Starting/charging system scope analysis

Jan. 1, 2020
How many times have we had a vehicle in for what the customer thought was a charging system problem? We hook a voltmeter to the electrical system and find low charging voltage and possibly excessive voltage ripple. We replace the generator, only to f

What seems to be isn't always the case.

Motor Age Garage 2005 Ford Freestyle scope analysis no starts vehicles won't start fixing vehicle repair shop training technician training automotive aftermarket

How many times have we had a vehicle in for what the customer thought was a charging system problem? We hook a voltmeter to the electrical system and find low charging voltage and possibly excessive voltage ripple. We replace the generator, only to find that the real problem is a defective battery or a poor electrical connection. Wouldn't a picture of what is happening in the starting and charging system be a great way to keep mistakes like these from happening? Over the years, I have tested a lot of batteries with both a carbon pile tester and a capacitance tester. I have had many batteries pass these conventional tests, only to fail a few days or weeks later. This has driven me to rethink starting and charging system testing. I am always looking for quicker and more accurate way to test in order to gain more information faster.

Let's take a look at a 2005 Ford Freestyle. The vehicle is in the shop for an oil change service. The odometer shows 60,000 miles. There are no starting complaints, though when we started engine in the parking lot, we noticed a slower than normal cranking speed (the engine did not spin up as fast as it should have). The vehicle is 5 years old with the original battery. In the shop, we hooked a voltage lead to the battery terminals and clipped a high amp current probe around the negative battery cable, and the engine started.

I use an oscilloscope, high amp current probe and a voltage test lead. This testing takes approximately five minutes, including hooking the current probe around a battery lead, testing the voltage across the battery terminals and running the engine for two minutes while graphing the voltage and current that is going either in or out of the battery. The current probe needs to be hooked to a place that will be able to test all of the current going in and out of the battery, but it makes no difference whether it is on the positive or negative cable. In my experience, the negative battery cable is the best and easiest place to hook the current probe.

With these two quick hookups, you are able to run a relative compression test, test the ability of the battery to produce energy, check the integrity of the starter motor circuits and examine the charging ability and the integrity of the generator. Any graphing multimeter or scope will work for this test, so you can use what you have in the shop. Do not use a digital multimeter (DMM) with min/max, because the capture rate is too slow. Besides, a DMM will not draw a picture. I have labeled the scope a video voltmeter, because it draws a picture of the voltage and amperage of the starting and charging systems. In this case, a picture is worth a thousand words.

With this testing, we are looking for the minimum battery voltage when the starter solenoid is closed (inrush voltage), and for the maximum amperage that the starter will draw before the motor starts turning. We can zoom in and take a look at the integrity of the starter solenoid contacts as they close, the operation of the starter motor, the maximum charge rate of the generator when the engine starts and the minimum charge rate after the engine has been running for two minutes. With all of this information, it is easy to see exactly what is happening in the starting and charging system. It also is easy to perform a relative compression test while the scope is hooked up. All that is needed on most cars is to depress the throttle to the floor and crank the disabled engine for a few seconds.

There are two important numbers I look for with this test: an inrush voltage of not less than 8.5 volts, and a minimum charge rate of 3 amps or less after charging for two minutes. An inrush voltage below 8.5 volts is a good indication of a weak battery, and a continual charge rate above 3 amps is a great indication of a sulfated battery (bad battery). The only way a sulfated battery can be found in its early stages is with a current probe monitoring the current going into the battery. A carbon pile load test or a capacitance tester will not find this problem.

Consider what an extra 10 or 15 amps of current load will do to the life of a generator. If a sulfated battery is found, more testing needs to be done to find out the reason the battery sulfated. As batteries age, this is a normal process. But if the battery is only a year or so old, most likely a problem in the electrical system has caused this condition. Some of the causes could be the parasitic draw is too high, which is discharging the battery over night with the generator having to charge the battery once the engine is started or a voltage regulator that is causing the generator to over charge the battery.

The Ford's battery, as seen in Figure 1, has an OCV (open circuit voltage) of 12.54 volts, which is good, but a minimum inrush voltage of 7.83 volts, which is not good. As the starter is energized, the starter current spiked to 475 amps. The maximum charge rate is 20.6 amps, and after charging for two minutes, the charge rate is considerably high at 6.4 amps. In this case, there are two things that stick out. First, the inrush voltage is too low and the minimum charge current is a little too high.
Figure 2 shows a closer view of the starter just as the solenoid contacts are closed and the motor starts turning. This view allows a better look at what is happening when the solenoid contacts closed and the starter motor started turning the engine. The solenoid contacts close nice and clean, and we also can see as the motor started turning the engine that the amperage spike is a little lower than expected. There are no problems seen in the starter motor.
In Figure 3, the battery was load tested with a carbon pile load test to verify the accuracy of the previous test. Sometimes breaking with traditional testing methods can be hard, but as a person becomes comfortable with new testing methods, the old way of doing things seem to fade away. The battery was loaded to half its CCA rating for 15 seconds. With this waveform it is easy to see that the battery has failed its load test. The minimum voltage of 8.92 volts is .58 volt below the 9.5-volt minimum of a good battery load test.
In Figure 4, we can see how the new battery performs. With the new battery, the minimum voltage is 8.84 volts, maximum amperage is 504 amps, maximum charge rate is 12.4 amps and the minimum charge rate is 2.1 amps. This procedure clearly shows the difference between the old and new batteries. Figure 5 is an overlay of both the new and old batteries as the starter was turned on. This view shows clearly the differences between the two and how the original battery was failing.
Here are some of the things to look for with this test. First, we want to see the open circuit voltage (OCV) of the battery. Normal is 12.6 volts, but if the OCV is higher than 12.6 volts, the cause could be surface charge. At this point a carbon pile battery load test will not be a good test. We want to see the OCV after the vehicle sets for 10 minutes. If it is too high, (above 13 volts) or too low, (below 12.4 volts) this is an indication that there is a battery problem.

You also can turn on the headlights for a few minutes to draw off this surface charge. At this point we can start the engine and be able to see how the battery reacts to the starter load. Most 4- and 6-cylinder engines will draw close to 500 amps when the starter solenoid contacts close. With some practice you will soon get a feel for what the peak turn on current should be on the different vehicles that frequent your bays.

Once the starter starts turning, the current requirements diminish, but that peak current is the killer. I find that the minimum voltage of less than 8.5 volts (inrush voltage) will prove out as a weak battery. I have also found that as a battery ages, the minimum charge rate keeps increasing. Most fully charged new batteries will have a continual charge rate of 2 to 3 amps. At times, a sulfated battery can have a minimum charge rate of 10 to 15 amps, or higher and never be satisfied. If this happens, the life of the generator can be shortened, because this 15 or 20 extra amps of current added to the normal loads of the vehicle can overload, and overheat, a generator.

This also is a very important test to do when a generator is replaced. With any generator replacement, the question should always be asked, "Why did the old part fail?" Was it from general wear where the brushes and slip rings finally wore out, a diode failure or was it from an overcharge condition? This quick test will most times answer those questions.

A system retest is easy to do with a new battery installe. Maximum starter amps have gone up to 502 amps and the inrush voltage is 9.23 volts. To make the difference even easier to see, an overlay of both the new and old battery will help, as seen in Figure 6. With the two waveforms overlaid the things that pop out are inrush volts are less on the good battery, the amperage the starter draws is more, the peak charge current is lower with the new battery and the minimum charge rate also is lower. What a great way to show a customer how the new battery that was installed is putting less stress on the starting and charging system of their car. This test not only can be used to prove out a failing battery, starter or generator, but allows us to see if any of the other components in the starting and charging system are functioning properly, or if failures in any other components are eminent.

I always have liked testing systems dynamically under real-world working conditions while leaving the system intact. Being able to test a system where it lives and works has a lot of advantages. This has led me to use a little different way of testing starting and charging systems in which I utilize an oscilloscope for both speed and accuracy. In the last few months, I have tested more than 100 vehicles with the same test and have found some rather interesting results. Very few of the vehicles I tested have come to the shop with any starting or charging complaints, but rather were in for some sort of routine service or repair. With this testing, I have sold a lot of batteries and a starter or two.

Albin Moore opened Big Wrench Repair in Dryden Wash., in 1992. Since opening the shop, he has moved the business to specialize in drivability problem analysis, both with gasoline and diesel vehicles. Albin is an ASE CMAT L1 technician, and brings with him 40 years of analyzing and fixing mechanical and electrical problems.

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