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Our trucks run on direct current (DC) which means that the voltage does not alternate like house current or AC. The nominal voltage of the batteries and alternators that run our trucks is “12V” but that’s a misnomer because a fully charged battery is around 12.6VDC and alternator output is above 13.5VDC
Ohms law:
The most basic concept in electrical circuits is ohms law: V=I x R or voltage equals current times resistance. Voltage is in volts, current in amps and resistance in ohms. Ohms can also be shown also as R or the greek letter omega. You can rearrange the terms in this equation to also say I=V/R and R=V/I so this way you can solve for any of the three variables if you know the other two values. Voltage is the potential to supply current; it’s like the water pressure in plumbing - a common analogy. Resistance is just that, the resistance to that pressure or potential. Smaller pipes, more resistance, less water flow. More resistance means less current at a given voltage.
Let’s look at a simple circuit such as a single load on a fused wire. The battery is the source of the voltage and is at 12.6V when fully charged. The load has a certain resistance and so you can compute the current through the circuit. You can also compute the resistance with current and voltage and so forth. Current leaves the battery from the positive terminal, goes through the load (headlights, motor, sensor, etc) and has to return to the battery negative terminal to complete the circuit. Not just any ground will do, it has to be that battery ground on that truck. This ground return path can either be a dedicated wire or can use the truck frame or engine block which is always connected to the battery negative terminal. Some loads have no negative wire at all because the body of the device is grounded directly to the frame or engine block, providing the return path to the battery. The stored charge flows out of the battery positive terminal, through the circuit and back into the battery negative terminal. They got it wrong when electricity was invented so the electron flow is actually the other way around but circuit current flow is always discussed as positive to negative. Resistance can be intentional like the windings of a motor, incidental like the resistance of the power wire and fuses or accidental like corrosion at the battery terminals. The incidental and accidental resistance elements cause the current to the load to drop as total R goes up, sometimes until the circuit does not function correctly. The losses from all of these resistances cause the available voltage and current to the load to drop and this effect is more dramatic with high current loads like starters and headlights which is why they are more sensitive to circuit problems with increased resistance.
In accordance with ohms law, there is a voltage “loss” and reduction in circuit current with each new resistive element in the circuit as current passes through it, they are additive within that circuit loop. This is why a circuit should be tested with the load connected if possible so that all circuit problems are revealed such as excess resistance at a corroded fuse block. If the load is off or disconnected the voltage may look ok at the load but at full power, the net voltage at the load may be too low. My example circuit below has some made up problems included to help make my point. Some loads like light bulbs will continue to work as system voltage drops all the way to zero but some components like say the ICM will eventually stop working as voltage drops long before zero volts occurs. Power is current times voltage or P=IV or I=P/V or V=P/I.
Example circuit:
In this circuit, the battery is at 12.6V and ground right at the battery is by definition at 0V. The same current flows in the positive lead from the batt + terminal to the load and also in the ground wire from the load to the batt – terminal. The current around this loop is the battery voltage divided by all resistive elements in the circuit loop. I show what happens if you have corroded fuses, frayed wires and corrosion at the connectors. At every point there is a voltage drop equal to I x R. If you put your meter leads across that element with the load on you would read that voltage drop/loss. If you ground the meter to battery ground and test both sides of the load with the positive meter lead, your two readings would be different by the voltage drop/loss across that element. You can see in this circuit that there is so much voltage drop in the fuse, wire and corrosion (even at their relatively low resistance readings) that the voltage at the load is way below a healthy value. Note that in this example, there is corrosion in the ground wire so there is even a voltage drop on the ground side. This would have the effect of “lifting” the ground by 5V. In this example, the voltage of the load would be the difference between 5V on the ground side and 8.9V on the positive side. For extra credit, what is the resistance of the load? The answer will be at the end of this article. In the second diagram I add some measured voltage points referenced to battery ground:
Batteries:
A typical automobile battery is a lead acid storage device. A series of metal plates sits in a solution of sulfuric acid of a certain specific gravity (concentration). The plates are grouped into 6 discrete battery cells. When a high enough controlled voltage is placed on the battery terminals a chemical and electrical process takes place where charge (electrons) is stored in the battery and the voltage rises up to the fully charged state. As the battery approaches the fully charged state the apparent internal resistance goes up and the current into the battery goes down. One measure of battery state of charge is resting voltage with no load. You should turn on the headlights for around 15 seconds after charging the battery to knock the “surface charge” off of the internal plates before measuring the resting battery voltage to assess its state of charge:
12.6V 100%
12.3 70%
12.1 50%
11.8 30%
10.5 0%
Overcharging a battery is bad because it overheats it and boils out the electrolyte water and can actually crack the case or cause it to explode. Letting a battery run down to zero volts is also bad because it causes the internal plates to sulfate which creates a deposit on the lead plates causing the battery to have problems accepting a charge and delivering its rated output. The battery CCA rating is the ability to deliver the very high current needed for starting and reserve capacity is the amount of time a battery can deliver a working voltage at a standardized current flow. Batteries do eventually die from use and heat. Here in Florida they last 3-5 years. They are properly tested by charging them completely and then applying a very high current (low resistance) load of a std value and seeing how far the voltage drops. A failing battery has increased internal resistance so there is an unacceptable voltage drop inside the battery at high currents causing the output voltage available to the load to drop. This also prevents the battery from being able to deliver its rated output current. A battery can be rehabilitated to some extent by applying a temporary controlled high voltage for a period of time called equalization or conditioning which helps mix the sulfuric acid electrolyte in the cells and get the sulfation buildup off of the plates. Some sophisticated chargers like in newer GM vehicles do this periodically to “condition” the battery and increase battery life.
Alternators:
In olden days the charging device in a vehicle was a generator which provided the DC charging voltage directly. In today’s vehicles we have alternators which actually produce AC like in house current and diodes rectify the AC waveform to provide DC voltage and the battery acts as the smoothing capacitor reducing AC ripple to an acceptable level, properly charging the battery and providing system voltage. There is a regulator internal to the alternator case along with the diodes which controls the output for battery charging and system operation. In older vehicles like ours the charging is crude and the alternator provides a high charging voltage of around 13.5 – 14.5 volts all the time, and the current drops as the battery gets fully charged. More sophisticated chargers change voltages in different charging stages starting higher for a depleted battery at say 14.5 volts and only supply a float voltage at the end which is around 13 volts. In some cases these new alternators actually drop the float voltage down to the minimum to run the system properly and not discharge the battery in fuel economy mode (around 12.8 V). This improves battery life and fuel economy and reduces stress on components. Improper charging voltages for our Blazers can be caused by a bad serpentine belt, bad alternator windings, bad internal diodes, bad regulator, wiring problems or a bad battery. An alternator is capable of charging current far in excess of most homeowner battery chargers, up to 100 or even 160 amps in a heavily depleted battery with an upgraded alternator.
Wire gauge:
You have to use the right wire for the job because every wire has a certain amount of resistance per foot. Ohms law tells us that as current goes up there is more voltage loss in that wire. We also know that wire with a lower gauge number is thicker and will have less voltage loss per foot. That’s why higher current circuits and loads need lower gauge number wires. Look at the starter cables, they handle 200 amps! This is particularly important for inductive loads like motors and electronics because they are the most sensitive to dropping voltage. You can find wire gauge charts online that help you choose the correct gauge depending on the current in that circuit and the limit on voltage loss, usually stated in percent loss. The more voltage sensitive the load, the more tightly controlled the loss should be. If you don’t know the total circuit current to decide on the wire gauge then you can look at the fuse value or the existing wire gauge. Thicker wires with lower gauge numbers won’t hurt anything and always use stranded wire, it handles more current than solid of a similar gauge because of the individual strands.
Sensors:
Sensors that vary resistance usually either get a voltage applied to them and the ECM measures the changing current to imply resistance or there is a constant current applied to the resistive sensor and the ECM measures the changing voltage.
Frequency:
Some devices such as our MAF sensors actually output a frequency measured in Hz or hertz which is cycles per second. In this case the output changes (modulates) frequency with airflow quantity. The ECM converts that frequency to an airflow amount.
THanks. I know that understanding electrical circuits is the hardest concept for many so I hope it’s helpful and doesn’t just make your brain explode. Lol
I added an important concept on wire gauge to this white paper, here is the additional paragraph:
Wire gauge:
You have to use the right wire for the job because every wire has a certain amount of resistance per foot. Ohms law tells us that as current goes up there is more voltage loss in that wire. We also know that wire with a lower gauge number is thicker and will have less voltage loss per foot. That’s why higher current circuits and loads need lower gauge number wires. Look at the starter cables, they handle 200 amps! This is particularly important for inductive loads like motors and electronics because they are the most sensitive to dropping voltage. You can find wire gauge charts online that help you choose the correct gauge depending on the current in that circuit and the limit on voltage loss, usually stated in percent loss. The more voltage sensitive the load, the more tightly controlled the loss should be. If you don’t know the total circuit current to decide on the wire gauge then you can look at the fuse value or the existing wire gauge. Thicker wires with lower gauge numbers won’t hurt anything and always use stranded wire, it handles more current than solid of a similar gauge because of the individual strands.
03-20-2021, 10:08 PM
12V Primer for Blazers
