making them gas excessively. It's best to
stick with what the manufacturer says. Some manufacturers specify a higher
maximum than others.
I answered a question about this that was emailed to
me, so I'm posting my
answer here to further explain...
It's not that a C/4 of C/5 charge rate will do
something awful to your batteries right away. There are just some issues to be
aware of. If you have 440 AH of batteries and a 100 Amp charger I'm not
saying you need to toss the charger and buy something smaller.
First, charging a standard flooded cell battery bank
at a C/4 rate (capacity divided by 4) will shorten the life of the batteries.
Depending on other factors you might originally have gotten 6 years from the
batteries. Now, maybe you'll get 4 or 5 years.
Second, maybe more important, you will increase the
maintenance required for the batteries. They will bubble and gas more, which
means they'll use more water and, require more frequent monitoring, cleaning
posts, etc. Your battery compartment may also need to be cleaned more often,
depending on how it's constructed. All this is because of the excess gassing and
the greater corrosion possibility.
There will also be a slightly greater danger around
the batteries, as they'll be giving off more gas while charging. If the battery
compartment isn't very well vented you may want to install a small vent fan.
I'd also be concerned if they're cheap batteries. Something like a Trojan
T-105 will take more abuse than a Sam's Club Exide. If you severely over
charge and over heat them the cases might warp and/or crack and they could leak.
Not a good situation, and can lead to a fire.
Gel Cell batteries can only be charged safely at
C/20 -- and you can damage them if you push it. This is why they have
fallen from favor with most RVers and boaters alike.
On the other end of the scale are AGM batteries,
they can handle a much higher charge current than other batteries. Concord
says that its Lifeline batteries have no current limitations in voltage
regulated charging! For practical purposes, charging them at C/4 is pretty
Some large inverters include a built in charger.
Typically a 2000 Watt inverter will include an 85 to 100 Amp charger, and this is
adequate for a large battery bank (and overkill for a small one). Examples are the ProSine
2000, RS2000 and RS2500 inverter/chargers from Xantrex.
In summary, you need to consider the SIZE of your
battery bank in Amp Hours capacity, and the TYPE of batteries you have (Flooded
cell, Gel, or AGM) when choosing a battery charger.
Here are some links to learn more about battery
12 Volt Side Of
Live - Part 1 - Excellent article by Mark Nemeth on Batteries and
General FAQ about
Batteries - Overview of battery types and charging from Arizona Wind & Sun's
Sheet on Batteries - Good basic to intermediate primer on
Truecharge Battery Chargers - Some high-end chargers, and a good
overview of charger features
Continue on to Solar Panels
-- or choose another topic from the menu in the left sidebar at the
top of this page.