Every industry has its own set of standards; electronics is a science of many standards.  One particular standard used when measuring voltage in any circuit is what we call the reference point.  The reference point typically is your most common point within a circuit.  By definition, it is a terminal  that is common to both the input and the output circuits.  In forklift trucks and many other circuits, the most common point for reference is battery negative, simply referred to as “common”.  Without a common reference point, voltage readings don’t always mean the same thing.  Whenever you are measuring voltage in a forklift system, always (99.9% of the time, anyway!) connect the negative lead of your meter to battery negative.  This is the industry standard for the voltmeter reference point.  Let me explain. . .

Figure 1 is a simple DC circuit using two resistors, R1 and R2, for the load.  A, B and C will be our points for measuring voltage.  The current flowing through this circuit, although constant, will depend on the resistance values of R1 and R2.  The voltage measured at various points will be quite different.  Each resistor will drop a specific voltage across it, depending on the resistance value.  An important thing to remember is that all those voltage drops will add up to the voltage being supplied.  All that aside, let’s keep this as simple as possible.  If we measure the voltage from point C to point A, we will measure our voltage being supplied (in this case 36 volts).  Now, measure the voltage at point B.  Using our common reference point for our meter, negative lead, we connect it to point C.  If we measure point B we will measure a specific voltage depending on the resistance values of R1 and R2.  To simplify matters, suppose we measure 6 volts at point B.  Remember, we stated that the voltage drops will always add up to the voltage being supplied.  With that in mind, we’ll measure point B again but this time we will use point A as our reference point.  If we measure 6 volts across R2 we must measure 30 volts across R1 or from point A to point B.  Notice that if we add our voltage measurements, our sum is the 36 volt battery supply;  6 volts + 30 volts = 36 volts.

Now the $10,000 question; what is the voltage measured at point B?  We measured two different voltages, 30 volts and 6 volts, depending on where we referenced our meter.  The proper reference point is point C, battery negative, thus our proper voltage at point B is 6 volts.

We shall now look at a typical measurement used on an EV-1 system.  Figure 2 shows the accelerator circuit, which is a measurement commonly encountered on the EV-1 system.  What is the accelerator voltage, measured at creep speed, at terminal R5?  Most manuals  will give you a reading of approximately 3.5 volts.  Indeed, if you measure from point A, battery negative, to R5 you will measure approximately 3.5 volts.  What if we measure from battery positive?  With a 36 volt battery our reading will be 32.5 volts; 3.5 volts + 32.5 volts = 36 volts.  Remember, our voltage readings must add up to the supply voltage, 36 volts.  What is even more confusing is if you have a 24 volt battery system and you measure from battery positive to R5 you will measure 20.5 volts; 3.5 volts + 20.5 volts = 24 volts.  Therefore,  if you measure or reference your meter to battery positive, you can measure different readings for different voltage batteries.  So is the accelerator voltage 32.5 volts or 20.5 volts?  Neither voltage is correct because the proper voltage at R5 is 3.5 volts on any battery voltage system, when the proper reference point is used.  No matter what the system’s voltage, if you reference your meter to battery negative and measure R5 you still get 3.5 volts.

“Standard” by definition, is an acknowledged basis for comparing or measuring.  If you want to be successful in the electric forklift industry, you must follow certain standards.  When measuring voltages, use battery negative for your reference point. By doing so, all your readings will be consistent, not only for you but for the entire industry.
 

This article is taken from the "FORKLIFT ELECTRONICS TRAINING MANUAL".  
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