Ground Circuits: Part 8

Sept. 27, 2017
The current clamp is used to measure the electron current in one wire at a time, the preferred method, but it can also be used to measure more than one wire at a time provided the direction of electron current in each wire is known.

Understand Current Clamps
The current clamp is used to measure the electron current in one wire at a time, the preferred method, but it can also be used to measure more than one wire at a time provided the direction of electron current in each wire is known. Our illustration below shows that the current clamp must be connected to a voltmeter. The current clamp develops 1 mV for each amp of electron current flowing through the wire. The voltmeter indicates the number of millivolts which indicates the number of amps flowing.

To illustrate this concept, we have selected Figure 2-42 from our book “Vehicle Electrical Troubleshooting SHORTCUTS:” Measuring electron current in two wires at the same time.

Figure 2-24 from "SHORTCUTS"

In Example #1 the current clamp algebraically adds the two currents flowing in the same direction indicating the sum:

+5 (+) +15 = 20 and indicates the sum is 20 A.

In Example #2 the current clamp algebraically adds the two currents flowing in opposite directions indicating the difference:
 
-5 (+) +15 = 10 A and indicates the difference is 10 A.

We employ this feature of a current clamp to measure the electron current in the battery negative cables when the battery has a dual ground cable to determine how much electron current is recharging the battery.

The ground configuration at the battery negative cable in Figure 1 below is the most common arrangement with two battery ground cables. The heavy-duty cable (G100) carries starter motor electron current (100 – 300 A) during engine cranking and is referred to as the Engine Ground. It connects to the engine block. The lighter duty cable (G101) carries electron current to all electrical circuits grounded to sheet metal and is called the Accessory Ground or the “sheet metal ground.” The current clamp readings that are shown indicate important circuit conditions during the time the ignition key is on but the engine is not running. The current clamp will indicate the amount of electron current depending on the direction the electrons are flowing.

Figure 1

Back in Part 3, Figure 1 of this series, I explained how the electron current flows in this dual battery ground. Now we measure the electron current in the engine ground and the accessory or sheet metal ground.  This is where a clear understanding of current clamp operation helps to understand the reading obtained.

Notice Current Clamp #1 in Figure 1, has both engine ground and sheet metal ground inside the jaws of the current clamp. This means the current clamp will indicate the algebraic sum of the electron currents flowing through the two cables. The direction of the electron currents flowing in each cable adds together.

+A[engine gnd] (+) +A[acc.cable]  = Total battery discharge current.

Since the key is ON and the engine is not running in Figure 1, the battery (B-) is the source that provides the electron current being measured. This tells us that the electron currents in the engine ground and the accessory ground cables are flowing in the same direction (away from -BATT) as they leave the battery negative terminal.

Electron current from the battery flows down the accessory ground to supply electron current to all circuits grounded to sheet metal. A very small electron current flows down the battery ground and through the engine block to supply a very small electron current (usually milli-Amps) to initialize the generator’s voltage regulator. At this point in vehicle operation the driver has turned the key ON and his next action is to engage the starter motor and crank the engine.

At this point the reading in Current Clamp #1 has little significance in troubleshooting the electrical system other than to indicate how many amps flow through the accessory ground with the key ON. This reading is a reflection of how many circuits are turned on when the ignition key is ON. Current Clamps 2, 3, and 4 have been discussed in earlier parts of this series and that information remains the same.

In Figure 2 below we show the engine running (after cranking) and the generator online. The generator goes into operation during engine RUN and produces the electrical electron current needed by all vehicle circuits.

Figure 2

The generator produces a voltage higher than battery voltage which we call charging voltage and becomes the source of electron current needed by all operating circuits in the vehicle. (We will discuss charging voltage later on in this series.)

We continue to focus on the electron current that occurs when the engine is running and the generator is doing its job. The generator is the voltage supply for all vehicle circuits, electrons leave -GEN (B-).

The battery becomes an electrical load on the generator during engine run and draws electron current from the generator. As electrons are forced into the battery’s negative terminal, at the same time, an equal number of electrons are pushed out of the battery’s positive terminal to make room for the new electrons entering the battery. This recharge electron current replenishes the energy taken from the battery during the cranking process. What is often overlooked at this time in vehicle operation is the amount of electron current the battery draws to recharge. I refer to this current as “battery recharge current.”

+A[engine gnd.] (+) -A[acc. cable]  = Battery recharge current

Current clamp #5 is the key to measuring battery recharge electron current during engine RUN. Figure 2 shows electrons from the generator flowing UP the engine ground and DOWN the sheet metal ground. Since these two electron currents are flowing in opposite directions, one electron current, the lesser sheet metal electron current, is subtracted from the larger total current flowing UP the engine ground. The remaining electron current indicated by the current clamp is the battery recharge current.

Battery Recharge Current Example #1:
Let’s say the total electron current flowing up the engine ground is 50 A. Let’s also say the electron current flowing down the sheet metal ground to supply all vehicle circuits is 45 A. The current clamp will indicate the difference, in this example is 5 A.

+50 A – 45 A = 5 A

We can then say that the battery recharge current 5 A. Keep in mind the current clamp only indicates the 5 A flowing through the battery which is the difference between the two currents flowing in opposite directions in the two cables inside the current clamp jaws.

Battery Recharge Current Example #2:
Let’s say the total electron current flowing up the engine ground is 47 A. Let’s also say the electron current flowing down the sheet metal ground to supply all vehicle circuits is 30 A. The current clamp will indicate the difference, in this example is 17 A.

+47 A – 30 A = 17 A

We can then say that the battery recharge current is 17 A. Keep in mind the current clamp only indicates the 17 A flowing through the battery which is the difference between the two currents flowing in opposite directions in the two cables inside the current clamp jaws.

Battery recharge electron current should be as low as possible. I like to see it at less than 10 A within five minutes of engine run. Battery recharge electron current is determined solely by the battery’s internal resistance which varies from battery to battery. That’s why you could test the battery recharge current on 10 different batteries and get 10 different readings. The amount of battery recharge current only becomes an issue when it is high enough to cause the diode bridge inside the generator to overheat and fail.

A good rule of thumb would be to test battery recharge current any time you replace a failed generator on a vehicle. It could be the battery suddenly increased its recharge electron current and overheated the generator. Expect the possibility of excessive battery recharge current causing the generator failure. In my experience of performing battery recharge tests for over 25 years, I have experienced batteries with a recharge current level exceeding 25 A and smoking a generator every three days due to the length of time the customer drove the vehicle each day.

In another example, I once tested a battery drawing 52 Amps of recharge electron current and the generator would fail in two hours of engine run. Don’t overlook this test of a battery during engine run. I would suggest you perform this test every time you replace the battery. I have experienced new batteries with higher than normal recharge current. I prefer to see a battery drawing less than 10 A within five minutes of engine run.

More details about evaluating a battery’s recharge current is covered extensively in our book “Vehicle Electrical Troubleshooting SHORTCUTS” in Sections 4 and Section 6.

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.