05 Mar 2024

A month in the life of a vehicle technician

A cross-section of repair challenges seen at my current workplace, Eldon Street Garage, probably reflect common issues dealt with by most workshops in our industry. With this in mind, it became increasingly apparent from the outset that little or no consideration was given to batteries.
    
A simple battery test tool was available. However, none of the results were reliable in my opinion as nobody was checking to see if the battery was the correct specification for the vehicle it was fitted on.

Basic test options
A good place to start would be to review some of the basic test options and tools required for accurate assessment.
1: Check that the correct battery is fitted before testing. I found a high percentage were not correct. CCA rating was often too low, or the battery was of poor quality. I will expand on this later. I always prefer to fit the higher rated options available.

2: Always use a competent conductance test tool. Prices will range from £600-£1,000. Consider the cost as an investment in accuracy, easily recouped when replacing faulty batteries. Set the test parameters to the correct specification required not the data from the fit label. It is essential that the battery is able to supply sufficient current for crank start without voltage starvation to the remaining vehicle systems. Network systems are especially voltage-sensitive easily leading to false failures, as was seen in my article in the March issue.
    
Common rail diesel requires a nominal crank engine speed in order to produce the minimum rail pressure required. Expect 300bar or more. If it is below 250bar it is possible the injectors will fail to operate. The same principle applies to direct gasoline injection systems, although pressures will differ. Nominal cranking speed is 150/175 for a calcium battery, or 200-plus for a EFB/AGM battery.

3: Please refer to Fig.1, which shows the Midtronics battery test tool. There should be a choice of in or out of vehicle test. When only a battery assessment is required, then choose out of car. Otherwise follow on screen prompts for cranking and charging performance.

4: Print the test result, repeat the test if a good battery requires charging. A calcium battery requires 16v to full charge and drive sulphation from the plates. Poor quality batteries have insufficient plate volume to store and supply the correct current. In order to meet the CCA rating the electrolyte strength is increased, but corrosion and service life reduction will follow. I am sure you will have noticed the difference in weight.

Voltage drop testing
Let’s explore using a DMM for voltage drop testing. Please refer to Fig.2. I am using a fluke 187. While I am aware this has been replaced in the market by a more up-to-date model, the 187 does offer a number of very useful features, including relative and min/max recording.
    
This test eliminates the need for a second tech noting voltage or current values during variable circuit load testing. Voltage drops between the battery terminal and the starter motor high tension post, so connect the ground lead to the battery negative post. Then connect the positive lead to the starter motor post. Note the static voltage reading, select relative test function. The screen will show a zero value. Then conduct an extended no-start crank test noting the voltage drop value. This feature can be used across any circuit between source and consumer. You may also connect the black DMM lead to positive battery and read the actual voltage loss by repeating the above test. It will show the same differential value.
    
Use min/max to assess cranking charging performance. Connect the DMM across the battery terminals, select min/max, crank start the vehicle, raise the RPM allowing the alternator charge to take effect, switch off. Toggle the min/max button. The DMM will display the static value, 12.4v max value on charge,14v, then minimum value cranking,10v (average values).
    
It generally understood that the maximum current across the fluke 187 is 10amp with a 30 second 20 amp in-rush limit. However, if a 30/60-amp Hall Effect current clamp is used with a 4mm BNC adaptor, it is possible to connect directly with the DMM input jack. The jack spacing is a global standard. Check the range conversion value. It is normally 100mv/amp, however many have switchable ranges.
    
Please refer to Fig.3, which shows a selection of current probes. You can now measure current ranging at 10amps per volt, using a 2,000 amp clamp. The range conversion is normally 1mv/amp, so 1000 amp = 1 volt. With the relative or min/max function, you can now record current flow through any circuit you choose. When placing the current clamp jaws around the conductor under test, the arrow should follow the flow direction. I call it the consumer. Remember, during cranking the starter motor is the consumer. Then, while receiving charge from the alternator, the battery is the consumer.

Non-start condition
A Peugeot van was towed in recently in a non-start condition. When turning the ignition key to terminal 50, crank request, a distinctive click could be heard. My first test was battery performance, rated at CCA520. The battery was rather low for a diesel. It tested at CCA70 so it was scrap. Don’t bother charging a failed battery (I will come back to this).  I fitted a CCA850 and continued testing. The engine would only just rotate while cranking. The current clamp test using min/max displayed 2 volts.
    
That’s 2,000 amps. I repeated the test using a scope and current clamp confirming an almost instant rise to 2,000 amps then flat line at 2,000 amps. Just a thought, maybe it’s a seized engine?  However, it turned quite easily with a socket and ratchet on the front pulley. So, it’s a consumer problem. It just proves that a quality battery rated at 850 amps actually delivered 2,000 amps.
    
The owner didn’t think it was worth repairing. I did warn you previously that this garage is often presented with skips on wheels.
    
Battery voltage assessment; The minimum cell voltage without cell damage is 12.4 volts. The death zone is below 11 volts, so if you are faced with battery that is this severely discharged, it is scrap. You can put voltage back but not capacity, hence you should always use a conductance tester.
    
More workshop woes next month.