When a vehicle is in for a major service, a pre purchase inspection or an unexpected dead battery, we perform a quick charging system test. The equipment first logs the labelled battery capacity and checks the battery temperature. It then checks the battery capacity and graphs it according to the labelled capacity. Next a graph of the battery voltage is made while the engine is started, and a reading is given of the starting voltage after the initial spike from starter actuation. The charging system voltage at idle is measured. Then the charging system voltage is measured again at 1500 rpm with loads such as the headlights and heater fan turned on.
The results confirm that the alternator and starting systems are working correctly. But the symptoms and readings may indicate the likelihood of other problems, and we need to do a complete charging system test. If any cable voltage losses or a parasitic draw are present, we get the clues from low voltages on the basic test. Cranking more than 1 second before starting or a delay in the onset of cranking after turning the key to start may indicate cable failures. Cars more than 10 years old may have voltage losses on their battery cables or charging cable.
Understand that the battery capacity check of a quick check is considered an initial check, and any reading less than 80% capacity indicates that further testing may be necessary. The lower limit of battery capacity is around 66%, and a charge and retest may be needed to confirm that. Also know that 66% applies to the Bay Area. If one expected a cold winter night in the mountains, 75% would be a better lower limit. When your records, or ours, show a battery did not last the minimum average of 5 years, we may want to test for a parasitic draw, meaning abnormal and excessive milliamps of current draw. A parasitic draw could be light that was left on, a bad phone charger, or some electrical component that is not shutting down correctly when you turn off the car.
Typically, we find the culprit to be an aftermarket stereo or alarm that is incorrectly installed. Occasionally, we find a door latch microswitch that is misbehaving, causing one of the car’s computers to wait forever for the door to open as the driver gets out, or for the door to close. Some microswitch may be keeping the light on inside the trunk. That could be hard to find. We have had just a few parasitic draws that took some time to run down. How ’bout a leaking tail light causing a partial short inside a CD changer in the trunk?
Complete Charging System Tests Include:
Hydrometer test of electrolyte: The specific gravity, or density, of the battery acid indicates the state of charge. First the readings of the 6 cells are taken to get a picture of the current state of charge. Then the battery must be fully charged by hopefully an overnight slow charge. Charging can be done more quickly, but the time required depends mostly on the size and capacity of the battery, and how far it might be discharged. After charging, the electrolyte specific gravity indicates the condition of the battery: 1.250 to 1.275 is a usable battery. 1.225 is low, and maybe the battery has very little life left. 1.200 is just too low and maybe too old. More than 50 points difference between cells indicates a failure. The density or specific gravity of water is 1.000, the sulfuric acid ions make it heavier, or more dense. Temperature correction is + or – .004 for every 10 degrees °F above or below 80 °F.
Standing voltage of the battery: First the surface charge, or residual charge is removed by turning on the headlights for 30 seconds, then turning them off. More time under load is needed if the battery was just on a fast charger. After 2 minutes, the voltage is checked. The scale is as follows: 12.7 V = great battery. 12.6 V = good battery. 12.5 V = midlife, but OK battery. 12.4 V = old battery. 12.3 V = battery is too old. 12.2 V = battery life way past over. If the electrolyte readings are low, or the battery obviously needs recharging, then this test should be performed after charging.
Load test the battery: A battery must be able to stay at a high enough voltage during cranking, or the fuel and ignition system simply won’t work. We load test with a carbon pile variable resistance load tester to about 1/3 the CCA or Cold Cranking Amps stated on the battery label, generally loading to 225 to 250 Amps. The voltage under load must stay above 9.6 Volts, most good batteries are over 10 Volts during this test. If the electrolyte readings are low, or the battery obviously needs recharging, then this test should be performed after charging.
Battery Positive and Negative cables: While cranking the engine, voltmeter readings are taken from end to end on each of the the two big battery cables. Maximum voltage drop is .5 Volts. Readings should be taken after cranking for at least 1 second to avoid the initial voltage spike that a starter makes when cranking is initiated.
Charging system cable from the alternator to the battery: Voltage readings are taken between the alternator charging post and the battery positive terminal with the engine at 1500 rpms. All the accessories are turned on, meaning headlights, windshield wipers, heater blower, and air conditioner. Alternatively, the load tester can be attached to the battery and set to 30-40 Amps, imitating the accessories. The voltage loss across the charging system cable should not exceed .5 Volts.
Alternator test: At idle, the alternator should have 13.8 Volts minimum output, hopefully closer to 14.2 Volts. At 1500 rpms with all the accessories turned on or a load tester at 30-40 Amps, the voltage should not fall below 13.8 Volts. Also at 1500 rpms, and with the load tester applying enough load to lower the output voltage down to 12.0 Volts, the alternator output must be within 10% of the stated output, which is usually 90 or 120 Amps.
Parasitic draw test: We prefer to use a clamp-on amperage probe, but sometimes connect an inline amp meter. Once the vehicle has been shut off, and locked up, we have to wait for the sleep cycle of the alarm and/or convenience systems to finish. Then the amperage draw should be between 16 to 40 milliamps, depending on the vehicle. The sleep cycle can take up to 20 minutes, depending on the vehicle. We allow 1/2 hour to be certain. If the parasitic draw exceeds those specifications, then further testing is required to determine the cause.
Ultimately, there are 6 ways of testing a battery, and there are 6 ways that a battery can go bad. The basic charging system test includes battery capacity tested with a small AC current reflection. Our complete charging system test includes 3 more tests: electrolyte specific gravity, standing voltage and load test voltage. The fifth test is for intercellular voltage, meaning the exact voltage between cells. A 10% difference in intercellular voltage indicates failure. The last and sixth test is for internal battery resistance. There is a complicated relationship between the charging voltage, the state of battery charge, and the amount of current flowing through the battery. These last two tests are rarely needed.
The Automotive Battery
We call everything 12 Volt, but really that just refers to the type of system. A battery has about 2.1 Volts per cell, so it is a 12.6 volt battery. To get to a full state of charge requires an additional .2 Volts per cell, so 13.8 is the minimum voltage under load that an alternator must produce to get the battery completely charged up. Most systems run around 14.2 Volts with everything turned on. Systems that run 14.5 volts and above will cause high internal corrosion in a battery, especially at ambient temperatures above 86 °F.
The battery’s capacity is often referred to as the State of Charge (SOC). There is a precise relationship between State of Charge (SOC), Cell Specific Gravity, and Battery Steady State Voltage when the battery is new and fully charged. As the life of the battery proceeds, the steady state voltage decreases as the state of charge or capacity diminishes. As mentioned, when the battery’s capacity drops to about 2/3, the life is technically over. But often when a battery is being condemned, the specific gravity shows that it is simply discharged and needs a good recharge. All battery chargers are not alike, and often some shop is saddled with the cheap version that cannot distinguish when the battery is actually charged up. In fact, most smaller chargers are like that.
Batteries have lead negative electrodes or plates (Pb), and lead oxide positive electrodes or plates (PbO2). Typically these plates also have other metals for physical strength and durability. In more or less order by weight, antimony and calcium are the major players; then tin, selenium, cadmium and arsenic are used in modern batteries to help the plates last longer and adjust other battery properties. The plates are manufactured by making a grid first of the hardiest alloy, then coating it with lead and lead oxide, both made as foamy or as pastes. The plates are separated to prevent shorting using a variety of materials, aptly called separators. Various types of plastic are used in most automotive batteries. AGM batteries use a mat of glass fibers.
When immersed in sulfuric acid (H2SO4), electrons build up on the negative plates, creating an electromotive force, which is the voltage inside each cell between the two types of plates. As the electrons accumulate, they create an electric field which attracts the positively charged hydrogen ions and repels the negatively charged sulfate ions, leading to a stratified double layer near the surface of the negative plate. The hydrogen ions effectively keep the sulfuric acid solution away from the the charged negative plates. The effect is a reversed at the positive plate, which contributes to why positive plates tend to disintegrate.
When charge flows through a battery and its circuit, electrons are flowing from negative to positive, and the sulfur ions join with the lead and lead oxide plates to form lead sulfate (PbSO4). The hydrogen ion component of the sulfuric acid combines with the excess oxygen to form water (H2O). So if one completely discharges a battery, which is never a good idea, both the positive and negative plates would then be turned into lead sulfate, with the other alloy metals of course. The electrolyte would be mostly water instead of sulfuric acid, and the specific gravity will go down under 1.080 (1.000 is the density of water). As the battery recharges, the sulfate ions are driven back off the plates into solution, raising the specific gravity of the electrolyte, and building up charge on the plates again.
Batteries allowed to operate in a low state of charge will always have a buildup of lead sulfate, which will eventually harden and not allow charging or discharging of the affected areas of the plates. In the past, an equalizing charge was used to keep the cell specific gravity and voltage more equal by slightly overcharging to get rid of these sulfate deposits. With modern batteries, this is generally deemed unnecessary, which is essentially true unless one of the following conditions is happening: There is a parasitic draw constantly lowering the voltage and discharging the battery. The balance between storing and driving the vehicle allows the 5% to 15% per month normal self-discharge rate to overcome the charging while driving. Internal soft shorts are causing higher self-discharge rates and sulfation of the affected cells.
Self-discharge rates vary in regular flooded batteries due to differences and impurities in the alloys of the plates, as well as the age and condition of the battery. An older battery, especially of low manufacture quality, may very well have a high self-discharge rate due to soft shorts, which are mild internal shorts usually caused by the buildup of debris under the plates. Soft shorts can also result from a defective misalignment of the plates vs. separators that allows moss-like crystalline structures to grown on the plates until the plates are slightly shorted. Soft shorts change over time, always getting worse. They often account for the mystery of why a battery just up and failed with no previous history or signs of trouble.
These soft shorts are nothing like the rare occurrence of complete shorts when plates touch each other. A big accident can cause these shorts, but as a rule, they would be caused by manufacturing defects. Complete shorts cause thermal runaway, the battery boils and may well explode. There are a few cars known for exploding batteries, but it is much more likely that most explosions are caused by internal soft shorts making a hydrogen buildup, combined with low electrolyte exposing the plates, and the occurrence of a static-electricity spark discharge inside the battery. Let’s just say that none of those brands of cars are in our fleet.
Higher temperatures cause higher self-discharge. Around 77 °F is the sweet spot for lead-acid batteries. Above 86 °F raises self-discharge significantly, above 104 °F will cause self-discharge rates over 20% per month. As a battery gets colder, the current capacity drops by about 10% for every 15 °F below 80 °F. And it also recharges slower.
Speaking of cold, a fully charged battery would have its electrolyte freeze at about -90 °F. When a battery is at 40% of its normal state of charge, it will freeze at 16 °F. Water freezes at 32 °F. Do the math before you go skiing. If you leave your lights on or your charging system has a problem with cable line loss or parasitic draw, the battery could freeze and break during a butt-cold Tahoe night.
Although lead-acid battery life is significantly reduced by elevated temperatures, most formulas to that effect end up being off base and misleading. If they were all true, then all car batteries in Phoenix, Arizona would only last 2 to 3 years. The reality is that car batteries go through large temperature swings every day, and generally last 5 to 7 years given a battery of quality and a perfect life. In practice, other factors affect lead-acid battery life more than the ambient temperature.
Any flooded battery will loose some electrolyte over time. During normal usage, some water will disassociate into hydrogen and oxygen due to electrolysis. Charging at high voltages over 15 Volts will liberate more hydrogen, and cause faster electrolyte loss. Although high charge rates slow negative plate sulfation, high charge rates cause positive electrode grid corrosion. The sulfation is generally reversible, the corrosion is permanent. Lower charge rates increase battery lifespan but decrease performance, the biggest reason is from sulfation. An occasional equalizing charge of about 15 volts can eliminate this concern.
The quality and quantity of the lead alloys and separators determine the overall quality of the battery. The first evaluation of a battery is to pick one up in each hand. One quickly notices that a dealer battery generally weighs in at 10 to 20 lbs heavier than a parts-store equivalent.
The Deep Cycle Battery:
A main starter battery has the lead coated onto the plates with a foam like stratum. There are many of these foamy plates so more surface area is exposed which makes discharging and recharging are easier. A true deep cycle will have fewer plates and made much more like solid lead to allow more material for the actual power production. That will also make a true deep cycle weigh more. And a deep cycle battery will typically take much longer to recharge completely. Note that there are huge differences between batteries that are called “deep cycle”, some of which barely qualify.
Lead-acid batteries do not have any “battery memory”. Although not good practice, they can be deeply discharged and recharged, but it is not necessary to do so to maintain capacity. Battery life is decreased by more deeper discharges. The best life for a starter battery is normal usage like discharging 5% to 10%, then recharging soon thereafter. Discharging to 100% is bad for any battery and is often fatal to an old battery. A new starter battery can only be expected to be discharged by 100% about a dozen times before it drops to 2/3 capacity, which means the end of the battery’s life. And battery capacity life is not linear, once it reaches 2/3 capacity, it is usually only a matter of months before it is completely unusable. A real deep cycle battery can be 100% discharged at least 100 times, and an excellent brand can go twice that.
More commonly, auxiliary batteries in campers are discharged deeply, then recharged, typically between 30% to 50%. A starter battery would fail quickly. At 50% discharged, a starter battery gets 100 reliable cycles, and a deep cycle will last 400 to 500 cycles. At 30% discharged, a starter battery will go maybe 150 cycles, but a deep cycle will provide 1000 to 2000 cycles reliably before it reaches 2/3 capacity. These are generalities for a regular lead-acid battery, a high quality AGM battery will last way longer.
AGM and Gell Batteries
A very important note is that all these battery facts and tests are oriented towards flooded batteries, meaning the usual lead-acid batteries. Two other less common lead-acid battery types are AGM (Absorbent Glass Mat) and gel acid. These gel and AGM batteries MUST NOT be load tested or charged above 14.8 Volts, either situation can ruin areas of the electrolyte permanently. Gel batteries are generally not used for main starter batteries, but AGM may be. Both types of batteries are used for auxiliary batteries. AGM batteries are an excellent choice for both the main starter and auxiliary batteries, but the cost is higher. The two common brands are Optima, with its cylindrical shaped cells, and Trojan, the choice of many sailors for many years.
The term VRLA battery refers to both gel and AGM batteries. It means Valve Regulated Lead Acid and refers to the pressure valves that allow the vast majority of the hydrogen and oxygen to recombine inside the battery, sometimes also using a catalyst. Although 2 psi of pressure does the job, many brands use 5 psi valves, and some spiral cell batteries use up to 40 psi. The point is to keep the electrolyte from loosing any water, and allowing Gel and some AGM batteries to be installed and/or used in any position.
The flip side of not loosing any water means that no hydrogen is emitted in normal operation. That makes VRLA batteries good for confined space and indoor use where ventilation is a problem.
AGM batteries have a 1% to 2% per month self-discharge rate. Most battery companies noodle around giving a real specification for self-discharge. Self-discharge does change over time, and since it depends a lot on material quality, don’t expect good specs from a cheap battery.
A third kind of battery that may be soon on the market for cars uses a lead positive electrode and a dual lead and carbon negative electrode. Known as advanced lead-carbon, or ALC, they are currently in use for some vehicles that stop and start the engine anytime the vehicle comes to a halt. The ALC battery technology is aimed at improving the ability to charge and discharge rapidly without loosing battery life due to negative electrode sulfation.
The Different Methods of Charging a Battery:
Constant-current Charge: This describes how a car recharges its battery normally, and how most battery chargers work. Sometimes called Bulk Charging since most of the charging happens at this rate. The voltage starts out lower due to the load on the alternator. A battery discharged over 80% may take a while before taking much current. After the initial lag, the current starts out higher as the battery can accept and hold more electrons when discharged. As the charge proceeds, the voltage will go up and the current will go down. And a full bulk charge may take 12 to 20 hours. At higher battery temperatures, the charge rate also increases.
Topping Charge: As the battery reaches capacity, the cell voltage and the electrolyte specific gravity stabilize. Charging continues at a lower charge current and provides good saturation. There is a normal tendency for a car’s charging system and most battery chargers to taper off as the battery reaches full charge.
Float Charge: Maintains the battery at full charge, and compensates for the loss caused by self-discharge. The float charge will slightly lower the voltage after normal charging to prevent negative plate sulfation and positive plate grid corrosion. Most cars cannot do any function like float charging. This is the realm of great battery chargers and battery tenders.
Equalization Charging: This is controlled over-charging to try to get rid of sulfate deposits. It used to be a common procedure on new batteries just when sold, or any battery that had been stored for more than a few months. A car’s charging system has no way of doing an equalization charge. The voltage is ramped up to 15 Volts. Some expensive chargers have a setting for this sort of charging, but most chargers just have a high setting and must be monitored every few minutes because the voltage is going to keep rising. If it gets near 16 Volts, things may get dangerous.
Pulse Charge: Many larger chargers from earlier this century do a pulsed charge while on an automatic setting. The voltage rises and falls radically. Your car doesn’t do this type of charging ever. For a battery that is old and somewhat sulfated, pulsed charging is probably a good idea. For a battery in good shape, probably not. With expensive chargers, the pulses flatten out as the state of charge increases, and the battery gets bulk charged well. Cheap chargers may vary the voltage wildly for the whole charge and may radically reduce the battery life.
Conditioning Charge: Your car can’t do this, neither should you. A Conditioning Charge should only be done by trained professionals. And only when actually necessary. This is what we were talking about with the 16 plus volt thing. The idea is to soften the old and deep sulfate deposits that result from long term storage at a low SOC so that they may charge off later with lower voltages. Generally, a conditioning charge cycle lasts 2 to 5 minutes, and occasionally may be needed more than once. It is ONLY performed when the electrolyte specific gravity is very low (~ < 1.100), and normal charging procedures are not working to raise the specific gravity. The high temperatures and large volume of hydrogen gas produced can have serious consequences to put it mildly. Though it may save a sulfated battery, it will likely damage a battery in good condition. Lots of ventilation is required. Hydrogen gas gets to be an explosive mixture really quickly, at around 4%. Remember that hydrogen is much lighter than air and will accumulate at the ceiling level where you can't smell it. The hydrogen itself is odorless, but some hydrogen sulfide is usually produced that can be smelled like rotten eggs. If you can't spell "Hindenburg" forwards and backwards, you probably should not perform this Conditioning Charge. And to repeat, NEVER preform a conditioning charge on a gel or AGM battery, it WILL damage it.
SAFETY NOTES: One cannot stress the need for safety training enough. Batteries contain dangerous sulfuric acid and emit a VERY FLAMMABLE mixture of hydrogen with oxygen when charged or discharged rapidly. Safety glasses are mandatory. By law, all shops must have an eye wash station just in case. Gloves are a great idea. Unattended batteries can explode while charging. ALWAYS try to check the electrolyte level before charging a very discharged battery. Most times, the caps are not obvious and a label must be removed or cut to find the caps. Some caps are valves and hold up to 5 psi of pressure to help the hydrogen recombine to form water again. These batteries may spatter if the charger was already on it or the engine was running. ONLY FILL the electrolyte just enough to cover the plates before charging. Distilled water is best, highly filtered water will do in a pinch. Mineralized drinking water or hard city water will damage the battery. Adding Pepsi or Coca Cola to a battery is just friggin crazy stupid! Electrolyte expands as the battery charges. If the electrolyte does not cover the plates, any plate surface exposed to air will get oxidized and never work again. WORSE!!!!! A STATIC DISCHARGE SPARK MAY HAPPEN, AND THE BATTERY WILL EXPLODE!!!!!! Think dangerous tiny lightning.
Battery Chargers: The Big Differences
Most of the world is unaware of how pathetic most battery chargers have become. In the effort to equip the world with battery chargers that do not damage batteries by over charging, we now see huge variables in whether a charger can accurately finish a real charge. Almost all small chargers fail to bring the battery to its best state of charge, as evidenced by testing the specific gravity of the electrolyte. And if the battery is more than a few years old and comes in at less than an 20% state of charge, the effect is almost guaranteed. In fact, most of these chargers will try charging for a couple of minutes, then quit charging and light up some indicator light that says that the battery is bad. But who checks if the battery can actually be saved?
If the battery is more than a couple of years old and comes in at around a 50% state of charge, these crappy chargers will charge for a short period of time, then stop and declare that the battery is at 100% state of charge. A quick load test and check of the electrolyte specific gravity shows that the charger is lying, and appears to have no clue. But who checks? The engine now starts, the car can drive. And the battery may never get charged all the way up ever unless the customer takes a really long trip.
The deal is that old time battery chargers just never quit trying to bulk charge a battery. It was up to the mechanic to monitor the electrolyte specific gravity and decide when to stop charging, usually trying to get the battery back up near 1.275, and at least 1.225. If the battery would not get to 1.200, then that was one way a battery could go bad, and we sold the customer another battery. The idea was to charge like crazy until the electrolyte hit 1.250, then change to the lowest setting and see if it eventually hit 1.275. And if one ran out of time, at least the battery got charged to 90% capacity (SOC).
If the mechanic spaced out and left the battery on the charger, the charge voltage would taper off. Once the electrolyte reached ~ 1.280, then the battery would start overcharging. If the charger was on a high setting and got left overnight, the next morning the whole shop smelled of rotten eggs, there was a possible explosion issue, and the battery was pretty much ruined. If the charger was on a low setting, the battery would have lost some lifespan, but nobody seemed worried.
So what does it take to get a great charger? Remember what we said about internal battery resistance and the complex relationship between the charging voltage, the state of battery charge, and the amount of current flowing through the battery? The best chargers are constantly checking several aspects of the battery and modifying the charge voltage. Given time, they can deal with a heavily sulfated older battery. And they generally can give the best charges in the shortest amount of time. And the short answer to the first question is: At least $200.
We are not saying that everybody needs to buy the best battery charger. Most chargers will do a decent job of getting 90% of all batteries back to at least 80% of their capacity if possible. Just be wary if someone is selling you a new battery and your dead battery is just 2 or 3 years old. Odds are that there is either something wrong with the car or the pilot does not drive enough, and maybe there is something not good enough about their charger or procedure.
Like the name sounds, alternators are related to the Alternating Current, or AC voltage found in the home. Briefly, Current, or Amps, refers to the amount of electrons moving in a circuit. Voltage is the force pushing those electrons. Resistance, or Ohms, is like it sounds; the resistance to the movement of the electrons in a circuit. The basic relationship is described by Ohm’s Law: 1 Volt of force on a circuit with 1 Ohm of resistance will have 1 Amp of current moving through it.
Power, or Watts, is the voltage force multiplied by the current, in DC circuits. AC circuits get a little more complicated because the current changes due the rise and fall of the voltage, but watts still applies. A single phase (2 wire) AC device, Watts is Volts times Amps times a Power Factor multiplier of .8 to account for said rise and fall. Most houses don’t have any 3 phase AC appliances, but another multiplier of the square root of 3 is used to calculate watts of power between the 2 voltage legs of a 3 phase circuit. Which has no bearing on our automotive discussion.
For our discussion, 12 Volt DC, for Direct Current, is the automotive standard. This means all the electrons have 12 Volts of force, and move in the same direction all the time. In the home, 120 Volt AC means all the electrons have 120 Volts of force and oscillate back and forth, alternating the current direction at 60 times per second, or 60 Hertz.
Although the final output of an alternator is 12 Volts DC, internally it has 3 separate poles that each produce 12 Volts of alternating current. These 3 poles are the stator windings inside the alternator housing. The AC voltage in the stator windings is created by spinning the rotor on the alternator shaft. A DC voltage is applied to the rotor windings by a voltage regulator, which makes the rotor winding generate a series of magnetic fields, each with a North and a South pole. As those magnetic fields whirls past the stator windings, 12 volts of AC voltage is generated in each stator winding. A pair of diodes on each circuit rectifies the 12 Volt AC into 12 Volt DC voltage. Those 3 poles are out of phase with each other by 120 degrees, so as they are rectified, then combined, a relatively smooth DC voltage line is produced.
Not in all cars, but in some other DC output alternator applications, there are 4 poles, making the final current even smoother. Also know that the term “alternator” began to be used late in the 19th century in reference to the power generating devices that had AC voltage as the final output. They could have up to 40 poles, and the frequency, or cycles per second, was set by the rotational speed.
One uncommon form of alternator failure is AC voltage on the DC line, called ripple. Ripple is the result of leaking diodes, allowing some of the 12 VAC to leak through. Normal amounts of AC voltage are in hundredths and up to one-tenth of a volt AC. An initial measurement can be made with just a voltmeter, anything over 1/10 of a volt AC needs checking, anything over 1/2 volt AC is a possible problem. An oscilloscope is needed to perform an accurate ripple test due to the exact shapes of the waveforms produced by leaking diodes. A complete discussion of ripple gets very complicated. Here are the high points about ripple testing.
Ripple testing should be done right at the alternator post to the alternator case.
The battery dampens ripple, functioning like a large capacitor. A battery in a low state of charge, or a failing battery with low capacity, can cause high ripple readings.
Digital voltmeters can show high AC voltage when the actual waveform on an oscilloscope shows a short tail on normal AC pulses. Try using an analog AC voltmeter, it is not trying to do the math like a digital voltmeter. Better yet, use an oscilloscope.
Load the system slightly by turning on the headlights. An unloaded system can read a false high ripple.
AC ripple may show itself as headlights that flicker constantly with the engine running, but not with the engine off. There may also be a whining noise on the stereo that goes up and down with the engine. Many alternators and most stereos have a capacitor that removes normal ripple noise, but high ripple voltages may overcome them.
AC ripple may cause false codes and incorrect sensor readings with engines and transmissions. Very large amounts of ripple can ruin electronics and/or interfere with cell phone transmission or keyless entry systems.
Be aware that modern cars with complicated power control modules and function can have bad ripple as a response to a particular type of battery failure. Just add a jumper pack to the original circuit and measure ripple again. If it changes the ripple reading, change the battery and retest.
A florescent droplight can cause high AC voltages on a voltmeter from the EMF created by the ballast. Keep it away from the measuring equipment.
Just a note from the past. Old time automotive generators put out DC voltage directly from the armature, the part that spins. And the voltage and amperage was controlled by the field coils, which are the windings on big steel plates bolted inside the housing. So the brushes that connected to armature had to be huge since all the power went through them. Generators needed the brushes replaced every 4 to 5 years. Alternators have tiny brushes by comparison, because just the control voltage and amperage from the regulator goes through the brushes. The main power is produced in the stators, which are sitting still.
Alternator Clutch Pulley
Inside the pulley of some alternators is a one-way freewheel clutch called a Sprag Unit or Sprag Clutch. It is built a bit like a bearing that can only turn in one direction. Instead of rollers or balls, it has sprags, which are like a slightly warped figure eights. Rather than tax the imagination, watch the short video for a good explanation. What may not be obvious is that the sprag unit itself is not load bearing, so it has a ball bearing on either side of it to hold the side load of the pulley on the shaft.
The alternator clutch pulley has two distinct functions. Most importantly on high torque engines like diesels, the clutch pulley smooths out the pulses of an idling motor. Especially on a 4 cylinder motor, the crankshaft speed rises and falls rapidly with each engine combustion pulse. Without the clutch pulley, the alternator will skid the pulley on the serpentine belt with each pulse, causing a chirping noise. Rubber is being worn off the belt and onto all the pulleys at a slow rate. The other function of the alternator clutch pulley is to allow the alternator to over speed when the engine speed changes rapidly as the transmission shifts gears. Again, the main effect is to reduce serpentine belt wear.
Some mechanics maintain that the clutch pulley is unnecessary, or may not even know that it exists. The replacement alternator they sell may be cheaper, but they are not there to pay the one to two hours needed to clean the rubber chunks out of the pulleys, left behind as the serpentine belt fails more than twice as fast as normal. And when the serpentine belt breaks and you can barely steer the car, or not at all, the higher cost of the proper alternator with a clutch pulley looks well worth it.