The following , and the related Car Battery FAQ are the work of Bill Darden ,who welcomes your comments. All of the usual disclaimers apply.

DEEP CYCLE BATTERY
FREQUENTLY ASKED
QUESTIONS 1.2
November 1, 1998

 
A word of caution.  Batteries contain a sulfuric acid electrolyte which is 
a highly corrosive poison, that will produce gasses when recharged and 
explode if ignited.  This will hurt you--BAD!  When working with 
batteries, you need to have plenty of ventilation, remove jewelry, wear 
protective clothing and eye wear (safety glasses), and exercise caution.  
Whenever possible, please follow the manufacturer's instructions for 
testing, jumping, installing and charging.  This FAQ assumes a  six cell 
battery commonly used for 12 volt negatively grounded system in most 
recreational applications.  For six volt batteries, divide the voltage by 
two. 

The technical stuff is in [brackets].  


CONTENTS

1.  WHAT IS THE BOTTOM LINE?
2.  WHY BOTHER?
3.  HOW DO I TEST A DEEP CYCLE BATTERY?
4.  WHAT DO I LOOK FOR IN BUYING A BATTERY?
5.  HOW DO I INSTALL A BATTERY?
6.  HOW DO I CHARGE A BATTERY?
7.  HOW DO I INCREASE THE LIFE OF A BATTERY?
8.  WHAT ARE THE MOST COMMON CAUSES OF BATTERY FAILURES?
9.  WHAT ARE THE COMMON MYTHS ABOUT BATTERIES?
10.  WHERE CAN I FIND MORE INFO ON BATTERIES?
 11. Isolators


1.  WHAT IS THE BOTTOM LINE?

     A.  Remove the surface charge before testing and check specific 
gravity in each cell,  (See Section 3)

     B.  Recharge as soon as possible after discharge.  (See Section 6)

     C.  Size charger so that it will recharge over a 10 to 12 hour period
at 13.8 to 14.6 volts,   (See Section 6)

     D.  Buy the freshest and largest Reserve Capacity (or Ampere Hour)
battery that will fit your requirements,  (See Section 4)

     E.  Perform preventative maintenance, especially during warm weather, 
and  (See Section 7)

     F.  Shallower the average discharge, the longer the battery life.  
(See Section 7).


2.  WHY BOTHER?

Because only the rich can afford cheap batteries.....

A good quality deep cycle battery will cost between $50 and $100 and, if
properly maintained, will give you at least 200 deep discharge cycles of
service.  The purpose of a deep cycle battery is to provide power for
trolling motors, lighting and other accessories for motor home and
marine applications.  Dead batteries almost always occur at the most
inopportune times, for example, across the lake or during bad weather.

Normally a battery "ages" as the active plate material sheds (or flakes 
off) due to the expansion and contraction that occurs during the discharge 
and recharge cycles.  Heat and vibration accelerate this "aging" process.  
Eventually, the sediment builds up and can short the cell out.  Another 
major cause of faulty batteries is sulfation.  When batteries are stored 
discharged or for over six months, lead sulfate makes the plates very hard 
and dense and the battery less capable or unable to be recharged.  When 
the active material in the plates can no longer sustain a discharge 
current and the battery "dies". 

Most of the "defective" batteries returned to the manufacturer  are good.  
This suggests that most SELLERS of new batteries do not know how or take 
the time to properly test or recharge batteries. 


3.  HOW DO I TEST A BATTERY?

     A.  Visually inspect for obvious problems, for example, damaged case,
corroded terminals or cables, loose hold-down clamps or cable terminals, 
or low electrolyte.

     B.  If you have just recharged your battery, then eliminate any
surface charge by one of the following methods; otherwise, go to the next
step:

          1.  Allow the battery to sit for two to three hours,

          2.  Apply a 25 amp load for three minutes and wait five minutes, 
or

          3.  With a battery load tester, apply a 150 amp load for 10-15
seconds.

     C.  Use the following table, determine the battery's state-of-charge.
The best way to measure the state-of-charge is to check the specific
gravity in each cell with a hydrometer.  A temperature compensating
hydrometer will cost approximately five dollars at an auto parts store. If
the battery is sealed, then the correct procedure to test it is to measure 
the battery's voltage with a good quality digital DC voltmeter with an 
accuracy of .5% or better.  

     Open Circuit          Approximate           Average Cell
    Battery Voltage      State-of-charge       Specific Gravity

         12.65+               100%                  1.265+

         12.45                 75%                  1.225

         12.24                 50%                  1.190

         12.06                 25%                  1.155

         11.89                  0%                  1.120

[If the temperature of the electrolyte is below 70 degrees F (21.1 degrees 
C), then add .012 volts (12 millivolts) per degree below 70 degrees F to 
the reading.  A 100% state-of-charge for a AGM (absorbent glass mat) 
battery will be approximately 12.80 VDC and 12.90 VDC for a gel cell.]

Check both the specific gravity in each cell with a external hydrometer 
AND the battery terminal voltage with a digital voltmeter without the 
engine running.  For sealed batteries, measuring the battery's voltage 
without the engine running with a digital voltmeter is the only way you 
can determine the state-of-charge.  Some batteries have a built-in 
hydrometer which only measures the state-of-charge in ONE of it's six 
cells.  If the indicator is clear or light yellow, then the battery has a 
low electrolyte level and should be refilled before proceeding, or if 
sealed, the battery should be replaced.

If the state-of-charge is BELOW 75% using either the specific gravity or 
voltage test or the built-in hydrometer indicates "bad" (usually dark), 
then the battery needs to be recharged BEFORE proceeding.  Replace the
battery, if one or more of the following conditions occur:

           1.  If there is a .050 or more difference in the specific 
                 gravity reading between the highest and lowest cell, you 
                 have a weak or dead cell(s), 

           2.  If the battery will not recharge to a 75% or more 
                 state-of-charge level or if the built-in hydrometer still 
                 does not indicate "good" (usually green, which is 65% 
                 state-of-charge or better), 

           3.  If digital voltmeter indicates 0 volts, you have an open
                 cell, or

           4.  If the digital voltmeter indicates 10.45 to 10.65 volts, 
                 you have a shorted cell.  [A shorted cell is caused by 
                 plates touching, sediment build-up or "treeing" between 
                 plates.



4.  WHAT DO I LOOK FOR IN BUYING A NEW BATTERY?

     A.  Reserve Capacity or Ampere Hour Rating 

The most important consideration in buying a deep cycle battery is the
Reserve Capacity (RC) or Ampere Hour (AH) rating that will meet or exceed 
your requirements.  RC is the number of minutes a fully charged battery at 
80 degrees F is discharged at 25 amps before the voltage falls below 10.5 
volts.  Some deep cycle batteries are rated in Ampere Hours.  To convert 
Reserve Capacity to Ampere Hours, multiple RC by .6.  For example, a 
battery with 120 minute RC will have approximately 72 Ampere Hours.  This 
means that the battery should produce one amp for 72 hours of continuous 
use.  Since shallower the average discharge increases the battery life, 
more RC is better in every case.

[If more RC is required, two six volt batteries can be connected in series 
or two (or more) 12 volt batteries can be connected in parallel.  Within a 
BCI group size, generally the battery with larger RC will weigh more 
because it contains more lead.]


     B.  Type

Car batteries are especially designed for high initial cranking amps 
(usually for five seconds) to start a car and no deep cycle discharges.
Deep cycle (and marine) batteries are designed for prolonged discharges at 
lower amperage.  [The plates in car battery are more porous and thinner 
than in a deep cycle battery.]  A deep cycle battery will typically outlast 
two to four car batteries used in deep cycle applications.  A "dual marine" 
battery is a compromise between a car and deep cycle battery and is used 
to start small engines and to provide deep cycle discharge capability. 


Using two battery setups through a diode isolator is popular in 
recreational vehicle (RV) applications.  A car battery is used to start
the engine and deep cycle battery is to supply power to the accessories.
When purchasing a isolator, be sure that it matches your alternator or
charging system.  [The batteries are connected to a diode isolator and 
both are automatically recharged by the RV's charging system when engine 
is running.  For additional information on multi-battery applications, 
call (800) 845-6269 or (503) 692-5360 and request a free copy of 
"Introduction to Batteries and Charging Systems" by Ralph Scheidler.]

The two most common types of  deep cycle batteries are flooded ( a.k.a. wet 
or liquid electrolyte) cell and valve regulated (VR).  

           1.  Flooded cell

                 Flooded cell deep cycle batteries are divided, like their 
                 car battery counterparts, into low maintenance (the most 
                 common) and  maintenance free based on their plate 
                 formulation.  [Low maintenance batteries have  
                 lead-antimony/calcium (dual alloy or hybrid) plates; 
                 whereas, the maintenance free batteries use 
                 lead-calcium/calcium.]  The advantages of maintenance 
                 free batteries are less preventative maintenance, longer 
                 life, faster recharging, greater overcharge resistance, 
                 reduced terminal corrosion and longer shelf life, but are 
                 more prone to deep discharge (dead battery) failures due 
                 to increased shedding of active plate material and more 
                 expensive.  

               2. Valve Regulated

                 Valve Regulated (VR) batteries are divided into two 
                 groups, gel cell and Absorbent Glass Mat  (AGM).  VR 
                 batteries are spill proof, so they can be used in closed 
                 areas, are totally maintenance free, and have a longer 
                 shelf life.  Their greatest disadvantage is the high 
                 initial cost (two to three times), but arguably could 
                  have an overall lower cost due to a longer lifetime.   

     C.  Size

An internationally adopted Battery Council International (BCI) group 
number (24, 27, 31, etc.) is based on the physical case size, terminal 
placement and terminal polarity.  Within a group, the RC ratings, 
warranty and battery type will vary in models of the same brand or from 
brand to brand.  Generally, batteries are sold by model, so some of the 
group numbers will vary for the same price.  This means that for the same 
price you can potentially buy a physically larger battery with more RC 
than the battery you are replacing.  Be sure that the replacement battery 
will fit, the cables will correct to the correct terminal, and that the 
terminals will not touch anything else.

The battery manufacturers publish application guides that will contain the
BCI group number replacement recommendations, and battery size, and RC
specifications.  Manufacturers might not build or the store might not carry 
all the group numbers.  

Battery manufacturers or distributors will often "private label" their 
batteries for large chain stores.  A list of the largest domestic 
battery manufacturers/distributors in North America and my understanding 
of some of their brand names, trademarks and private labels can be found
in Car Battery FAQ

     D.  Freshness

Determining the "freshness" of a battery is sometimes difficult. Never buy
a flooded battery that is more than six months old.  Stamped on the case 
or printed on a sticker is the date of manufacture.  It is usually a
combination of alpha and numeric characters with letters for the months
starting with "A" for January (generally skipping  the letter "I") and 
digit for the year, for example, "F5" for June, 1995.  Like bread, fresher 
is definitely better.

     E.  Warranty

As with tire warranties, battery warranties are not necessarily indicative
of the quality or cost over the life of the application.  Manufacturers will
prorate warranties based on the list price, so if a battery failed half
way or more through its warranty period, buying a new replacement might
cost you less.  The exception is the free replacement warranty period.
This represents the risk that the manufacturer is willing to assume.  A
longer free replacement warranty period is better.


5.  HOW DO I INSTALL A BATTERY?

     A.  Thoroughly wash and clean the old battery, battery terminals and
case or tray with water to minimize problems from acid or corrosion.  
Heavy corrosion can be neutralized with a mixture of baking soda and 
water.  Also, mark the cables so you do not forget which one it is which 
when you reconnect.

     B.  Remove the NEGATIVE cable first because this will minimize the
possibility of shorting the battery  when you remove the other cable.  
Next remove the POSITIVE cable and then the hold-down bracket or clamp.  
If the hold down bracket is severely corroded, replace it.  Dispose the 
old battery by exchanging it when you buy your new one or by taking it to 
a recycling center.  Please remember that batteries contain large amounts 
of harmful lead and acid.  

     C.  After removing the old battery, be sure that the battery tray 
and cable terminals or connectors are clean.  Auto parts stores sell a 
cheap wire brush that will allow you to clean the inside of a terminal 
clamps and the terminals.  If the terminals, cables or hold down brackets 
are severely corroded, replace them.  Corroded terminals or cables will 
significantly reduce starting capability.

     D.  Thinly coat the terminal and terminal clamps with a high 
temperature grease or petroleum jelly (Vaseline) to prevent corrosion.

     E.  Place the replacement battery so that the NEGATIVE cable will
connect to the NEGATIVE terminal.  Reversing the polarity of the 
electrical system will severely damage or DESTROY it.  

     F.  After replacing the hold-down bracket, reconnect the cables in 
reverse order, i.e., attach the POSITIVE cable first and then the NEGATIVE 
cable last.  

     G.  Before using the battery, check the electrolyte levels and
state-of-charge.  Refill or recharge as required.  


6.  HOW DO I CHARGE MY BATTERY?

In addition to the earlier cautions, some more words of caution:  

     A.  NEVER, NEVER disconnect a battery cable from vehicle with the 
engine running because the battery acts like a filter for the electrical 
system.  Unfiltered [pulsating DC] electricity can damage expensive 
electrical components, e.g., radio, charging system, etc.

     B.  Check the electrolyte level and be sure it is not frozen BEFORE 
recharging.  

     C.  Do NOT add water if the electrolyte is covering the top of the 
plates because during the recharging process, it will warm up and expand.  
After recharging has been completed, RECHECK the level. 

     D.  Reinstall the vent caps BEFORE recharging and recharge ONLY in 
well ventilated areas.  NO smoking, sparks or open flames because while
the battery is being recharged because they give off explosive gasses.  

     E.  If your battery is a VR type or a sealed flooded type, do NOT 
recharge with current ABOVE 12% of the battery's RC rating (or the 20% of 
the amp hour rating).

     F.  Follow the charger manufacturer's procedures for connecting and 
disconnecting cables and operation to minimize the possibility of an 
explosion, but generally you should turn the charger OFF before connecting 
or disconnecting cables to a battery.

     G.  If a battery becomes hot, or if violent gassing or spewing of 
electrolyte occurs, turn the charger off temporarily or reduce the 
charging rate.

     H.  Insure that in car charging with an external charger will not 
damage the electrical system with high voltages.  If this is even a remote 
possibility, then disconnect the car's negative battery cable from the 
battery BEFORE connecting the charger.

     I.  If you are recharging VR (AGM or gel cell) batteries, manufacturer's 
charging voltages can be very critical and you might need special 
recharging equipment.  In most cases standard car or flooded deep cycle 
battery chargers can not be used.

Use an external constant current charger which is set not to deliver more 
than 12% of the RC rating of the battery and monitor the state-of-charge.  
For fully discharged batteries, the following table, published by BCI, 
lists the recommended battery charging rates and times:

    Reserve Capacity         Slow Charge           Fast Charge
      (RC) Rating 

  80 Minutes or less      15 Hours @ 3 amps      5 Hours @ 10 amps

  80 to 125 Minutes       21 Hours @ 4 amps     7.5 Hours @ 10 amps

  125 to 170 Minutes      22 Hours @ 5 amps     10 Hours @ 10 amps

  170 to 250 Minutes      23 Hours @ 6 amps     7.5 Hours @ 20 amps

  Above 250 Minutes       24 Hours @ 10 amps     6 Hours @ 40 amps
    
The BEST method is to SLOWLY recharge it using an external constant
voltage (or tapered current charger) because the electrolyte has more time
to penetrate the plates.  A constant voltage "automatic" charger applies
regulated voltage at approximately 14.4 volts.  A 10 amp automatic charger
will cost between $30 and $60 U.S. at an auto parts store. 

[An excellent automatic constant voltage battery charger is a 15 volt 
regulated power supply adjusted to 14.4 volts at 80 degrees F (26.7 
degrees C).  If 32 degrees F (0 degrees C), then increase the charging 
voltage to 15.3 volts.  When charging a maintenance free battery, add .2 
volts.  ]

If left unattended, a cheap, unregulated trickle battery chargers can
overcharge your battery because they can "boil off" the electrolyte.  Do
NOT use fast, high rate, or boost chargers on any battery that is sulfated
or deeply discharged.  This condition requires a constant current from one
to two amps for 60 to 120 hours.  The electrolyte should NEVER bubble
violently while recharging because high currents only create heat and
excess explosive gasses.


7.  CAN I INCREASE THE LIFE OF MY BATTERY?

Recharging slowly and keeping your battery well maintained are the best
ways to extend the life of your battery.  For cold climates, keeping the
battery fully charged will help.  In the warmer climates and during the 
summer, check the electrolyte levels more frequently and add distilled 
water, if required.  Never add acid--just distilled water and do not 
overfill.  The shallower the average discharge, the longer the battery life.

Recharge a deep cycle battery as soon as possible after each use.
Maintaining the correct electrolyte levels, tightening loose hold-down
clamps and terminals, and removing corrosion is normally the only
preventative maintenance required for a battery.


8.  WHAT ARE THE MOST COMMON CAUSES OF BATTERY FAILURES?

     A.  Loss of electrolyte due to heat or overcharging,

     B.  Sulfation in storage,

     C.  Undercharging with voltages less than 13.8 volts,

     D.  Old age,

     E.  Vibration,

     F.  Freezing,

     G.  Using tap water,

     H.  Corrosion.

    
9.  WHAT ARE SOME OF THE MYTHS ABOUT BATTERIES?

     A.  Storing a battery on a concrete floor will discharge them.

Modern lead acid battery cases are better sealed, so external leakage 
causing discharge is no longer a problem.  [Temperature stratification 
within large batteries can accelerate the internal "leakage" or self 
discharge if the battery is sitting on an extremely cold floor in a warm 
room or installed in a submarine.]

     B.  Driving a vehicle will fully recharge a battery.  

There are a number of factors affecting alternator's ability to charge 
a battery.  The greatest factors are how much current from the alternator 
is diverted to the battery to charge it, how long the current is available 
and temperature.  Generally, running the engine at idle, short 
"stop-and-go trips", or  during bad weather at night will not recharge the 
battery.  

     C.  A battery will not explode.  

While recharging, a battery produces hydrogen and oxygen gasses.  If a
spark occurs, an explosion can occur.  Remember the "Hindenburg"?

     D.  A battery will not lose its charge sitting in storage.  

A battery has internal electrical leakage that will cause it to become
fully discharged and sulfated over time.  Prior to storing a battery, it
should be fully charged and recharged when it reaches  80%
state-of-charge or six months, whichever occurs first.

     E.  How long will a deep cycle battery last?

Discharging, like charging, depends on a number of factors.  The most
important ones are the initial state-of-charge, capacity of the battery,
load and temperature.  For a fully charged battery at 80 degrees F, the
ampere hour rating divided by the load in amps will provide the estimated
life of that cycle.  For example, a 72 ampere hour battery with a 10 amp
load should last approximately 7.2 hours.


10.  WHERE CAN I FIND MORE INFO ON BATTERIES?

Additional information sources about deep cycle batteries can be found in the links page.

Copies of the Car Battery FAQ and the Deep Cycle Battery FAQ are also available by requesting them by e-mail from Bill Darden.


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