BATTERY 411:Testing, charging and replacing a battery

BATTERY 411:Testing, charging and replacing a battery

In recent years, the electronic content in vehicles has multiplied several times over. More electronics means more demand on the battery and charging system.

In recent years, the electronic content in vehicles has multiplied several times over. More electronics means more 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.

For example, low voltage may cause the airbag or ABS warning lights to come on. The turn signals may not blink normally when the switch is flipped to either side. Electronic gauges 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 its normal charging output.

Many so-called battery problems are not the battery, but a charging fault. The alternator’s job is two-fold: to supply current for the vehicle’s electrical system and to maintain the battery at full charge. Normally, the battery is only used to crank the engine, to provide power for lights and accessories when the engine is not running and provide supplemental power when the demands of the vehicle’s electrical system exceed the output of the alternator.

The alternator’s output is lowest at idle, and increases with engine speed. The powertrain control module in most late-model vehicles controls charging output, so the PCM can boost the charging curve a bit when demands are high at low engine speed. Even so, most alternators can’t 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 the headlights, A/C, defrosters, radio or other accessories on, it can overtax the charging system and drain the battery.

Police cars are murder on alternators and batteries because they spend so much time idling with 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 charging system to bring the battery back up to full charge. It might take 20 to 30 minutes or more of normal driving to fully recharge the battery.

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

The average service life of a conventional lead-acid car battery is only about four to five years, 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 by evaporation. Even so, their average service life is typically five to six years depending on use.


Allowing a vehicle to sit for a long period of time without being driven (say a week or more) can allow the battery to run down. The electronic modules in today’s vehicles draw a small amount of power from the battery to keep their memories alive when the vehicle isn’t running. Many go into sleep mode and shut down after a certain period of time to reduce the power draw, but others (such as the antitheft system, keyless entry system and PCM 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). This can run the battery down fairly quickly if the vehicle sits for long periods of time, is driven only 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 can drain a battery fairly quickly. Interior lights, or a trunk or underhood light that fails to go out can also sap power from the battery when a vehicle sits overnight. Sometimes a power relay may stick on, or a module may fail to go to sleep after the engine has been turned off, causing a higher than normal key-off power drain. Any of these can run the battery down and increase the load on the charging system when the engine is first started. The result can be a chronic undercharging condition if the vehicle isn’t driven long enough to fully recharge the battery, and shortened battery life.

Any problems in the charging system itself can also allow the battery to run down and/or shorten battery life. A bad alternator, voltage regulator, faults in wiring harness or PCM voltage control circuit, or even a slipping alternator drive belt can all cause low or no charging output.


The output of the charging system can be easily checked with a voltmeter while the engine is idling. The actual output voltage produced by the charging system will vary depending on temperature and load, but will typically be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems will produce 13.8 to 14.8 volts with no lights or accessories 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 voltage capacity 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-output unit. Now, alternators that produce up to 120 to 155 amps are used in many vehicles. The current output can be measured with a charging system tester, or on a test bench if the alternator has been removed from the vehicle.

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

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


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

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

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

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

When charging a battery, do not turn the charger on until after the charger has been connected 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 is frozen, do not attempt to jump it or recharge it. Remove the battery from the vehicle, bring it indoors and allow it to thaw before recharging it.

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

The time it takes to recharge a battery will depend on the battery’s reserve capacity (RC) rating, it’s state of discharge, and the output of the battery charger. The charging rate (in amps) multiplied by the number of hours of charging time should equal the reserve capacity of the battery. For example, a dead battery with a RC rating of 72 will take about 12 hours to fully recharge with a 6 amp charger. 


A load test will tell you if a battery is good or bad. The test is done by applying a calibrated load to the battery and noting how much battery voltage drops. The test requires a 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 load testing.

The test requires loading the battery to 1/2 of its CCA rating for exactly 15 seconds. This is done by adjusting the carbon pile setting on the tester. The battery must maintain a minimum post voltage of 9.6 Volts at 70 degrees F during the test to pass. If the voltage drops 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 electronic battery conductance tester. Conductance is how much current the battery can conduct internally. 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 be fully charged for accurate test results.


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) as the original, and should have the same or higher Cold Cranking Amp (CCA) rating as the original battery. Most V6 and V8 engines require 600 CCA for reliable cold weather starting. Many diesel pickup trucks have a dual battery setup for added cranking power, so if 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 battery’s location. It may be sealed up inside a fender panel (many Chrysler cars) or in the trunk or under the back seat. If the vehicle is a hybrid, it may require a special gel cell 12-volt battery rather than a wet cell lead-acid battery. Also, use extreme caution around high-voltage hybrid batteries. Follow the vehicle manufacturer’s safety precautions. The high voltage hybrid battery is usually covered by a 10-year warranty and is a dealer-only replacement item.

Here’s another precaution that is often overlooked: Disconnecting a battery that still has voltage can wipe the memory in some modules in many late model vehicles. The resulting memory loss in the affected modules may prevent certain systems from functioning until a special relearn procedure has been performed (some of which may require using a scan tool to reset the module).

To prevent unwanted memory loss in modules, connect a “memory saver” to the electrical system before the battery is disconnected. These devices typically plug into the cigarette lighter 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) battery charger to the battery cables while the battery is being replaced.

Be extra careful when reconnecting battery cables to not reverse polarity. Reversing the connections can damage the battery, charging system, and on-board electronics (including the PCM). Except for some antique vehicles, all modern vehicles have a negative ground electrical system. The negative battery post is marked with a minus (-) sign, while the positive battery post is marked with a plus (+) sign. The battery cables may be color coded red 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 load on the charging system). Batteries are “dry charged” at the factory, but can discharge over time as they sit on the shelf. Your battery inventory should be arranged so your oldest batteries are the first on the shelf, with the newest batteries in the back. Use a voltmeter to check the charge level on your batteries, and use a charger to bring any low batteries up to full charge before they go out the door.

Battery customers should also be reminded to check the condition of the battery cables on their vehicle. A new battery can’t crank the engine normally or maintain its charge if the battery cables are loose, badly corroded or undersized. Watch out for cheap replacement battery 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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