Battery Service & Diagnosis – UnderhoodService

Battery Service & Diagnosis

A good battery with an adequate charge is absolutely essential for reliable cold starting. A weak battery or one that is run down may not deliver enough amps to crank the engine when temperatures plunge and the oil thickens. Cold weather can be hard on batteries, but so can hot weather because it increases water loss from the electrolyte inside the battery. That’s why batteries in hot climates usually don’t last as many years as those in more moderate or colder climates.

The battery must also be capable of providing backup power for the vehicle’s electrical accessories and electronic modules. When the engine is running, the charging system can usually produce enough current to meet all of the vehicle’s electrical needs. But when the engine is idling and the lights, heater, defroster, radio or other accessories are placing high loads on the system, additional power may be drawn from the battery if the alternator can’t produce enough juice. And when the key is off, the battery has to provide a steady supply of voltage to any onboard modules or other accessories that need power.

When a battery is disconnected or goes completely dead, the loss of voltage can cause a variety of problems. Radio presets and memory seat/mirror settings are lost, and so are the adaptive and learned memories in the PCM and other modules. The PCM can forget fault codes and other diagnostic information (long term fuel trim adjustments, the readiness status of OBD II system monitors, electronic transmission shift adjustments, etc.). Some climate control systems (1996 and up Dodge minivans, also Cadillac Escalade, Chevy Avalanche and Suburban) can forget the position of flow control doors and actuators. And on some late-model vehicles with Controller Area Network (CAN) electrical systems, some modules may go into a default mode, standby mode or sleep mode — which may require pinging or reprogramming with a scan tool to restore normal function. We’ve even heard of 4×4 modules on some GM SUVs never waking up if the battery is disconnected (which requires installing a new module to fix the fault!).

For this reason, never disconnect a battery on a late-model car without first attaching a backup power source to the vehicle’s electrical system. This could be another battery, a battery charger or a “Memory Saver” that plugs into a power outlet (make sure the outlet remains “hot” when the key is off otherwise it won’t feed voltage from the auxiliary battery back into the system).

Checking Battery Charge
The first thing that needs to be checked when testing a battery is its state of charge. A fully charged battery in good condition should read 12.66 volts at 80° F (deduct one-tenth of a volt for every 10° drop in temperature). If the battery reads 12.45 volts or higher, it’s at least 75% charged and should be capable of functioning normally. So if the vehicle has a starting or charging problem, it’s not the battery. If the battery reads less than 12.4 volts, it is low and needs to be recharged.

Battery voltage can be measured with any voltmeter (analog or digital). You can also use the voltmeter to check the charging voltage when the engine is running (which should be about 13.5 to 14.5 volts on most vehicles depending on the battery’s state of charge and temperature). If the charging voltage is low, that would probably explain why the battery is low.

But a simple voltage check doesn’t tell you anything about the condition of the battery, or why it is low if it less than 75% charged.

As a battery ages, the lead plates in the cells become sulfated if the battery is not maintained at or near full charge. Sulfate reduces the battery’s ability to accept and hold a charge, as well as the battery’s power output. Recharging the battery reverses the chemical reaction and forces the sulfate back into solution in the acid or gel between the plates. Even so, after many discharge-recharge cycles, the sulfate continues to accumulate preventing the battery from fully recovering. The battery then produces less and less power, and eventually it fails.

Average battery life for most lead-acid batteries is only about four years, and may be as short as three years in hot climates. Absorbent Glass Mat (AGM) gel cell batteries are somewhat better and typically last a couple of years longer than liquid acid lead-acid batteries. But no battery lasts forever.

You can’t really estimate a battery’s condition by its age alone because you don’t know if the battery has been maintained at or near full charge for most of its life. If a vehicle sits for many days at a time without being driven, or the vehicle is not driven far enough to fully recharge the battery, the battery can become sulfated very quickly. Consequently, you need to test the battery with some type of battery tester to see how it responds.

Load Testing
Load testing a battery with an adjustable carbon pile battery tester is one way to check the condition of the plates inside a battery. A load tester applies a calibrated load and displays how the battery responds. The load, which is set to half the battery’s cold cranking amp (CCA) rating, is applied for 15 seconds. As long as battery voltage remains above 9.6 volts, the battery is considered “good.” But if it drops below 9.6 volts by the end of the test, the battery may be “bad” — or it may have to be recharged and retested if it was not fully charged prior to the test. That’s one of the drawbacks of a traditional load test. For accurate results, the battery must at or above 12.45 volts (75% charged) before it is load tested — otherwise it may fail the test.

Another way to check the battery is with a small handheld electronic load tester. This type of tester does not require a fully charged battery because it only applies a small amp load to the battery. The electronics in the tester monitors how the battery responds to the load and then decides if the battery is good or bad.

Battery Conductance Testers
Another way to test a battery is to measure its “conductance.” When a low frequency alternating current (AC) signal is sent through a battery, it can reveal how much plate area is available to hold and deliver power. As a battery ages, its conductance declines. Shorts, opens and other cell defects also affect conductance, so measuring conductance gives an accurate indication of battery condition.

Some battery testers use a slight variation of this technique and send a high frequency signal through the battery to measure its internal resistance.

One of the main advantages of conductance testing is that it can test batteries that are low or almost completely discharged. It can be done with the battery in the vehicle and without disconnecting the terminals (though the terminals must be clean and tight). If the posts or side terminals on a battery do need to be cleaned (which is recommended), remember to use a battery backup to maintain voltage to the onboard electronics.

A conductance test can also be repeated multiple times on the same battery without draining the battery (unlike a traditional load test which pulls a lot of amps out of the battery). That also saves time because there’s no need to recharge the battery before or after testing.

Some conductance testers also analyze the battery’s CCA capacity, which can be used to estimate the battery’s remaining service life. Some also allow you to measure the amps drawn by the starter while cranking the engine, and analyze the charging system’s output under load once the engine is running.

If the tester reads cold cranking amps, you can also use it to diagnose bad ground connections by running a battery CCA test at the battery terminals, then repeating the same test using a ground point on the engine or elsewhere. A difference of more than 25% in the CCA readings between tests indicates a bad ground.

Key-Off Current Drains
If a vehicle has a battery that keeps running down, but the battery and charging system both test OK, the problem may be a key-off current drain that’s sapping the battery of its reserve power.

As a rule, the parasitic drain on most late-model vehicles should be less than 50 milliamps one hour after the vehicle has been shut off and left undisturbed. But this is a rule of thumb only. Always refer to the vehicle manufacturer’s key-off electrical drain specifications if available (some vehicle manufacturers have no published specifications).

Keep in mind, though, that opening a door, the trunk or turning anything on can wake up various modules and start the timer countdown all over again. So if you want to check the parasitic draw on the battery, leave the hood open (or the trunk if the battery is located in the back).

If the key-off current reading is above specifications, it’s time to start pulling fuses and relays to isolate the fault. Refer to the vehicle owner’s manual or a wiring diagram to identify the fuses and relays. Then pull the fuses and relays one at a time until the current reading drops. Avoid pulling the fuses for the PCM or other KAM-sensitive modules until you have checked all of the other circuits. Once you’ve found the circuit that is causing the excessive current drain, check the relay, switch, module or other components in the circuit and replace as needed.

Battery Service Precautions
Handle batteries carefully because lead-acid batteries that contain liquid electrolyte can spill. The acid in the electrolyte can burn your skin and eat holes through your clothing (sometimes not immediately but when the clothes are laundered). Gel type batteries are safer in this respect because they contain no liquid and can’t spill.

If carrying or replacing a battery that contains liquid electrolyte, use a carrying strap that attaches to the battery posts or the strap on the battery case (if it has one). Or, lift and support the battery from underneath. Don’t squeeze the sides of the battery because the plastic case can flex and force acid out of the vents on top. Wear rubber or nitrile gloves to protect your hands. Avoid direct skin contact with the battery itself, and wear eye protection.

Warning: Batteries can explode. When the electrolyte inside the battery breaks down, it releases hydrogen gas. Hydrogen is flammable and can explode if ignited by a spark. Do not smoke around a battery, or use anything that produces an open flame or spark. When charging a battery, do not turn the charger on until after the charger has been connected to the battery.

Jumping a Dead Battery
When a battery is being jump-started by another vehicle, the positive jumper cable should be connected first between the two batteries. The negative cable should then be connected to the good battery, and the final connection made to metal ground on the vehicle being jumped — not the negative post on the dead battery. Why? Because the final jumper connection will usually spark, and you want to keep the spark away from the battery.

The safest way to jump-start a vehicle with a dead battery is to use a pair of “smart” jumper cables that automatically recognize the polarity of the post connections. This prevents reversed connections that could damage the charging or electrical system on either vehicle. The smart cable module also delays the completion of the circuit after the final connection has been made to eliminate any sparking at the battery post.

Do not attempt to jump start or recharge a frozen battery. Remove the battery from the vehicle, bring it into a warm room and let it thaw before charging or testing. Generally, any battery that has been frozen will require replacement.

Always wear safety glasses when charging or jump-starting a battery to protect your eyes, and wear gloves to protect your hands.

Shock Hazards
Most automotive batteries only produce 12 volts, so there is no danger of being shocked by the battery, charger or jumper cables. The one exception is high voltage hybrid batteries, which pack as much as 300 volts and are capable of delivering a lethal shock. On hybrid vehicles, don’t touch any of the hybrid electrical components without first isolating (disconnecting) the hybrid battery. Most have a safety switch that disconnects the battery from the rest of the electrical system. Refer to the vehicle owner’s manual or the OEM service procedures for how to do this as procedures vary from one vehicle to another. Wear gloves rated to withstand 1,000 volts and don’t touch any orange color-coded cables until the high voltage battery has been disconnected.

Ordinary 12-volt batteries may not shock you, but they can produce currents of several hundreds amps, which is equivalent to an electric welding arc. Never allow positive and negative jumper cables to touch each other, or metal objects to come into contact with the battery posts.

To prevent accidental shorts, the battery negative cable should be disconnected prior to making electrical repairs or replacing electrical or electronic components.

Caution: Never disconnect a battery while the ignition is on or the engine is running. This can damage electrical and electronic components.

Battery Charging Tips

  • Do not turn the charger on until both leads have been connected to the battery. Connect positive to positive, and negative to negative. Do not reverse the connections as doing so may damage the charger and battery.


  • Slow-charging at 6 amps or less is best because it develops less heat inside the battery (which can damage cells and increase evaporation). A slow charging rate will also break up the sulfation on the battery plates more efficiently to bring the battery back up to full charge.


  • “Smart chargers” also work well because they automatically adjust the charge rate to the battery’s state of charge. Most start out with a charging rate of 15 amps or higher, then taper off the charging rate as the battery charge comes up.


  • Fast-charging a battery saves time, but risks overheating and damaging the battery. If using a fast-charger, don’t use the highest setting. Use the “boost” setting only when cranking the engine.


  • Refer to the battery’s reserve capacity (RC) rating to determine charging times and rates. The charging rate (in amps) multiplied by the number of hours of charging time should equal the reserve capacity of the battery. (Example: a dead battery with a RC rating of 72 will require charging at 6 amps for 12 hours.)


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