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the volts are in



Text by Joel Donaldson

When it comes to getting maximum enjoyment from your RV good "house" batteries are absolutely essential. A properly sized, well-maintained
battery bank makes your RV feel at home virtually anywhere — even when there are no AC hookups to be found. Good batteries can make all the difference between having cold drinks or melted ice cream in the fridge, staying up late with a good book or going to bed at sundown, and staying warm in cool weather. Without adequate DC power, most of the creature comforts in your RV quickly come to a screeching halt.

Sadly, the house batteries are usually the weakest link among all the typical RV's self-containment systems. While you might be able to dry camp
for a week or so before the holding tanks require service (or up to several months between propane refills), you'd be doing well to wring more
than several days out of the typical house battery bank. Undersized batteries are the bane of the RV industry, many of which are further hampered by improper battery care and recharging. In this article, we'll explain how to put together a battery bank that won't tire of dry camping before you do.

battery types compared

House batteries are currently available in several distinct technologies. The original flooded-cell design uses flat lead plates that are immersed
in an electrolyte solution comprised of sulfuric acid and water. Inexpensive and widely available, these batteries are very forgiving of overcharging, since the non-sealed construction makes it easy to replenish any water lost to evaporation. Although not very glamorous by contemporary standards,
some of the better brands are capable of delivering long, trouble-free service when properly maintained.

Some flooded cell batteries are also available in semi-sealed, quasi maintenance-free packages. By using special plate chemistries and large electrolyte reservoirs to reduce gassing and water loss, these designs virtually eliminate the need to add water for the entire life of the battery.
However, the sealed construction requires tighter control over the recharge process to avoid permanent damage due to water evaporation. 
Gelled-cells are finding favor among many RVers. The use of a gelled electrolyte makes gelled-cells virtually spill-proof, and the sealed
construction does away with any need for periodically adding water. It also eliminates the generation of explosive hydrogen gas under normal
conditions, helps keep the battery terminals clean, and allows the battery to be mounted at odd angles — even upside down in some
applications. As with all sealed batteries, proper recharge techniques are essential in preventing water loss.

Like gelled-cells, most absorbed-electrolyte glass mat (AGM) batteries are completely sealed. However, instead of gelling the electrolyte, these batteries suspend it inside porous glass mats stacked against the lead plates. In some models, the plates are not flat, but are instead rolled in
a spiral around the mats. Like gelled-cells, the close contact between the plates and electrolyte allows AGM batteries to rapidly accept or deliver
current. This makes fast recharge times possible, assuming good recharge capability.

Most batteries are designed for a specific application, and deliver the best performance when used for their intended purpose.
Starting/lighting/ignition (SLI) batteries are built to briefly deliver a large burst of engine starting current, which is almost immediately replenished
by the engine's charging system. These engine starting batteries are engineered for maximum power in the smallest, lightest package.
However, the plates tend to be very thin, making them prone to cracking and buckling during heavy discharges. Some flooded cell SLI batteries
won't withstand more than a few complete discharge/recharge cycles without sustaining permanent damage, making them very poor choices for
use as RV house batteries.

This contrasts with deep-cycle batteries, which are specifically designed to be heavily discharged, and recharged many times without suffering
plate damage. Even the most inexpensive of these batteries will far outlast most SLI batteries in typical RV house battery service.

rate your batteries BY THE numbers

It's important to understand how house batteries are measured and compared. The most common specification is the amp-hour (AH) rating,
which defines how many amps the battery will deliver over a 20-hour period before the voltage drops to a level many 12-volt DC appliances will
not tolerate (usually 10.5 volts for 12-volt batteries). For example, a fully-charged battery that delivers 5 amps for 20 hours might earn a 100 AH
rating. This battery could also deliver 1 amp for 100 hours (or more), or 2 amps for 50 hours, and so on.






Lifetime Cost





Per 1000 AH

Flooded-Cell Deep-Cycle



Group 24





Group 27





GC-2 Golf Cart





Group 8D





Gelled-Cell Deep-cycle



Group 24





Group 27





GC-2 Golf Cart





Group 8D





AGM Deep-Cycle




Group 24





Group 27





GC-2 Golf Cart





Group 8D





SLI (Starting) Battery



Group 24





Group 27





Group 8D





Note: Prices shown for 6-volt batteries are per pair






Chart 1

Reserve capacity is another rating that describes how many minutes the battery can continuously supply 25 amps of current. Multiplying the
reserve capacity by 0.6 provides a rough estimation of the battery's equivalent amp-hour rating.  In order to estimate your battery requirements,
it's helpful to do an amp-hour "budget forecast." To do this, determine the number of amps consumed by a 12-volt DC appliance, and then
multiply it by the anticipated number of hours of operation between battery recharges. For example, if you use a 2 amp reading lamp 3 hours
every day for 6 days between recharges, your batteries will have to supply 36 amp hours for the lamp. Do the same thing for all your other 12-volt
DC appliances, and add the amp-hour results together to get your total AH requirements. Ideally, your battery bank should be at least twice as
large as your amp-hour   requirements, for several reasons. First, the life expectancy of any battery is drastically shortened by heavy, repeated discharges. For example, regularly draining 80 percent of a battery's charge will wear it out up to six times faster than draining it just 50 percent.
Next, all batteries temporarily lose some of their capacity at low temperatures, just when heavy use of the furnace places extra demands on them. Finally, capacity drops off gradually as the battery ages, leaving you with less reserve than you used to have.

Most RVers will need more than one battery to meet their total amp-hour requirements. Note that connecting multiple batteries in parallel
effectively adds the amp-hour capacity of each battery together, but connecting them in series (as when combining two 6-volt batteries to obtain
12 volts) does not.

installation considerations

It's not uncommon to discover that insufficient space exists in the current RV battery compartment for your required battery bank size. In the
case of minor discrepancies, it's sometimes possible to squeeze slightly bigger batteries into existing compartments. For example, Group 29,
30 or 31 batteries often have significantly more capacity than their similarly sized Group 27 cousins.

For more ambitious upgrades, it may be necessary to fabricate a second battery compartment. This enclosure must provide proper ventilation,
easy access for routine maintenance, and sufficient
mechanical strength to support up to several hundred pounds of extra battery weight. (See Chart 1).

How long will they last?

Although it isn't widely advertised, some battery manufacturers publish typical life expectancies for their products. This data is usually expressed
in terms of cycle life — in other words, how many discharge/recharge cycles the battery can withstand before reaching the end of its useful life.
By definition, draining the battery down to 20 percent of rated capacity constitutes a discharge, and the battery is considered to be worn out
when it will no longer recharge to more than 80 percent of rated capacity. Chart 1 describes the approximate cycle life expectancies of some
common batteries, as well as typical costs when averaged over their total life spans.

Chart 1 shows that flooded-cell golf cart batteries generally deliver the best bang for the buck. They also have the advantage of being easy to
obtain (many wholesale clubs and golf cart shops stock them), and their size allows them to take the place of a pair of conventional Group 24
batteries in some battery compartments. Gelled-cell and AGM batteries offer the advantage of dramatically reducing exterior corrosion
(no electrolyte spillage) and, for the RV owner who demands maximum usable capacity from a limited number of batteries, the advantage of
higher recharge acceptance (see High Performers sidebar, end of document).

when & How to recharge

For maximum battery longevity, it's important to recharge batteries before they become excessively discharged. In order to do this, you'll need a
reliable method of determining battery state of charge. Using a digital voltmeter is one method. The voltages shown in Chart 2 indicate the
battery's approximate state of charge, although differences in battery designs can affect these readings somewhat. Another method involves the
use of a hydrometer, which measures the specific gravity of the battery's electrolyte. Note that this method is only useful for flooded-cell batteries
with removable filler caps. The hydrometer's suction bulb is used to draw enough electrolyte out of each battery cell to lift the float tube off the
bottom of the glass enclosure. The level at which the tube floats is an indication of the electrolyte's specific gravity. A chart similar to Chart 2 can
then be used to convert specific gravity to battery state of charge. Some hydrometers or conversion charts include provisions for taking readings
at various temperatures, thereby improving measurement accuracy. For best accuracy, the battery should sit in a "resting" state (no large
discharge or recharge currents) for several hours prior to taking either voltage or specific gravity measurements.




depth of




12.66 or more


0% (full)







11.79 or less


100% (dead)




Chart 2

The other trick to maximum battery life involves prompt, complete recharging. Flooded cell batteries in particular are prone to permanent loss of capacity after being left in a dead or partially discharged state. Gelled-cells and AGM batteries are more forgiving of this kind of abuse, but
can be damaged by it as well. Ideally, you should fully recharge your batteries just as soon as it's possible to do so.

In order to fully recharge a battery in the shortest time, a "multi-step" approach is preferred. First, a "bulk" charge quickly brings the battery up to approximately 70 percent of full capacity. The charging current during this phase is limited only by the size and kind of battery bank, and can be
up to 20 percent of the bank's total AH capacity for  flooded-cell batteries, 35 percent for gelled-cells, or 50 percent for AGM batteries.

The charger constantly measures battery voltage, which gradually rises to approximately 14.4 volts for flooded and AGM batteries (14.1 for gels)
as the battery reaches the 70-percent level. At this point, the charger switches to an "acceptance" charge, which supplies what ever current is
necessary to maintain the battery at 14.4 volts. This phase gradually brings the   battery up to approximate 95 percent capacity.

As the battery reaches a full state of charge, the current required also drops off to just a  few amps, at which point the charger switches to a final maintenance or "float" charge, which supplies only enough current to keep the battery at approximately 13.5 volts for flooded-cell batteries, or
13.4 volts for gelled-cell or AGM batteries.


Since exposing the battery plates to air may permanently reduce capacity, it is essential that the electrolyte level in flooded-cell batteries be periodically checked. Only the water in the acid/water electrolyte is lost to evaporation, so only distilled water should be used to replenish it.
Batteries under light usage will typically require water every several months. More frequent watering may indicate heavy service or battery
overcharge; less frequent watering may indicate chronic undercharging.

The occasional application of an equalization charge is beneficial in extending the life expectancy of nonsealed flooded-cell batteries. Some
multi-stage battery chargers have a built-in equalization function. Typically performed once every few months, this procedure supplies a
deliberate overcharge that lasts for up to 8 hours, thereby helping to remove plate sulphation and thoroughly "stir" the electrolyte. Equalization
may cause the battery voltage to rise above 16 volts, so it's important to disconnect any 12-volt DC loads beforehand. Also, significant gassing
and water loss will occur, so provide adequate ventilation and top off the water levels frequently. Equalization should never be used on sealed
or gelled-cell batteries, which would suffer irreparable water loss; AGM batteries should be equalized annually for 1 to 3 hours while limiting
 voltage to 15.5.

safety first

Battery electrolyte is poisonous, corrosive, and can cause severe skin and eye bums. Flooded-cell batteries can produce hydrogen gas in
potentially explosive concentrations during the recharge process, so it's essential to provide adequate ventilation, avoid electrical sparks and
open flames. Finally, most batteries are capable of supplying enough current to instantly melt metal hand tools or jewelry, so care should be
taken to prevent accidental contact with exposed terminals and wiring. Always wear face, hand and clothing protection when working around
 batteries. TL


AC Delco; (810) 257-8512, www.acdelco.com . circle 222 ON reader service card.

East Penn Mfg. Co. Inc.; (610) 682-6361, www.eastpenn-deka.com.  circle 223 on reader service card.

Exide Corp.; (610) 378-0500, www.exide.com . circle 224 on reader service card.

Interstate Battery System Of America; (972) 991-144, www.interstatebatteries.com.  circle 225 on reader SERVICE CARD.

Lifeline Batteries; (626) 969-6886,
www.lifelinebatteries.com.  circle 226 on reader service card.

Optima Batteries Inc.; (303) 340-7440, www.optimabatteries.com . circle 227 on reader service card.

Thermoil; (407) 786-7479, www.thermoilbatteries.com  circle 228 on reader service card.

Trojan Battery Co.; (800) 423-6569, www.trojan-battery.com . circle 229 on reader service card.

WestCo Battery Systems Inc.; (800) 214-8040, www.westcobattery.com . circle 230 on reader service card.

high performers

RV owners who hook up in RV parks most of the time may not know the difference between a car battery and a top-notch RV deep cycle battery.
The boondocker, however, undoubtedly has seen the lights go dim around 10p.m. on a chilly evening after several hours of using 12-volt DC
appliances, and the battery question becomes connected to the challenge of staying warm through the night.

Deep-cycle batteries of good quality and capacity are the answer (see main article). For the owner who wants the racehorse of the deep-cycle
breed — the battery that lasts the longest and can be recharged the fastest — absorbed glass mat (AGM) batteries fit the description.

Lifeline AGM batteries, developed originally for military aircraft, are available in various popular sizes, including the Group 27 unit featured here,
and they offer several advantages over gelled-cell and conventional flooded lead-add batteries. The most important aspect for the boondocker is low internal resistance (recharge current lost to gassing or breaking up of water). Batteries with this capability can be charged rapidly from typical
sources, including solar panels, an AC generator (via a converter/inverter) or an alternator while driving.

Conventional flooded batteries have the highest resistance, listed by Lifeline at 26 percent or higher (the battery must be charged to 126 percent
of rated capacity to achieve full recharge); gel batteries are listed at 16 percent and AGM batteries at 2 percent. Lifeline claims that this
translates to 20 to 25 percent faster recharge than conventional flooded or gelled-cell batteries.

Maximum charging voltage is about the same as that recommended for flooded batteries — 14.4 volts at 70 degrees ambient temperature —
 with no amperage maximum as long as voltage limits are observed. Maximum recommended recharge voltage for gel batteries at the same temperature is 14.1, so they are more easily overcharged, according to Lifeline.

As one might expect, high performance is not cheap. The Lifeline AGM units are priced substantially higher than conventional flooded-cell
batteries ($ 162 for a Lifeline Group 27 battery, rated at 100 amp hours, or reserve capacity of 186 minutes at the 25-amp rate), but they offer a
 number of advantages. In addition to fast recharge capability, they offer extended life-cycle durability (the number of times the battery can go
through discharge/recharge cycles without serious deterioration), a very low self-discharge rate while in storage and no venting of explosive
hydrogen gases.

Lifeline batteries ore sealed, with the electrolyte held in place by silica gloss mat, which allows oxygen produced during charging to recombine
into water rather than being expelled. Thus, there can be no spillage of electrolyte, and the battery can be mounted in any position. It has the
ability to handle relatively large loads with comparatively little voltage drop even after substantial power has been drown out.

The fast-recharge attributes of Lifeline batteries are exceptional in the deep-cycle battery field, not to mention the other advantages. This kind of performance is what it takes to keep the lights on, the furnace working and all the toys in motion while camping without hookups. — TL

Lifeline Batteries; (800) 527-3224,  www.lifelinebatteries.com .




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