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Battery Testing


A good battery is essential for reliable starting, especially during cold weather. Low temperatures increase the strain on the battery, starting and charging system and tends to bring out any weaknesses in these components.

Cold weather thickens the oil and makes the engine harder to crank. Normal cranking loads can require 125-200 amps or more from the battery depending on engine displacement, compression and temperature. At 0 degrees F, that number can increase 200-250% depending on the viscosity of the oil in the crankcase.

At the same time, freezing temperatures also sap the battery's ability to supply amps. At 0 degrees F, most batteries can only deliver about 65% of their normal cranking amps. At -20 degrees, battery power is cut in half!

A battery can't deliver maximum cranking power if it is not maintained at or near full charge - especially when outside temperatures drop and reduce the battery's amp output. So reliable starting also requires a good charging system that can keep the battery fully charged and also supply enough amps to meet all of the vehicle's other electrical needs.

test battery
If the battery is low or is getting old, the starter is weak or there is too much resistance in the starting circuit, the combination of increased cranking load and reduced battery capacity may prove to be too much when temperatures drop. The engine may not crank fast enough to start or it may not crank at all. Either way, the vehicle is not going to start.

Test Battery
Test the battery to determine its condition.
If a battery is dead or low, the first thing you have to figure out is whether the battery is still good or has failed and needs to be replaced. A good battery is one that can accept and hold a charge, and deliver the rated number of amps on demand. A bad battery is one that won't accept a charge or can't supply its normal dosage of amps due to cell damage or deterioration. Good batteries can be recharged and returned to service but bad batteries need to be replaced.

Even if a battery has a built-in charge indicator, use a voltmeter to check the battery's charge. The reason? Built-in charge indicators only read one cell, not all six cells. If another cell is bad, you would not know it by looking at the charge indicator.

A fully charged battery should read 12.6 volts. A reading of 12.4 volts equals about a 75% charge and is good enough for further testing. But anything less means the battery is low and needs to be recharged.

Battery Voltage and State of Charge:
12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%
(NOTE: these readings are at 80 degrees F. Battery voltage readings will
drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.)

Lead-acid batteries must be maintained at or near full charge to prevent deterioration of the lead plates inside. If the battery is allowed to sit more than a couple of days in a discharged condition, the plates can become sulfated and may not fully recover when the battery is recharged. This will reduce battery output as well as shorten its service life.

test battery
If the battery is run down or dead, test it to see if it is good or bad. If you still test batteries with a carbon pile tester, you will have to recharge the battery first, a process that may take half an hour to overnight depending on your charger's output and the rate at which the battery accepts the charge. Do not attempt to recharge the battery if it is frozen. Let it sit until it thaws, then check the electrolyte level. If the level is OK, then hook up your charger and see if it will accept a charge.

To get accurate results with a battery load tester, the battery must be fully charged otherwise it may fail the test. When the load is applied to the battery (typically half the battery cold cranking amp [CCA] capacity or three times its amp/hour rating), battery voltage should remain above 9.6 volts. If it can't maintain the minimum voltage, the battery is probably bad. To be sure, you can recharge it some more and retest it, or give it a "three-minute charge test."

A three-minute battery charge test checks for a sulfated battery. This battery test requires slow charging the battery at 40 amps for six minutes, then checking the voltage across the terminals with the charger on. If the voltage is above 15.5 volts, the battery is not accepting a charge. Slow charging for 20 hours can sometimes reverse the sulfate condition and save the battery, but if it cannot, the battery will have to be replaced.

Don't have time for all this testing? Then use an electronic battery tester that does not require a fully charged battery for accurate test results. Some electronic battery testers measure battery "conductance" to reveal battery condition. Sending a frequency signal through the battery reveals 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.

Many electronic battery 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 charging system output under load once the engine is running. Some testers even provide a built-in voltmeter for checking connections.

If the tester reads CCAs, 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. More than a 25% difference in the CCA readings between tests indicates a bad ground.

You also can use a voltmeter to check for voltage drop across all circuit connections, too. Ideally, you should see less than 0.1 volt drop across a good connection. More than 0.4 volts drop indicates high resistance and a dirty or loose connection.

Regardless of the test method or equipment used to conduct a battery test, make sure the battery is fully recharged before returning it to service. The alternator is designed to maintain a battery charge, not to recharge a dead battery. Overloading the charging system with a dead battery can tax it to the point of where it may damage the alternator.

NOTE: If a vehicle has a history of repeat alternator failures, it might mean the battery is not building up normal resistance as it accepts a charge. This, in turn, makes the alternator keep charging the battery at a higher than normal rate. The result is that the alternator runs hot, overheats and eventually fails from being over worked. The battery charging current should gradually decrease after the engine starts, and taper off to less than 10 amps at idle (with no lights or accessories on) after five minutes of running. If a fully-charged battery is still pulling 20 or more amps after five minutes of idling, the battery is defective and needs to be replaced.

Replacement Batteries
A replacement battery must have the same post configuration as the original (top post or side post), and fit the battery tray. Your first order of business, therefore, is to figure out the correct "group size" for the vehicle.

Next, you have to figure out how many CCAs the vehicle needs for reliable cold weather starting. The replacement battery should have the same or higher CCA rating as the original battery. Bigger is usually better, but keep in mind that some batteries sacrifice "reserve capacity" to achieve higher CCA numbers.

Another number that is important is the months of prorated warranty coverage provided by the battery manufacturer. As a rule, the higher the warranty months on the battery, the higher the battery CCA rating and the better the battery. So consider upgrading from a basic 36-month replacement battery to a premium 72-month battery.

There are also differences in battery technology and design. Though all automotive batteries today are still based on lead-acid chemistry, redesigned grids, thinner plates and new connectors allow more amps to be packaged into smaller cases. Some new batteries use a "spiral wound" cell configuration instead of flat plates to achieve a higher packaging density, more power output and increased durability.

Some batteries also use a "gel" electrolyte or "recombination" technology that replaces the liquid acid. Some have "absorbent glass mat" (AGM) separators between the plates that hold the acid much like a paper towel soaks up water, making the battery "spill-proof" even if the case is punctured. AGM technology also makes batteries more resistant to vibration damage and helps extend battery service life.

Understanding Battery Ratings

Before you can test or replace a battery, you need to know something about battery ratings:

Cold Cranking Amps (CCA) This is the most common battery capacity rating. The rating is the number of amps the battery can deliver for 30 seconds at 0 degrees F. while maintaining post voltage of 7.2 Volts. For reliable cold weather starting, most vehicles require 400 to 600 cold cranking amps. Larger displacement engines require more cranking amps. Some batteries are rated up to 1000 CCA, but may sacrifice reserve capacity to achieve high short term outputs.

Cranking Amps (CA) This is a less meaningful rating. It is the same as CCA except it is measured at 32 degrees F. A battery's CA rating can be converted to CCA by dividing the number by 1.28 (Example: a CA rating of 500 amps becomes 390 CCA).

Reserve Capacity (RC) Think of this as the battery's staying power. This is the number of minutes the battery will deliver 25 amps and still maintain a post voltage of 10.5 Volts. The higher the reserve capacity rating, the longer the battery will last if the charging system fails.

* Amp Hour Rating (A/H) This rating is not used much any more. It measures low current draw for 20 hours while maintaining a minimum post voltage of 10.5 Volts at 70 degrees F. (Example: a drain of 3 amps for 20 hours = 60 A/H rating).

Battery Installation

When installing a new battery, clean the posts and inspect the battery cables. Also, check the negative battery cable ground connection and the integrity of any engine ground straps. Loose or corroded connections can cause starting and charging problems.

Charging System Checks

Always check the performance of the charging system when replacing or recharging a battery. A charging system that is working properly should produce a charging voltage of somewhere around 14 volts at idle with the lights and accessories off (always refer to the vehicle manufacturer specifications). When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage.

check battery and alternatorCharging output also can be checked with an adjustable carbon pile, voltmeter and ammeter. The carbon pile is attached to the battery and adjusted to obtain maximum output while the engine is running at 2,000 rpm.

The exact charging voltage will vary according to the battery state of charge, the load on the vehicle electrical system and the temperature. The lower the temperature, the higher the charging voltage, and the higher the temperature, the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.9-15.1 volts at 77 degres F. But at -20 degrees F, the charging voltage might be 14.9-15.8 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5-14.3 volts.

Charging amperage is another number that can reveal the condition of the alternator. With the engine idling and no load on the charging system (lights and all accessories off, battery fully charged), the amperage output should be relatively low (typically less than 10 amps). With the headlights and heater blower fan on and the engine running at 2,000 rpm, the output should jump to a higher reading, typically 25-30 amps or more.

test battery
Warning: Never disconnect a battery cable while the engine is running to "test" the alternator. Doing so can cause high voltage spikes that can damage the alternator as well as other electronics.

Another way to check alternator output is with an oscilloscope. Observing the "ripple voltage" pattern will tell you at a glance whether or not all the alternator windings are functioning. A "good" pattern should look like the top of a picket fence. If any of the humps are missing, it means one or more of the windings is grounded or open, or there is a bad diode. Most battery/charging system testers also have a test function that can detect bad diodes.

One way to check the integrity of the alternator and diodes on a Bosch alternator is to check the voltage readings at the D+ (blue wire) terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the alternator.

Bosch does not recommend full fielding as a procedure for testing alternator output because full fielding may damage onboard electronics.

Replacement Parts
Starters and alternators often have a high warranty return rate. Often there is nothing wrong with the part because the real problem was not diagnosed correctly. To reduce the risk of misdiagnosis, have your parts store bench-test the old starter or alternator to confirm your diagnosis if you do not have a starter/charging system tester. Also have them test the new or reman unit before you leave the store. Replacement alternators should always have the same or higher amp rating as the original. The battery also should be fully charged before installing an alternator.

If you are replacing a permanent magnet starter, handle with care because the magnets can be easily damaged if the starter is dropped. Your parts store might not give you credit for the old exchange core if it is damaged. And you certainly do not want to damage the new unit before it is installed.

Adapted from an article written by Larry Carley for Underhood Service magazine

 

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