Battery testing basics

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Battery testing basics

Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is a National Institute for Automotive Service Excellence (ASE)-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. Weber is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an automotive service technician, a shop manager and a regional manager for an automotive service franchise operation.

The cold snap that recently swept through most of the nation caught many folks unprepared as roads closed, pipes burst and batteries failed.

Many technicians began their career because they marveled at things mechanical. We like to watch things going ‘round and ‘round, and up and down. We understand the mechanical stuff, but when it comes to electricity, some techs tend to stumble. Electrical system diagnosis seems to be the bane of many technicians.

You can neither see electrons flowing through a wire nor the chemical’s ions moving in the battery. This is an area of auto service resulting in far too many guess-and-replace attempts at fixing. That includes batteries, alternators and starters.

It should not be so.

You need a window into the system. That window can be oscilloscopes, digital multimeters or special testing tools.

It usually starts with a customer on the other end of the phone complaining that their vehicle will not start (naturally, they’re referring to an engine no-start). The battery is dead. Even before you send your wrecker to get the car, you check your battery inventory to see if you have one to fit.

It may be a guess or a hunch, but not all starting problems are the fault of the battery. But how do you know?

You perform a diagnosis, just as you would with any vehicle problem. It helps to understand the basics of battery technology to understand what your tests are revealing, even if you use a modern battery-charging-starting system tester.


Battery basics

A battery’s chemistry is a storehouse of electrical potential. Upon request, ions move from one place to another and electrons flow from the battery post to power a circuit. Regardless of the item being powered, whether a lamp, a control module or a starter, it is referred to as a load. Batteries have a limit to how long they can supply a load before requiring recharge. That, of course, is the function of the alternator and its regulator. Electronic control modules that offer more precise control are quickly replacing discrete regulators. Someday, ECMs may even be able to predict impending battery failure and alert the driver.

So there we have the starting problem trinity: the battery, the starter and the alternator.

How do you test the battery? There are two common methods. The first is to use a voltmeter along with an ammeter. The second method is to use a dedicated battery system tester.

Voltage is the electromotive force (EMF) and amperage is the rate of flow. Using the analogy of water flowing through a pipe, voltage is the pressure and amperage is the rate of flow. For the first method, all you need is a digital multimeter and a current clamp to create your window.

First, check the open circuit voltage at the battery. With your DMM on the voltage scale, connect the leads to the battery posts, not to the cable clamps. Any corrosion between the clamps and posts will cause a bad reading.

But take care with side-post batteries found in GM vehicles. A common mistake is to screw a bolt into the battery’s terminal and clamp the leads to the bolt. This will give an erroneous reading. It also runs the risk of screwing the bolt all the way through the terminal and into the battery case. Instead, use screw-in lugs that contact the lead pads (these will serve as stoppers to prevent excessive depth).

Open circuit voltage is checked with no loads. A fully charged battery should read 12.66 volts. If it reads between 12.70 and 12.80 volts, you are probably seeing a surface charge created because the battery just came off a charger or because the customer just finished a long trip. Remove the surface charge by turning on the headlights for about 20 seconds. Then, after allowing a minute or so to stabilize, check the open circuit voltage again.

If the open circuit voltage is above 13.00 volts, the electrolyte (mixture of water and sulfuric acid in the battery) may be too strong. The battery may have been overcharged and the electrolyte may have cooked down. In this case, the battery’s plates may be exposed above the electrolyte solution and the battery cannot be saved. It must be replaced.

If the voltage is between 12.58 and 12.66, the battery is fully charged. With maintenance-free batteries, it is not uncommon to see 12.70 to 12.80 at full charge, however.

If the open circuit voltage is below 12.58 volts, the battery must be recharged and retested. These voltage readings are often the result of frequent short trip driving where the alternator does not have time to fully charge the battery.


If the battery’s open circuit voltage is zero, there is likely an internal short. Shorted batteries are about 10 times more common than sulfated batteries. It is usually due to material breaking off the plates and falling to the bottom where it may create a bridge (a short). In years gone by, this may have been diagnosed by shaking or dropping the battery a few inches to the floor to jar the material loose. It should be obvious that it’s a bad idea to place a previously shorted battery back in service.

Clamp your current probe around the battery’s negative cable. If there is more than one cable, put the clamp around all. If your current clamp has an arrow on it, be sure the arrow is pointing away from the battery. The arrow points in the direction of current flow, away from the battery’s negative post.

Start the engine and observe the current reading. Do not be alarmed if it jumps up to 50 to 60 amps, as this is the surge of power to recharge the battery following the starter’s draw. The amperage reading should quickly drop down to about 10 amps. The battery essentially becomes a load on the charging system when the engine is running. That is in addition to the other loads such as the fuel pump, electronic control units, ignition, fuel injectors and so on.

By the way, the battery has a dual personality. Not only does it supply power, it acts as a capacitor to cushion power spikes that could cause damage to other systems or circuits.

If the resistance does not drop, the alternator will eventually fail due to burned out diodes. Unless the battery is replaced and creates sufficient resistance to charging, the new alternator will soon fail as well.

At this point, we should caution that it is a bad practice to connect a carbon pile to the charging system and crank the resistance up to test the alternator. Doing so will almost certainly damage the alternator. Old-timers used to swear by this test, but do not do it.

An easier way

Although testing a battery using a DMM and current clamp is not very difficult, it takes a few minutes. There is an easier way. Hand-held battery and charging systems are readily available from many companies. Generally, all you need to do is hook the equipment to the battery and push buttons to get the battery’s state of charge. Some, to cite the MDX series testers from Midtronics as one example, report the battery’s state of health by using AC voltage conductance to come up with various battery ratings.

The beauty of the new generation of testers is not only their ease and speed, but also the reports they generate.

After testing, the results can be printed out and shared with the customer. People seem to trust machines’ reports over a service technician’s oral report. However, it is up to a skilled technician to explain what the report means.

Replacement considerations

You can’t go wrong by replacing a battery with one of identical specs. Find the Group Number, select the same Cold Cranking Amps (CCA) rating and Reserve Capacity (RC) rating. Choose one with top post or side post or universal battery with both configurations.

What if your customer isn’t satisfied with an original equipment, direct fit replacement?

Historically, we would sell up to a battery with a higher CCA rating. After all, it provides that oomph required when the weather turns cold. But that is history. The cold cranking amp test is done at zero degrees for 30 seconds. Very few engines crank for 30 seconds unless there is something seriously wrong. It was once commonplace to hear starters whining away for 10, 15 or 30 seconds at a time. But electronic fuel injection, coil-on-plug ignition, electric fuel pumps, computer engine controls and gear reduction starters have reduced cranking times to just a couple revolutions of the crankshaft — or less.

Growth of absorbed glass mat (AGM) batteries is also the result of the growth of stop/start vehicles. AGM batteries are very charge-receptive and are able to handle what is referred to as high cycling. While starting demands have been reduced, power demands from comfort and convenience accessories, infotainment systems and other accessories have grown exponentially. It was not long ago that power locks and power windows were bragged about in used car sales ads. Today these features are common even in entry-level vehicles.


Today, many components and systems are kept alive long after the engine has been shut off. You may discover current draws in the 500-milliamp range even after the vehicle has been sitting for nearly a half-hour. In addition, there are the drivers who leave their proximity sensing key fob transmitters near, or even in the vehicle, where they constantly ping the system. Added to that may be a factory update delivered or downloaded telemetrics while the owner sleeps. A car that is not driven often enough could easily suffer a discharged battery and a no-start.

If your customer wants a better battery, suggest one with more reserve capacity.

Even carmakers are touting RC over CCA. Reserve capacity provides more key-off energy. That means the battery can hold a higher state of charge for a longer time. In addition, the battery does not get cycled too deeply too many times, which can lead to premature failure.

AGM batteries are another upgrade over flooded wet cell batteries. The electrolyte in AGM batteries is held like a sponge. There is no free liquid sloshing around. Even if the battery case is punctured or cracked, the electrolyte is unlikely to leak.

As such, AGM batteries are great if the automotive designer chooses to locate the battery somewhere other than the engine compartment. They can be cycled more deeply than a flooded battery without sulfating and failing.

Replacing the battery

Once you have decided which battery to install, test the replacement before you install it. Batteries lose charge while sitting on the shelf. You’ll never hear the end of it if a customer encounters a no-start the day after the installation.

When installing a replacement battery, check the condition of the battery tray or box and replace questionable ones. If the cables are shabby or there is corrosion around the insulation near the clamp, replace the cable. Band-aid type replacement cable ends are not the proper choice. They invite future problems and are to be used only as a temporary measure.

Unless you provide external power during the battery swap, many memories including the radio presets and various ECM memories may be lost. In some cases, power equipment such as windows will need to be re-initialized. One carmaker (Mercedes) even requires that the replacement battery information is recorded with the ECM.

A typical 9-volt dry cell battery memory keeper that plugs into the cigarette lighter may not be sufficient. This circuit is typically fused at around 10 amps or less. Memory savers the plug into the OBD II connector are facing a five amp fused circuit.

A regulated power supply is the best choice to maintain memories. If you choose to use your jump-start box or battery charger, be extremely careful to insulate the positive battery cable after it is disconnected. A couple shop rags and a rubber band is not enough, nor is it safe.


Routine testing benefits

Automotive batteries are designed to provide a short burst of high current. It may take up to 500 amps to start an engine. But they cannot keep doing this forever. The typical life expectancy of an automotive battery is roughly 500 cycles at a 20% discharge.

Batteries face their roughest times during very hot and very cold weather. Replacing a marginal battery before leaving your customer stranded may make you a hero. Offer free battery testing as a seasonal promotion, perhaps as an add-on to other routine services such as oil changes.

Like tires, engine oil and antifreeze, batteries wear out. Testing your customers’ batteries on a regular basis may reveal when the battery is approaching the end of its life. Not only is this information useful to your customers, it will result in increased battery sales.   ●

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