Battery, Starter & Charging System

By Ken Layne

The relationship of the battery to the starting and charging system is really a continual cycle of converting one form of energy to another and then back again. Itís a chicken-and-egg sort of relationship in which the mechanical energy of the engine drives the alternator, which forces electrical energy (current) into a battery, where itís stored as chemical energy. The chemical energy of the battery then gets changed back to electrical energy when it supplies current to the starter motor, which uses mechanical energy to crank the engine. Then the engineís mechanical energy again drives the alternator to recharge the battery so it can supply more current to the starter when needed. It doesnít matter where you pick up the cycle as a starting point, as long as you recognize how the battery, alternator and starter relate to one another.

Battery Basics

Most complaints that point to the battery or the charging or starting system include a symptom of hard starting or failure to start. Slow cranking, long cranking times or failure to crank can have their root causes in anyóor alló of these electrical areas. Often, the job is to fix a no-start problem, period. So where do you start troubleshooting such a problem? With the battery.

Beyond a simple blown fuse or a broken wire, you canít troubleshoot any electrical problem without a fully charged battery. A good battery has two characteristics: First, it must deliver the electric current demanded by the starter and other electrical devices on the car. Second, it must maintain enough voltage to force that current through the carís circuits. Basic battery tests include an open-circuit voltage check, a load test and maybe a three minute charge test. Most batteries sold today are maintenance-free units, with non removable vent caps. If the battery does have removable caps, though, get out your trusty hydrometer and check the specific gravity of the electrolyte.
Remember, battery electrolyte is sulfuric acid and water. So checking the specific gravity of the electrolyte simply means ďweighingĒ it. The specific gravity of water is the baseline and, therefore, has a value of 1.000. Sulfuric acid is heavier than water, while electrolyte is about 35% to 40% acid in a fully charged battery. That works out to a specific gravity of 1.260 to 1.280. When the battery is fully charged, all of the sulfate part of the acid stays in the electrolyte. As the battery discharges, sulfate ions move from the electrolyte to the lead plates, and the electrolyte becomes more water and less acid. And as it gets diluted, the batteryís state of charge drops as follows:

Specific Gravity State of Charge (at 80įF) (%)
1.280 - 1.260...................................100
1.250 - 1.230.....................................75
1.220 - 1.200.....................................50
1.190 - 1.170.....................................25
1.160 or lower ...................Discharged

If the battery in a problem car is a maintenance-free unit (as most are today), you canít use a hydrometer. As an alternative, start by checking the opencircuit (no-load) voltage. To do this, remove the surface charge from the battery by turning on the headlamps for about 10 seconds. Then turn the lights off and connect your voltmeter across the battery terminals. The voltage reading will indicate the approximate state of charge, as follows:

Voltage % of Charge
12.72 - 12.60...................................100

If either the specific gravity test or the open-circuit voltage test indicates that the battery is 75% charged or better, you can continue with a load test. If the battery is less than 75% charged, you should roll out the battery charger before doing more testing. In either case, this is a good point for a basic inspection.

Inspect the Nuts & Bolts & Belts

You can find and fix lots of basic problems with a simple inspection during the early steps of your troubleshooting. Now is a good time to look for loose, glazed or otherwise damaged drivebelts. Remember, the alternator canít get up to speed and keep the battery charged if its drivebelt is slipping.

Neither the alternator nor the battery can move enough current through wiring thatís frayed or otherwise damaged. Loose or corroded wiring terminals and ground connections also add resistance to circuits and reduce current flow. And then there are the battery terminals themselves. Many no-start problems have been fixed just by cleaning and tightening battery connections that had a great growth of ďgray fuzz.Ē

The Load Test


A load (capacity) test indicates how well the battery can deliver high current while still maintaining enough voltage to operate the ignition. A load test is the basic way to test a maintenance-free battery and an important test for any battery. Check the open circuit voltage again before loading the battery to be sure itís fully charged.

To determine the amperage load for the test, check the battery top to see if itís printed there. If itís not, divide the cold-cranking amps rating by 2 or multiply the ampere-hour rating by 3. You also can use these guidelines:

Engine Test Amperage
4-cyl., small 6-cyl............................170 - 190
Small 8-cyl. (up to 5 litres).............175 - 250
Large 8-cyl. (above 5 litres)...........225 - 300

If the battery was charged just before this test, donít forget to remove the surface charge by turning on the headlamps for 10 to 20 seconds. The traditional load-test method requires a voltamp tester (VAT) with a carbon pile to apply the test load. Connect the tester to the battery and turn the control knob to draw the desired current for about 15 seconds. Note the voltmeter reading and turn the control knob to Off.

During the test, voltage should stay above 10 volts. The customary acceptable minimum voltage is 9.6 at 70į to 80įF. If the voltage stays above 10.0 with the full current load for 15 seconds, the battery is okay. If the voltage drops below 9.6 or if the specified current canít be applied, the battery can be tested further, although the usual conclusion at this point is that it has earned honorable retirement. If battery voltage is between 9.6 and 10.0, charge it and retest before deciding its fate.

If you donít have a VAT, you can still do a load test with a digital voltmeter. This works particularly well if your meter has a Min/Max recording function. Simply connect the meter across the battery terminal clamps and select the Min/Max function. Then disable the ignition, turn on the headlamps and crank the engine for about 10 seconds. Check the recorded minimum and maximum readings on your meter. Once again, the battery voltage should not drop below 9.6 during cranking.

Although you donít measure the current load during this test, itís a realistic measure of the batteryís cranking ability under the load of its own engine and electrical system. The test is easy, and itís quickówell under a minute. As evidence that old-fashioned, basic tests have a place in the world of high-tech, Fluke programmed this load test into the menus of its top-of-the-line Model 98 Scopemeter. You can even graph the voltage drop if youíre so inclined.

Starter Current Draw, Voltage Drop & Speed Tests

If the battery qualifies as okay, you can move on to some basic starting system tests. To check cranking current draw and rpm, you basically repeat the alternative load test with an ammeter connected to the starter motor circuit and a tachometer connected to the engine. Crank the engine for about 15 seconds and note the voltmeter, ammeter and tach readings. Again, the voltage shouldnít drop below 9.6. If it does, youíll have to return to square one with the battery or look for a whopping current draw.

Cranking current should be within manufacturerís specs. If itís above, look for a short in the starter or an engine thatís binding for some reason. If amperage is below specs, look for high resistance in the starting system or recheck the battery.

Cranking speed for most engines is about 200 620 rpm. Low cranking speed plus high current draw points you to the possibility of a binding engine. High cranking speed with low current draw points to a badly worn engineóburned valves or pistons, or something else that drastically lowers compression. Interestingly, nothing makes an old pushrod V8 crank faster or smoother than a jumped timing chain...but itís not going to start.

The cranking current draw, voltage drop and speed tests will lead you to pinpoint voltage drop tests for the starting system. For these tests, we divide the system into the control circuit and the insulated and ground sides of the motor circuit. Test points will vary from one vehicle to another, so you should have an accurate wiring diagram from a manual, unless you know the system by heart.

The starter control circuit is made up of the ignition switch, the neutral safety switch (or switches) and the coil side of the starter relay or solenoid. Disable the ignition, then crank the engine over with the ignition switch during these tests. Donít use a remote starter switch because you want to include the voltage drop across the ignition switch and its wiring to completely satisfy the test requirements.

Youíre going to use your voltmeter to check voltage drops around the circuit to pinpoint high resistance. Remember, according to Ohmís law, every point of resistance in a circuit will drop part of the source voltage. Excessive resistance at any point drops more than its share of voltage and doesnít leave enough to push current through the circuit. High resistance in the control circuit wonít usually cause a slow-cranking problem. More likely, it will keep the engine from cranking at all because there wonít be enough current to close the relay or solenoid and turn on the starter motor. Sometimes the relay or solenoid will chatter as borderline current tries to energize it but just canít quite get the job done.

While cranking the engine, connect your voltmeter across all the switches and coils in the control circuit to measure voltage drop. Just as important, check voltage drops across all the connectors and all the lengths of wiring in the circuit. These are the maximum allowable voltage drops that you should see:

Any length of wire
or cable ......................200mV (0.2 volt)
Any switch ....................300mV (0.3 volt)
Any ground connection..................100mV (0.1 volt)
Any other circuit connection ........................0mV (0 volt)

If you havenít solved the no-start problem after checking the starter control circuit, or if youíre troubleshooting a slow-cranking problem, move your voltmeter to the motorís power circuit. The insulated side of the motor circuit is the part that supplies battery voltage (B+) to the motor. It contains the positive terminal of the battery, the heavy cables, the power contacts of the relay or solenoid and the motor itself. The ground side of the starter motor circuit starts with the motorís ground to the engine and includes the low-voltage ground path through the frame or body, plus the ground cable to the battery negative terminal.

For the motor circuit tests, disable the ignition and use a remote starter switch to crank the engine. For the insulated side of the circuit, put your voltmeter positive lead on the positive battery terminal (not the cable clamp). Then probe backward through the circuit with the negative meter lead from the high-current cable connection at the motor to every high-current connection on the solenoid and back to the battery positive cable clamp. Look for the same voltage dropsóor lessó listed for the control circuit.

For the ground side of the circuit, put your voltmeter negative lead on the negative battery terminal (not the cable clamp). Then probe backward through the circuit with the positive meter lead from the ground connection of the motor to the engine, then back to the battery negative cable clamp. Again, look for excessive voltage drops. If it looks like weíre dealing with a lot of small voltage and resistance readings, we are. But whatís a few tenths of a volt or ohm among friends? The answer is, plenty! Using good old Dr. Ohmís law (E=IxR), you can calculate that as little as 0.01 ohm of resistance in a starter motor circuit causes a 2-volt loss of electromotive force. And thereís your slow cranking problem.

Charging System Output Test The final steps of this electrical exercise will ensure that the charging system can put back into the battery what the starting system took out. After cranking the engine, the battery is slightly discharged, and this is a good time to check the alternator because it will deliver high current and voltage as soon as the engine starts.

With your voltmeter and ammeter connected to the engine, turn the ignition on but donít crank the engine. Read the discharge current on the ammeter. This is the ignition primary current and, on some vehicles, the alternator field and blower motor current. Now start the engine and run it at 2000 rpm, then read the charging voltage and current. Next, hold engine speed at 2000 rpm until the current drops below 10 amps. Then check the voltage again and return the engine to idle.

Add the current reading taken with the engine off to the highest output current reading with the engine running. This is the total output current and should be within 10% to 20% of the alternatorís rating. The regulated voltage should be from 12.6 to 15.5 volts. When the current drops below 10 amps, the voltage should be at the regulated maximum. Check a manual for exact specifications. If current and voltage are outside the general limits of this test, youíll want to go for pinpoint current and resistance tests.

Because of the variety of alternators and regulators on late-model vehicles, itís a good idea to check the specs and wiring diagrams in a manual for these tests. When you test charging system circuits, however, youíll still be taking resistance and current measurements that are applications of basic electrical diagnosis. Remember what one technician said about troubleshooting: ďBasic testing will find 99% of whatís wrong, after which youíve earned the privilege of using the remaining 1% of your knowledge.Ē



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