Battery FAQ

by William Darden

JUNE 24, 1995

A word of caution. Batteries contain a sulfuric acid electrolyte which is a highly corrosive poison and produces gasses that will explode if ignited. When working with batteries, you need to remove jewelry, wear protective clothing and eye ware, and exercise caution. Follow the manufacturer's instructions for testing, jumping, installing and charging.


      A.  Check specific gravity in each cell and remove surface charge
             before load testing,
      B.  Retest after deep discharges or jump starts,
      C.  Recharge slowly at 14.6 volts,
      D.  Buy the freshest and largest Reserve Capacity, non-sealed
             (low maintenance) car battery that will fit with a CCA
             rating for your climate that meets or exceeds the car's
             OEM cranking amps requirements, and
      E.  Perform preventative maintenance, especially during warm

 A good quality car battery will cost between $50 and $100 and, if properly
 maintained, will give you four to eight years of service.  The purpose of
 a battery is to start the car; to provide power for the ignition, lighting
 and other accessories when their combined load exceeds the capability of
 the alternator; and to filter the power.  Dead batteries almost always
 occur at the most inopportune times, e.g., returning home from a long
 trip, during bad weather, late at night in a dark parking lot or when you
 battery or more for an emergency jump start or tow.

 Most store employees do not know how to correctly test a battery.  The
 national average of "defective" batteries returned to the manufacturer
 that are in fact good is 50%.

      A.  Visually inspect for obvious problems, e.g., damaged case,
 corrosion, loose hold-down clamps or cable terminals, or low electrolyte.
      B.  If you have just recharged you battery or driven your car,
 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.  Turn the headlights on high beam for three minutes and
                 wait five minutes before further testing, or
           3.  With a battery load tester, apply a 150 amp load
                 for 10-15 seconds.
      C.  Using 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 can be purchased at a auto parts store for
 approximately five dollars.  If the battery is sealed (maintenance free),
 the correct procedure to test it is to measure the battery's voltage
 WITHOUT the engine running with a good quality digital DC voltmeter.  Some
 sealed batteries have built-in hydrometers.  They are not good testing
 devices because they only measure the state-of-charge in one of the six

 If the state-of-charge is BELOW 75% using either test, then the battery
 needs to be recharged BEFORE proceeding.  If there is a .050 or more
 difference in the specific gravity reading between the highest and lowest
 cell or the battery will not recharge to 75% or higher, then the battery
 should be replaced.
           Battery            Approximate           Average Cell
           Voltage          State-of-charge       Specific Gravity
            12.66                100%                  1.265
            12.45                 75%                  1.225
            12.24                 50%                  1.190
            12.06                 25%                  1.155
            11.89                  0%                  1.120
 Note:  If the temperature of the electrolyte is below 70 degrees F, then
 add .012 volts (12 millivolts) per degree below 70 degrees F.
      D.  If the battery's state-of-charge is at 75% or higher, then load
 test the battery by one of the following methods:
           1.  Turn the headlights on high beam for six minutes,
           2.  Disable the ignition and turn the engine over for 15
                 seconds with the starter motor,
           3.  With a battery load tester, apply a load equal to one half
                 of the Cold Cranking Amp (CCA) rating of the battery, or
           4.  With a battery load tester, apply a load equal to one half
                 the Original Equipment Manufacturer (OEM) cranking amp
 9.7 volts with the electrolyte at 80 degrees F.  (If the electrolyte
 is above 80 degrees, add .1 volt for every 10 degrees above 80 until you
 reach 100 degrees.  If below 80 degrees, subtract .1 volt for every 10
 degrees until 40 degrees.)  After the load is removed, the battery should
 "bounce back" to the 50% state-of-charge level or above.  If the battery
 drops below minimum test voltage, does not bounce back or will not start
 the engine, then you should replace it.  If it passes this test, you
 should recharge your battery to restore it to peak performance.
 When the charging system fails, usually the alternator light will come on.
 With a good battery and the engine running at 2000 RPM or more for two
 minutes, depending on the load and ambient temperature, the voltage will
 increase to between 13.0 and 15.1 volts.  Other factors affecting the
 charging voltage are the battery's age, state of charge, and electrolyte
 level and purity.  A loose alternator belt will significantly reduce the
 alternator's output.
 This assumes a 12 volt negative grounded system found on almost every car
 in the United States.
      A.  Determine that the electrolyte is NOT frozen.  If frozen, allow
             to thaw.
      B.  Turn off all unnecessary accessories and lights on both cars.
      C.  Start the car with the good battery and let it run for at least
             two or three minutes at fast idle BEFORE attempting to start
             the disabled car.
      D.  Connect the POSITIVE terminal on the disabled battery to the
             POSITIVE terminal on the good battery.
      E.  Connect the NEGATIVE terminal on the good battery to a clean,
             unpainted area on the ENGINE BLOCK on the disabled car.
      F.  Let the good car to continue to run for five minutes or more.
             This will allow the dead battery to receive some charge and
             to warm it's electrolyte.
      G.  Start the disabled car and allow to run at fast idle.
      H.  Disconnect the jumper cables in the reverse order, starting
             with the ENGINE BLOCK on the disable car.
      A.  Cold Cranking Amps
 The most important consideration is the battery's CCA rating.  CCAs are
 the discharge load measured in amps that a fully charged battery at 0
 degrees F can deliver for 30 seconds and while maintaining the voltage
 above 7.2 volts.  Batteries are sometimes advertised by their Cranking
 Amps (CA) measured at 32 degrees or Hot Cranking Amps (HCA) measured at 80
 degrees, which are not the same as CCA.  Do not be mislead by CAs or HCAs.
 To convert CAs to CCAs, multiply the CAs by .8.  To convert HCAs to CCAs,
 multiply HCAs by .69.  In hot climates, buying batteries with double or
 triple the CCA ratings that exceed the OEM requirement is a WASTE of
 money.  However, in colder climates the higher CCA rating the better, due
 to increased power required to crank a sluggish engine and the
 inefficiency of the cold battery.  One of the major manufacturers, Exide,
 publishes the following table:
       Available Power        Temperature        Power Required
        From Battery           degrees F         To Crank Engine
            100%                  80                  100%
             65%                  32                  155%
             40%                   0                  210%
             25%                 -32                  350%
      B.  Reserve Capacity
 The next most important consideration in buying a battery is the Reserve
 Capacity (RC) rating because of the effects of an increased parasitic or
 "key off" load produced by electrical devices, e.g., fans, clocks,
 computers, etc., that operate after the engine is stopped.  RC is the
 number of minutes a fully charged battery at 80 degrees F can be
 discharged at 25 amps until the voltage falls below 10.5 volts.  More RC
 is better in every case.  For example, if your car has a 360 OEM cranking
 amp requirement, then a 450 to 500 CCA rated battery with 120 minute RC
 would be more desirable in a warm climate than one with 700 to 800 CCA with
 90 minutes of RC.
      C.  Type
 A sealed or "maintenance free" battery will NOT allow you to test the
 specific gravity with a hydrometer or add DISTILLED water when required.
 Sealed batteries are more prone to deep discharge failures, but require
 less preventative maintenance.  Car batteries are specially designed for
 high initial cranking amps (usually for five seconds) to start a car;
 whereas, deep cycle or "marine" batteries are designed for prolonged
 discharges at lower amperage.  A "dual marine" battery is a compromise
 between a car and deep cycle battery.  However, a CAR battery will give
 you the best performance in a car.  Some manufactures have introduced a
 "dual" battery that combines a standard battery with emergency backup
 cells.  For about the same cost a better approach is to buy two batteries
 and isolate them.
      D.  Size
 Manufacturers build their batteries to an internationally adopted BCI
 group number (24F, 35, etc.) specification, which is based on the physical
 case size, terminal placement and terminal polarity.  The OEM battery
 group number is a good starting place to determine the replacement
 will vary in models of the same brand or from brand to brand.  Batteries
 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 the hood when
 The battery manufacturers publish application guides that will contain the
 OEM cranking amp and group number replacement recommendations by make,
 model and year of car, and battery size, CCA and RC specifications.
 Manufacturers might not build or the store might not carry all the group
 numbers.  To reduce inventory costs, dual terminal "universal" batteries
 that will replace several group sizes are becoming more popular.  The four
 largest domestic battery manufacturers are Johnson Controls (Interstate,
 Motorcraft, Energizer, older Diehards), Delco (Sears, newer Diehards), GNB
 (Champion) and Exide (NAPA).
      E.  Freshness
 buy a battery that is MORE than SIX months old.  The date of manufacture
 combination of alpha and numeric characters with letters for the months
 starting with "A" for January (skipping "I") and digit for the year, e.g.,
 "F5" for June, 1995.  Like bread, fresher is definitely better.
      F.  Warranty
 As with tire warranties, battery warranties are not necessarily indicative
 of the quality or cost over the life of the car.  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.
      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.  Auto parts stores sell a cheap wire brush that will allow you to
 clean the inside of a terminal clamps.  Mark the positive cable so you do
 not forget which one it is when you reconnect.
      B.  Remove the NEGATIVE cable first, the POSITIVE cable and then the
 hold-down bracket or clamp.  Dispose the old battery by exchanging it when
 you buy your new one or at a recycling center.  Batteries contain large
 amounts of lead and acid.
      C.  After removing the old battery, be sure that the battery tray and
 cable connectors are clean.  If the cables are corroded or damaged,
 replace them; otherwise, they will significantly reduce starting
      D.  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 it.
      E.  After replacing the hold-down bracket, reconnect the POSITIVE
 cable first and then the NEGATIVE cable.
      F.  Before starting the engine, check the electrolyte levels and
 state-of-charge and refill or recharge as required.
      G.  Coat the terminals with a high temperature grease to prevent
 If you do not want to lose your car's computer memory, security codes or
 radio settings, a second battery can be temporarily connected to the
 electrical system in parallel before disconnecting the first one.  A
 cigarette lighter plug can easily connect a parallel battery.
 Some more words of caution.  Do NOT ever disconnect a battery cable from
 car with the engine running because the battery acts like a filter for the
 electrical system.  Unfiltered electricity can damage the electrical
 components, for example, computer, radio, etc.  Check the electrolyte level
 before recharging.  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 and the electrolyte has
 cooled, RECHECK the level and add DISTILLED water to 1/8" BELOW the bottom
 of the filler tube (vent wells) or to the level indicated by the battery
 manufacturer.  Reinstall the vent caps before recharging and recharge ONLY
 in well ventilated areas.  NO smoking, sparks or open flames because while
 being recharged, batteries give off explosive gasses.  If your battery is
 the sealed, do NOT recharge with high current.
      A.  Usually, a car is jump started and run to recharge the battery,
 which might NOT fully charge it.  The length of time to fully recharge the
 that is diverted to the battery, how long the engine is run, RPM, and
 temperature.  That is, an alternator is sized by the car manufacturer to
 carry the maximum accessory load and maintain a battery, NOT recharge a
 dead one.  If you jump start your car, test your battery after you have
 finished driving it, and recharge if the state-of-charge is below 75%.
      B.  A better method to recharge batteries is to use an external
 constant current charger which is set not to deliver more than 1% of the
 CCA rating of the battery.
      C.  The best method is to use an external constant voltage or tapered
 current charger.  A constant voltage "automatic" charger applies regulated
 voltage at approximately 14.6 volts.  A 10 amp automatic charger will cost
 between $30 and $50 at an auto parts store.  To prevent damage to the
 first 30 minutes.  With a taper charger, a high current, up to 30 amps, is
 applied to the battery for a short period up to 30 minutes maximum and
 then is regulated downward until the charge state reaches 100%.
 For batteries with a state-of-charge of 25%, the following table,
 published by Interstate Batteries, lists the recommended battery charging
 rates and times:
      Reserve Capacity            Slow Charge          Fast Charge
        (RC) Rating                @ 5 Amps             @ 20 Amps
      80 Minutes or less           10 Hours             2.5 Hours
      80 to 125 Minutes            15 Hours              4 Hours
      125 to 170 Minutes           22 Hours              5 Hours
 If left unattended, cheap, unregulated trickle battery chargers can
 overcharge your battery, because they can "boil off" the electrolyte.  Do
 or deeply discharged.  This condition requires a constant current from one
 to two amps for 60 to 100 hours.  The electrolyte should NEVER bubble
 violently while recharging.  High currents only create heat and excess
 Keeping your battery well maintained is the BEST way to extend the life of
 your battery.  For cold climates, keeping the battery fully charged and
 warm will help.  In the warmer climates and during the summer, the
 electrolyte levels need to be checked more frequently and DISTILLED water
 added, if required.  Batteries last approximately two thirds as long in
 hot climates as cold ones.  The parasitic load will determine how long a
 car can sit and still be started.  Disconnecting the NEGATIVE cable will
 extend the "life" of the battery.  Turning off unnecessary accessories and
 lights BEFORE starting your car will decrease the load on the battery
 while cranking.  Leaving your lights on and fully discharging the battery
 can ruin it, especially if it is the sealed or maintenance free type.
 Should this occur, you should test the battery after it has been recharged
 to determine if there is permanent damage.  NEVER add acid or
 additives--just water.
 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.
      A.  Using an undersized battery,
      B.  Loss of electrolyte due to heat or overcharging,
      C.  Overcharging with voltages greater than 15.1 volts,
      D.  Undercharging,
      E.  Old age,
      F.  Vibration,
      G.  Deep discharges (leaving your lights on),
      H.  Using tap water,
      I.  Corrosion,
      J.  Freezing, and
      K.  Overheating.
      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.
      B.  Driving a car 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
 and temperature.  Generally, short trips during bad weather will not
 recharge the battery; whereas, a long daytime drive in good weather will.
      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 it's 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 the 50%
 state-of-charge level.  Batteries on store shelves should be checked
 periodically and recharged if necessary.
      E.  Maintenance free battery never requires electrolyte.
 In warm climates, the electrolyte could be "boiled off" due to the high
 underhood temperatures.  It could also be lost due to excessive charging
 voltage or using high charging currents.

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