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 other problems may indicate the 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 is considered an initial check, and any reading less than 90% capacity indicates that further testing may be necessary. 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 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: 1250 to 1275 is a usable battery. 1225 is low, and maybe the battery has very little life left. 1200 is just too low and maybe too old. More than 50 points difference between cells indicates a failure.
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. 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 to 14.2 Volts minimum output. At 1500 rpms with all the accessories turned on or a load tester at 30 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.
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.
Batteries have lead negative electrodes or plates, and lead oxide positive electrodes or plates. 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 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, the 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 hydrogen ions and repels the sulfate ions, leading to a stratified double layer near the surface of the negative plate. The hydrogen ions effectively keep the 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. 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. And the electrolyte would be mostly water instead of sulfuric acid and the specific gravity will go down close to 1000 (1.000 is the density of water). As the battery recharges, the sulfate ions are driven back 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 build up 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 now deemed unnecessary, which is essentially true unless there is a parasitic draw constantly lowering the voltage, or the balance between storing and driving the vehicle allows the 5% to 15% per month self-discharge to overcome the charging from driving.
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. Higher temperatures cause higher self-discharge. Around 68 °F is the sweet spot for 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 night.
Lead-acid batteries do not have any “battery memory”. Although not good practice, they can be 80% discharged and recharged. Battery life is decreased by more deeper discharges. The best life is normal usage like discharging 5% to 10%, then recharging soon thereafter. Discharging to 100% is bad for any battery and is usually fatal to an old battery.
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 30 lbs heavier than a parts-store equivalent.
A main starter battery has the lead coated onto the plates with a foam like stratum, so more surface area is exposed which makes discharging and recharging are easier. A true deep cycle will have plates much more like solid lead to allow more material for the actual power production. That will also make a true deep cycle weigh more. Note that there are huge differences between batteries that are called “deep cycle”, some of which barely qualify.
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 Different Kinds or 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. The current starts out higher as the battery can accept and hold more electrons when discharged. A battery discharged over 80% may take a while before taking much current. As the battery warms up, 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: Controlled over-charging to try to get rid of sulfate deposits, it used to be common on new batteries just when sold, or any battery that had been stored for more than a few months. This never happens in normal operation with a car. If the specific gravity varies more than 30 points between cells, then an equalizing charge is an option. But a normal charge should be tried first. 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 will get dangerous.
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 Volt thing. The idea is to soften old and deep sulfate deposits from long term storage, so that they may charge off later with lower voltages. The high temperatures, and large volume of hydrogen 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”, you probably should not do this.
Pulse Charge: Many larger chargers from earlier this century do a pulsed charge while on an automatic setting. The voltage rises and falls radically. 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. Cheap chargers may vary the voltage wildly for the whole charge and may radically reduce the battery life.
SAFETY NOTES: One cannot stress the need for safety training enough. Safety glasses are mandatory. 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 to cover the plates before charging. Electrolyte expands as the battery charges. If the electrolyte does not cover the plates, any plate surface with be oxidized and never work again. WORSE!!!!! A STATIC DISCHARGE SPARK MAY HAPPEN, AND THE BATTERY WILL EXPLODE!!!!!! Think dangerous tiny lightning.
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 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.
AGM batteries have a 1% to 2% per month self-discharge rate. Most battery companies noodle around giving a real specification for self-discharge. Since it does change over time, and 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.
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 to 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 8 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 magnetic field. As that magnetic field whirls past the stator windings, 8 volts of AC voltage is generated in each stator winding. A pair of diodes on each circuit rectifies the 8 Volt AC into 4 Volt DC voltage. Those 3 poles are connected in series to form 12 Volts of DC power.
Each battery is a load, just like each headlight is a load. So campers with an auxiliary battery need to be sure that each battery is getting its 13.8 Volt minimum when every other load is turned on. Cars with huge sound systems have a higher load, same with foglights, inverters, refrigerators and on and on.
Just a note from the past. Old time 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. 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.