BATTERY BASICS
Battery selection questions seem to top the list of many RV owners today. With 6 volt, 12 volt,
Marine/RV deep cycle, cold cranking amps, marine cranking amps, amp hour ratings and the
like, it can be somewhat confusing. To know what battery is best for you, and to make your
existing batteries perform as designed, you have to ask yourself some questions.
 How often do I dry camp?
 How many 12 volt operated items do I have?
 Do I plan on using my inverter when dry camping?
 If I do use an inverter, what 110-volt items do I plan on using, and how long?
These are just some of the questions you have to ask yourself. The refrigerator alone on 110-volt
power will completely drain a typical 6 volt, 4 battery system running off the inverter in about 12
hours. So having a total battery capacity of only 200 amp hours may not work for you, but may
suit a non-inverter equipped vehicle just fine.
Types of batteries
Generally you may run into three types of RV deep-cycle batteries (never install an automotive
cold cranking amp-only battery into a deep-cycle designated location). Wet cell, Gel cell and
Absorbed Glass mat (AGM) batteries. The most common used is the Wet cell battery. Thick
internal plates that are alloyed in antimony characterize true deep-cycle wet cell batteries
(automotive cold cranking amp batteries have thinner plates, which deteriorate at an accelerated
rate when discharged too often). Wet cell batteries will give off gas as a natural result of
charging and will experience some water loss. It is very important that the electrolyte level be
checked frequently and topped off with distilled water when necessary. Follow the battery
manufacturer’s recommendations for maintenance. Do not allow the top of the battery plates to
be exposed to air, as contamination of the cell will result.
Gel cell batteries are lead-acid batteries similar to the common wet cell battery, but differences
in the chemistry and construction provide some unique features. Gel cell batteries have no
maintenance, a low self-discharge rate and low internal resistance. The battery compartment
should still be ventilated, even though gel cell batteries are sealed.
Absorbed Glass Mat (AGM) batteries are lead-acid and maintenance free also, but the
performance is similar to gel cell batteries and the charge parameters are similar to wet cell
batteries.
Battery selection
When selecting your new deep-cycle batteries you should look at the amp-hours the battery is
rated for, not the cold cranking amps (CCA). Hybrid batteries will have both ratings. Stay away
form batteries that just list the CCA, as they are designed for starting, not deep discharging.
When you shop for a particular battery, keep in mind that the method of installation can double
the available amp-hours and/or voltage. When batteries are installed in series, the voltage is
doubled and the amp-hour rating remains the same. In other words, two 6 volt, 220 amp-hour
batteries installed in a series will put out 12 volts of power and still only 220 amp-hours. See
figure 1. When batteries are installed in parallel, the amp-hours are doubled and voltage
remains the same. Two 12 volt, 220 amp-hour batteries installed parallel will put out 12 volts
and 440 amp-hours. See figure 2.
FIG. 1 Series Hookup
+
+
+
6 volt
6 volt
Converter/inverter
_
__
__
Each Battery Capacity:
220 Amp-Hours
6 Volts DC
Total Battery Capacity:
220 Amp-Hours
12 Volts DC
FIG. 2 Parallel Hookup
+
+
+
12 Volt
12 Volt
Converter/inverter
__
__
__
Each Battery Capacity:
Total Battery Capacity:
220 Amp-Hours
440 Amp-Hours
12 Volts DC
12 Volts DC
(note that the positive line leads from one battery and the negative from the other)
Only similar batteries should be connected together in one bank. Do not connect old and new
batteries, or wet and gel cell batteries together. Deep-cycle batteries are usually rated in Amphours, which is based on a 20-hour discharge rate. Therefore, a 100 amp-hour battery can
deliver 5 amps for 20 hours. Deep-cycle batteries can be discharged about 80% of capacity
before damage occurs. Shallow cycling (50%-60% drain before recharge) will result in much
longer battery life. To find out how long your batteries will last when using various equipment
in your coach, research the amperage of the items in use, and use that number to gauge what size
amp-hour battery you think you need. Keep in mind that you should recharge your batteries
when they are 50% discharged, so only half of the amp-hour rating is actually used.
Battery Charging
Completely charging wet cell deep-cycle batteries requires the battery voltage to be raised
beyond what is known as the gassing point. This is the voltage at which the battery begins to
bubble and gas is given off. If charging stops short of this point, sulfate is left on the plates and
deterioration of the battery begins. The gassing point will vary with battery temperature. At 77
degrees F, the gassing point of a 12-volt battery is about 14.0 volts.
Inverters
For those of you who have an inverter-equipped vehicle, the above information is pulled directly
from the inverter owners manuals. Read the manual carefully for more information on proper
use of your inverter and how to extract the most power from your batteries. For those of you
without inverters, the above information should give you some educative insight on exactly how
much power an inverter can supply, and what its limitations are.