Vehicle Electrical System - Explaination and Modification

[tim292stro]

There have been many forum threads that have members poking at electrical modification projects, and tiny "I did this, this way" comments buried in those threads. After being asked about one of those type of projects, I felt there should be a more involved and dedicated thread that deals with understanding the electrical systems of vehicles, so that everyone can benefit and make some better decisions or at least frame questions with a more informed starting point.

I'm going to make an attempt at explaining electrical systems and what makes them tick, and things that you can do in order to modify them - I won't be making recommendations that are "cheats", but I will very specifically point out improvisations that can be made AT YOUR OWN RISK (this will be called out every time). I will have to get into some complicated topics, but I will also try to explain things in a way that people who don't have a Master's Degree in Electrical Engineering can understand.

Any others who wish to add for the good of the collective may of course chime in, that's the nature of the forum - think of this as Free and Open Source Software, with public contributions. The uncontrolled and limited editing of the information in this thread suggests that disclaimers be made first at the outset, so that expectations for the readers are set correctly.

And so here is the first disclaimer: "As with anything free, I should warn that the advise given is worth what you paid for it. I'm a human, I make mistakes, I own them and attempt to learn from them - but verify the information here with other sources before you commit to potentially irreversible damage, life safety system modifications, or big $$$ purchases. I will probably miss something in an explanation, this potential omission is not necessarily intentional, obvious, or of a malicious nature. Anyone else who catches it is perfectly eligible to correct me, please do so."

Here is the second disclaimer: "You are human. This is a complicated subject based on hard learned physics and engineering. Not everyone will understand this subject, and many will ASSUME they understand it in error. This is not a class where you are given a written exam - this is life, and there are sometimes pass/fail tests where failing equals death, maiming, or dismemberment - not only to yourself, but others either relation or complete strangers. Read that last sentence again. You must ultimately be responsible for the use of the information that is put forth in this thread. If you can't handle taking responsibility for your actions at every level, I'd ask that you stop reading this thread and forget you ever saw it."


With these disclaimers I'm not trying to be mean, or a jerk, I'm attempting to limit my personal liability, and impress upon you the importance of this topic to safety.


My last disclaimer for now is simple, however I reserve the right as of 3/13/2014 @ 04:30hrs Zulu time, to make alterations to all of the above disclaimers or additions to the collections of disclaimers. Here's my third disclaimer: "I have a personal life outside of this thread, I offer my time and thoughts at no cost, so no contract or obligation shall be implied - meaning, I reserve the right chose to reply to questions or comments at my own discretion (dumbed-down: sometimes I won't reply, or I may stop all-together)."

Also, read my signature, that's not at all a joke - though you may find it funny.

 

[tim292stro]

Electricity is often taught with a water analogy since water is easy to see and most people have obvious daily interactions with it, where "volts"="water-pressure", "current"="water-flow-rate". A "switch"="valve", a "battery"="water-tank", an "alternator/generator"="water-pump", "wires"="pipes", and so on.

There are also some basic laws that need to be accepted: Ohm's law for example instructs us that Voltage (Volts) = Current (Amps) x Resistance (Ohms)

You can use Ohm's law to figure out one of the three (Volts, Ohms, Amps) if you know the other two:
Volts = Amps x Ohms
Amps = Volts / Ohms
Ohms = Volts / Amps

Power (Watts) are another simple equation: Power (Watts) = Voltage (Volts) x Current (Amps)

Again here you can figure out one of the values if you know the other two:
Watts = Volts x Amps
Volts = Watts / Amps
Amps = Watts / Volts

 

[tim292stro]

There is one math terminology that I feel should also be put forth at the outset, metric notation, or exponent short-hand.

This would be Mega, Kilo, Giga, Tera, Pico, Nano, Femto, Micro. These should sound familiar due to terms like "gigabyte" or "micrometer"

Basically for every exponent of three we change the way we describe the number. You may say "10-thousand", which would be 10Kilo. Alternately you might say "3-thousandth", which would be 3mili. I don't really want to get to much into the explanation of this since it would be a duplication of other easy to find resources like this.

Just know if your number is rising order from one up (positive exponent) the number prefixes are none, kilo, mega, giga, tera, peta, exa, etc..... (yeah "etc" is not actually a prefix turkey ).
If your number is smaller than 1 but still larger than zero in falling order (negative exponent) the number prefixes are: mili, micro, nano, pico, femto, atto, etc... (again "etc" just indicates that it continues).

 

[tim292stro]

Electricity is the movement of electrons through a medium (conductive material) - electrons are those funny orbiting things you heard about on the outside of atoms in school (remember your valence shells?). Specifically, an Amp-Hour (electron flow rate) is a measurable number of electrons passing a point for a measured amount of time.

Let's get a bit crazy here. As small as an atom is, we can actually measure how many electrons are moving in an electrical circuit. A unit for counting electrons is defined as the Coulomb, which is roughly 6 and 1/4 quintillion electrons. An Amp-hour is 3600 coulombs, or 22.5 sextillion electrons, passing a measurement point over one hour. So take a moment to absorb that and think again to the water example. An electron is like a water molecule. An Amp-hour is like gallons-per-hour (who the heck wants to count water molecules when filling a swimming pool?!?!). Now you can ignore the Coulomb thing (water molecules) and focus on the Amps (gallons) and Amp-hours (gallons per hour).

This part is probably not obvious, but is very important. In an electrical system, any loss equals the creation of heat - this means that any inefficiency in a system that is not directly contributing to the execution of "work" is a loss.

This can be intentional, for example with a calibrated "resistor" of a known Ohm value (hey there's that Ohm thing and it's good). It can also be unintentional, for example a wire with a lose lug or corroded connection can have an unexpected Ohm value (oh wait, there's that Ohm thing again, but this time it sounds naughty). Did you know that even a brand new piece of pure copper wire has a measurable Ohm value per foot? As a wire corrodes, the Ohm value changes (always more Ohms). Also temperature can have an effect on the Ohm value of materials - some have Ohm values that increase when the material's temperature increases, other materials have Ohm values that decrease when the temperature increases.

So what is an Ohm? Simply the unit where a measured voltage difference of 1 volt across a medium that is allowing 1 Amp to pass when a current is applied to the medium. This is the basis of Ohm's law, remember Volts = Ohms X Amps. There is something important to notice here: the voltage is not created by the medium, there is simply a difference from one side to the other - this voltage is considered "lost", but we know where it went right? HEAT!!!

Think of a resistance like a known hole size in a pipe. if you force water into that pipe, on one side of the hole the water is constrained by the size of the hole so that only so many water molecules can squeeze through in a given time. If you increase the pressure (voltage) before the hole, you squeeze the water molecules through that hole harder, so more electrons get through in a given amount of time. Realize that you don't lose or gain water molecules due to this effect, you only limit the passage of the molecules (electrons) - and the "loss" or work that is happening is that the water molecules (electrons) are fighting each other for space in that tiny hole (resistance). This is what a resistance does to electrons, and that's the super simplified version of Ohm's law. Increase the pressure (voltage) with a fixed hole size (resistance), and you will get a known amount of water out (current). Keep the pressure the same (voltage), and increase the hole size (lower the resistance), and you will get a known amount of water out (current). If you know how much pressure (voltage) and how much water you got out (current), you can figure out your hole size (resistance).

 

[tim292stro]

Now forgive me if I just skip the super detailed explanation of Volts, suffice it to say that is is the energy or "force" that we impart into an electrical system that overcomes the resistance. I'm going to leave Joules and getting into the laws of thermodynamics on the sidelines for the sake of this thread. I just ask you to realize that not all of the energy that is put into a system is available to use. And with that I'll drop one last nugget before we start doing anything with this info: electrons are not created, they are only moved. If we could just "make more electrons" in an electrical system, you'd all be driving electric cars that never needed to be plugged in once they were made - and since I've told you that any loss or resistance creates heat do to the work involved in overcoming the force against the flow of electrons, now you know where a good portion of the electricity goes in an electric car.


How about an example that has a useful purpose for electrical systems in a car?

Let's talk first about an electric heater, a resistor in its simplest form, which is designed to act against ALL of the force of the electricity that is provide to it. In this example, I'll say we are using a 1000Watt heater (1kW). I'll also say that the voltage we are connecting the heater to is 24Volts (why not use a good NATO standard automotive voltage right?). If you refer to the equations I listed at the top, you'll see that in order to find out how much resistance this heater places on the voltage presented to it, we need Volts and Amps. We know volts because I told you what it is. There is another equation up there that tells you how to get Amps when Watts and Volts are known: Amps = Watts / Volts.

1000 Watts / 24 Volts = 41.67 Amps

So how many Ohms in this heater? Using the known two values Volts and Amps, we can now find the Ohms:

24 Volts / 41.67 Amps = 0.5759 Ohms <-- I'm going to use the metric notation here, moving the decimal place over three, I get 575.9miliohms.


So what happens if your voltage drops due to say, leaving a dome light on, and now your voltage is only 21 Volts (yeah, your batteries are pretty much dead here). Remember, we're only changing the pressure (voltage), not the size of the hole (resistance), so this will have an effect on the water flow (current).

We know the resistance is fixed by SPECIFICATION FOR THE HEATER (1kilowatt @ 24V), and we figured it out as 575.9miliohms. We know the voltage is 21 Volts, so we can again refer to the equations above an work out how many Amps the heater will now output.

21 Volts / 0.5759 Ohms = roughly 36.5 Amps.

Now we want to find out how many Watts that is (just be cause we're curious).

21 Volts x 36.5 Amps = 766.5 Watts


That makes some math really easy: ( 766.5 / 1000 ) * 100 = 76.65% of the nameplate rating. So now you know that you want to have nice charged batteries if you want your engine pre-heater to work when you're starting your truck, low or dead batteries means that you won't get as much heat out of glow-plugs or fuel heaters etc. You knew that already, but now you know why .

 

[tim292stro]

Esr

Continuing. I've done a basic explanation of DC power and a simpler part called a resistor. Now I wan to build on that concept by discussing something called Equivalent Series Resistance, or "ESR".

The concept of a resistor is simple because it has a known and usually calibrated Ohm value. This ohm value basically stays the same if we ignore the effects or age, and temperature once it's produced. However there are devices that have a resistance that is not fixed, rather it changes with other conditions the most common is temperature.

The most obvious part to consider this effect on is your standard light bulb, as this is also a component you will have in any historic vehicle. The operation of a light bulb is fairly simple:

1. pass power through it a wire
2. wire heats up due to more current passing through it than it can support without a significant heat gain
3. as wire gets hot enough, the heat produces light that starts with IR and slowly works down to visible spectrum

If you take a multimeter across a known good 24V-60Watt tungsten/halogen bulb you will likely see a resistance of 0.925Ohms. But wait, if you run that 60Watt bulb on your 28.8 charge voltage that doesn't make sense...

This is the "Powered On" math:
60W / 28.8V = 2.03A
28.8V / 2.083A = 13.824Ohms

This is the "Powered Off" math:
60W / 0V = 0A
0V / 0A = whatever your resistance value is at rest.


The key to remember here is the heating up filament, which has a Positive Temperature Coefficient of approximately 15:1. Positive meaning as the temperature increases, so the the resistance. This is an important effect in a light bulb, since if it were allowed to pass the same amount of current continuously as it heated up, the rate of heating would be directly controlled by the amount of current flowing through the filament. If the current doesn't change because neither the voltage or resistance did, eventually the filament would just melt and open up (i.e. "burn-out").

So with a 15:1 PTC, the resistance of the wire will increase as the temperature increases - this means and engineer can control the temperature by selecting a rated voltage (stay at the voltage as best as possible), then pick an operating temperature for the metal to heat to, select a thickness of wire that will support the desired temperature at the desired current, then build a bulb (in reality there is a bunch more work, but I don't want to bore too many with it).

As a light bulb that is off gets its first sip of power from being turned on, the resistance in the filament is very low and allows a lot of current to pass through it - this creates heat which due the the PTC and design rating causes the resistance to increase (i.e. it's harder for current to flow, kind of like turning the tap down a bit) so less heat is generated. At the rated voltage, and assuming there is nothing else in the electrical circuit interring with the current flow, the filament will reach a balance point - at which more heat causes the resistance to exceed the value at which the current required to keep the bulb hot - causing it to cool slightly and reduce the resistance heating it up - etc.


On the flip side, lets see what hapens if you have a 2Watt NTC thermistor and you apply DC power to it. NTC or Negative Temperature coefficient means that your resistance will drop with higher temperatures. If you think about the light bulb example where we are putting a butt-load of power into a tiny wire so that it glows white hot, and that it has a balance point that keeps it regulated - imagine if you will what happens if you have the exact opposite of a regulator.

For this example we'll assume we are using a 5Volt power supply to connect this NTC thermistor.

At 2Watts of power drop across this thermistor:
2W / 5V = 0.4A
5V / 0.4A = 12.5Ohms

This does mean however that you are able to take away 2Watts of heat continuously from this thermistor, as the flowing current is still generating heat. If you don't remove the heat and keep the voltage the same, you get this:

5V / 6Ohms = 0.833A
5V * 0.8333A = 4.167Watts aka


ESR is an important concept to understand in an electrical system especially if you plan to modify said electrical system. Things don't always stand still, so you need to figure out what your target is and design around that. ESR is used when we want to use simple DC math assuming the component is operating at its steady-point. For most components that have an unstable DC resistance but designed to have a steady state, the manufacturer will publish an ESR value for your calculations. Without an ESR value, you can power the part to its normal rated steady state (if known) and deduce/solve-for the ESR from the Ohms Law equations listed above.

 

[tim292stro]

Switches/Relays/Contactors - Reserved

 

[tim292stro]

Diodes/LEDs (Light Emitting Diodes) - Reserved

 

[tim292stro]

Wire - Reserved

 

[tim292stro]

Fuses/Circuit Breakers - Reserved

 

[tim292stro]

Batteries/Charging - Reserved