car battery 411

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Battery 411: Testing, Charging and

Replacing a Battery

In recent years, the electronic content in vehicles has

multiplied several times over. More electronics meansmore demand on the battery and charging system. A

weak battery or low system voltage due to a charging

problem can cause all kinds of havoc with the on-board

electronics.

Larry Carley

6/12/2012

In recent years, the electronic content in vehicles has multiplied several times over. Moreelectronics means more demand on the battery and charging system. A weak battery orlow system voltage due to a charging problem can cause all kinds of havoc with the on-

board electronics.

For example, low voltage may cause the airbag or ABS warning lights to come on. Theturn signals may not blink normally when the switch is flipped to either side. Electronicgauges may give strange or erratic readings. The engine may lack power, misfire or stall.Any of these things may occur if the battery is low or the alternator is not producing itsnormal charging output.

Many so-called battery problems are not the battery, but a charging fault. The alternatorsjob is two-fold: to supply current for the vehicles electrical system and to maintain thebattery at full charge. Normally, the battery is only used to crank the engine, to provide


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power for lights and accessories when the engine is not running and provide supplementalpower when the demands of the vehicles electrical system exceed the output of thealternator.

The alternators output is lowest at idle, and increases with engine speed. The powertraincontrol module in most late-model vehicles controls charging output, so the PCM canboost the charging curve a bit when demands are high at low engine speed. Even so, mostalternators cant achieve maximum output until engine speed reaches about 3,000 RPM or

higher. Consequently, if the engine is left idling for a long period of time with theheadlights, A/C, defrosters, radio or other accessories on, it can overtax the chargingsystem and drain the battery.

Police cars are murder on alternators and batteries because they spend so much time idlingwith high electrical loads on the charging system (lights, radios, heater or A/C, etc.).

If the battery is low when a vehicle is first started, it takes some time for the chargingsystem to bring the battery back up to full charge. It might take 20 to 30 minutes or moreof normal driving to fully recharge the battery.

Lead-acid battery technology is actually ancient. But it is simple, cost-effective andgenerally provides an adequate power for most automotive applications. But automotivelead-acid batteries must be maintained at or near full charge for the cells to last. If thebattery is allowed to run down or discharge excessively and is not fully recharged within afew days, the lead plates inside the battery can become permanently sulfated. This willreduce the batterys ability to accept and hold a charge, and drastically shorten thebatterys life.

The average service life of a conventional lead-acid car battery is only about four to fiveyears, and typically a year or so less in extremely hot climates. Gel-cell batteries that do

not contain liquid acid electrolyte are better in this respect because they are less affected byevaporation. Even so, their average service life is typically five to six years depending onuse.

Battery Power Drains

Allowing a vehicle to sit for a long period of time without being driven (say a week ormore) can allow the battery to run down. The electronic modules in todays vehicles drawa small amount of power from the battery to keep their memories alive when the vehicleisnt running. Many go into sleep mode and shut down after a certain period of time toreduce the power draw, but others (such as the antitheft system, keyless entry system andPCM keep-alive memory) are always on. Because of this, the key-off power drain can be

fairly high in many late model vehicles (80 milliamps to several hundred milliamps). Thiscan run the battery down fairly quickly if the vehicle sits for long periods of time, is drivenonly infrequently or for short trips, or has a weak battery or low charging system output.

Abnormal key-off power drains can also run down a battery. Leaving the lights on candrain a battery fairly quickly. Interior lights, or a trunk or underhood light that fails to goout can also sap power from the battery when a vehicle sits overnight. Sometimes a powerrelay may stick on, or a module may fail to go to sleep after the engine has been turnedoff, causing a higher than normal key-off power drain. Any of these can run the batterydown and increase the load on the charging system when the engine is first started. Theresult can be a chronic undercharging condition if the vehicle isnt driven long enough to


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fully recharge the battery, and shortened battery life.

Any problems in the charging system itself can also allow the battery to run down and/orshorten battery life. A bad alternator, voltage regulator, faults in wiring harness or PCMvoltage control circuit, or even a slipping alternator drive belt can all cause low or nocharging output.

Charging Checks

The output of the charging system can be easily checked with a voltmeter while the engineis idling. The actual output voltage produced by the charging system will vary dependingon temperature and load, but will typically be about 1-1/2 to 2 volts higher than batteryvoltage. At idle, most charging systems will produce 13.8 to 14.8 volts with no lights oraccessories on.

If the current produced by the charging system is not sufficient to recharge a low battery,the battery may never achieve full charge. This can lead to a permanent loss of voltagecapacity inside the battery as the plates become sulfated.

The current (amperage) produced by the charging system is also important to maintain a

fully charged battery. Not long ago, an 80-amp alternator was considered a high-outputunit. Now, alternators that produce up to 120 to 155 amps are used in many vehicles. Thecurrent output can be measured with a charging system tester, or on a test bench if thealternator has been removed from the vehicle.

Alternator power ratings can also be given in watts (which is volts times amps). Manyalternators in foreign vehicles are rated in watts rather than amps. The important point hereis to make sure a replacement alternator has the same power rating (in amps or watts) asthe original so the charging system can maintain the same power output as before, shouldthe alternator need to be replaced.

If your store has a bench tester or a portable charging system tester, you should alwaysrecommend testing a customers alternator if their battery keeps running down, is dead orhas failed prematurely. This can prevent unnecessary battery warranty claims if they buy anew battery only to have it run down or fail due to a charging fault.

Battery TestsBatteries need to be tested for two things: state of charge (a base voltage measurement thatshows if the battery is low or fully charged), and capacity (a load or conductance test thatchecks the condition of the plates inside the battery).

Connecting a voltmeter to the batterys positive and negative terminals (key off and alllights and accessories off) will reveal the charge level of the battery. A reading of 12.66volts indicates a fully charged battery. If the reading is 12.45 volts or less, the battery islow and needs to be recharged.

Some batteries have a built-in charge indicator. A green dot tells you the battery is 75percent or more charged. A dark indicator (no dot visible), means the cell is low and thebattery needs to be recharged. A yellow or clear indicator tells you the electrolyte levelinside the cell is low and the battery needs water. If the battery has a sealed top and watercannot be added to the cells, do not attempt to recharge the battery. The battery must bereplaced.


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If a battery is low, use a charger to bring it back up to full charge. Alternators are designedto maintain the battery charge, not to recharge dead batteries. A heavier than normalcharging load on an alternator may overheat and damage the diode trio (rectifier) in thealternator, causing it to fail.

When charging a battery, do not turn the charger on until after the charger has beenconnected to the battery. Sparks can be very dangerous around a car battery because lead-

acid batteries give off hydrogen gas, which is highly flammable. Also, if a battery isfrozen, do not attempt to jump it or recharge it. Remove the battery from the vehicle, bringit indoors and allow it to thaw before recharging it.

Slow-charging is usually better than fast charging. Fast-charging saves time, but risksoverheating the battery. Slow-charging at 6 amps or less develops less heat inside thebattery and breaks up the sulfate on the battery plates more efficiently to bring the batteryback up to full charge. Smart Chargers automatically adjust the charging rate. Most startout with a charging rate of 15 amps or higher, then taper off the charging rate as thebattery comes up.

The time it takes to recharge a battery will depend on the batterys reserve capacity (RC)rating, its state of discharge, and the output of the battery charger. The charging rate (inamps) multiplied by the number of hours of charging time should equal the reservecapacity of the battery. For example, a dead battery with a RC rating of 72 will take about12 hours to fully recharge with a 6 amp charger.

Testing Battery Condition

A load test will tell you if a battery is good or bad. The test is done by applying acalibrated load to the battery and noting how much battery voltage drops. The test requiresa carbon pile load tester, a volt/amp meter (if not part of the load tester), and a battery that

is 75 percent or more charged. If the battery is low it must be recharged prior to loadtesting.

The test requires loading the battery to 1/2 of its CCA rating for exactly 15 seconds. Thisis done by adjusting the carbon pile setting on the tester. The battery must maintain aminimum post voltage of 9.6 Volts at 70 degrees F during the test to pass. If the voltagedrops below 9.6 volts, the battery is bad and needs to be replaced.

A faster and easier method to check the condition of a battery is to use an electronicbattery conductance tester. Conductance is how much current the battery can conductinternally. Conductance is determined by sending an alternating frequency signal through

the battery. The main advantage with this method is that the battery does NOT have to befully charged for accurate test results.

Battery Replacement

If a battery tests bad, or it will not accept or hold a charge, it will have to be replaced.There is no way to rejuvenate an old sulfated battery or a battery with internal shorts,opens or cell damage.

A replacement battery must be the same group size (dimensions and post configuration) asthe original, and should have the same or higher Cold Cranking Amp (CCA) rating as theoriginal battery. Most V6 and V8 engines require 600 CCA for reliable cold weather


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starting. Many diesel pickup trucks have a dual battery setup for added cranking power, soif one battery has failed it is usually a good idea to replace both batteries at the same time.

Replacing a battery in some vehicles can be difficult because of the batterys location. Itmay be sealed up inside a fender panel (many Chrysler cars) or in the trunk or under theback seat. If the vehicle is a hybrid, it may require a special gel cell 12-volt battery ratherthan a wet cell lead-acid battery. Also, use extreme caution around high-voltage hybridbatteries. Follow the vehicle manufacturers safety precautions. The high voltage hybrid

battery is usually covered by a 10-year warranty and is a dealer-only replacement item.

Heres another precaution that is often overlooked: Disconnecting a battery that still hasvoltage can wipe the memory in some modules in many late model vehicles. The resultingmemory loss in the affected modules may prevent certain systems from functioning until aspecial relearn procedure has been performed (some of which may require using a scantool to reset the module).

To prevent unwanted memory loss in modules, connect a memory saver to the electricalsystem before the battery is disconnected. These devices typically plug into the cigarettelighter or power outlet, or attach to the battery cables, and use a 9-volt battery to supply

power to the modules. Another option is to connect a low amperage (3 amps) batterycharger to the battery cables while the battery is being replaced.

Be extra careful when reconnecting battery cables to not reverse polarity. Reversing theconnections can damage the battery, charging system, and on-board electronics (includingthe PCM). Except for some antique vehicles, all modern vehicles have a negative groundelectrical system. The negative battery post is marked with a minus (-) sign, while thepositive battery post is marked with a plus (+) sign. The battery cables may be color codedred for positive and black for negative (but not always, so watch out!).

Finally, batteries should be fully charged before they are installed (to reduce the initial loadon the charging system). Batteries are dry charged at the factory, but can discharge overtime as they sit on the shelf. Your battery inventory should be arranged so your oldestbatteries are the first on the shelf, with the newest batteries in the back. Use a voltmeter tocheck the charge level on your batteries, and use a charger to bring any low batteries up tofull charge before they go out the door.

Battery customers should also be reminded to check the condition of the battery cables ontheir vehicle. A new battery cant crank the engine normally or maintain its charge if thebattery cables are loose, badly corroded or undersized. Watch out for cheap replacementbattery cables that have undersized wire inside. It takes heavy gauge wire to handle all the

amps that many starting systems require.

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