CAPS vs Batterys

[dave_Edwards 1]

A carry over from Fordexplorer.net

Charging System Basics:

The electrical system in an automobile is said to be a 12 volt system, but this is slightly misleading. The charging system in most cars will generally produce a voltage between 13.5 and 14.4 volts while the engine is running. It has to generate more voltage than the battery's rated voltage to overcome the internal resistance of the battery. This may seem strange, but the current needed to recharge the battery would not flow at all if the charging system's output voltage was the same as the battery voltage. A greater difference of potential (voltage) between the battery's voltage and the alternator's output voltage will provide a faster charging rate.

As long as the engine is running, all of the power for the accessories is delivered by the alternator. The battery is actually a load on the charging system. The only time that the battery would supply power with the engine running is when the current capacity of the alternator is exceeded or when engine is at a very low idle.

Overview:

A basic alternator has 2 main electrical components. The rotor and the stator. The rotor is the part of the alternator that is spun by the drive belt. There are a group of electrical field coils mounted on the rotor. The stator is the group of stationary coils that line the perimeter of the inside of the alternator case. When current (supplied by the voltage regulator - to be explained later) is flowing in the rotor's coils, they induce current flow in the stationary coils. The induced current (and voltage) is an AC current. To convert this to DC, the current is passed through a bridge rectifier.

Dimming lights:

When you play your system at very high volumes and the lights on your vehicle dim slightly, it generally means that your alternator can not supply enough current for all of your electrical accessories (including your amplifiers). If you play a long bass note/tone and the lights get dim and stay dim until the note is over, your alternator clearly can not keep up with the current demand. If, on a long bass note, the lights dim just for a fraction of a second but return to their original brightness while the note/tone is still playing, the alternator's regulator may just be a little slow in reacting to the voltage drop. Since the lights return to their original brightness during the bass note, the alternator is able to supply the current needed by your power your amplifiers and other electrical accessories.

Capacitor:

A capacitor is an electronic device which consists of two plates (electrically conductive material) separated by an insulator. The capacitor's value (its 'capacitance') is largely determined by the total surface area of the plates and the distance between the plates (determined by the insulator's thickness). A capacitor's value is commonly referred to in microfarads, one millionth of a farad. It is expressed in micro farads because the farad is such a large amount of capacitance that it would be impractical to use in most situations.

Capacitor and DC voltage:

When a DC voltage source is applied to a capacitor there is an initial surge of current, when the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is 'charged'. If the DC source is removed from the capacitor, the capacitor will retain a voltage across its terminals (it will remain charged). The capacitor can be discharged by touching the capacitor's external leads together. When using very large capacitors (1/2 farad or more) in your car, the capacitor partially discharges into the amplifier's power supply when the voltage from the alternator or battery starts to fall. Keep in mind that the discharge is only for a fraction of a second. The capacitor can not act like a battery. It only serves to fill in what would otherwise be very small dips in the supply voltage.

Capacitors and AC voltage:

Generally, if an AC voltage source is connected to a capacitor, the current will flow through the capacitor until the source is removed. There are exceptions to this situation and the A.C. current flow through any capacitor is dependent on the frequency of the applied A.C. signal and the value of the capacitor.

ESR:

ESR is the equivalent series resistance of a capacitor. An ideal capacitor would have only capacitance. As you remember, all conductors have resistance. In a capacitor, there are multiple conductors like the wire leads, the foil and the electrolyte. The resistance of all of the conductors contribute to the capacitor's series resistance. It's essentially the same as having a resistor in series with an ideal capacitor. Capacitors with relatively high ESR will have less ability to pass current from its plates to the external circuit (to the amplifiers in the case of stiffening capacitors in car audio). Low ESR is desirable when using a capacitor as a filter.

ESL:

ESL is the equivalent series inductance of a capacitor. Since most electrolytic capacitors are basically a large coil of flat wire, it will have even more inductance than it would have if it were flat. This inductance, along with the small amount of inductance from the wire leads, will make up the ESL of the capacitor. The ESL is essentially the same as having an inductor in series with an ideal capacitor. Low ESL is desirable when using capacitors for filtering purposes.

Leakage:

Even though a capacitor's plates are insulated from each other, there is a small amount of 'leakage' current between its plates. This current is generally insignificant but will cause a capacitor to slowly discharge with no external circuit path between the capacitor's leads.

Note:

Some large capacitors used in car audio systems have a digital voltmeter on them. Some of these displays will have a remote turn on lead to turn on the LED display. Others will have a timer that will turn the display off after a few minutes. If, in either case, the capacitor's positive lead was removed from the power source (and the display remained on), the capacitor would be quickly discharged by the display. This is not the same as the leakage current that we previously discussed.

Electrolytic Capacitor Foil:

The aluminum foil that makes up the plates in the electrolytic capacitor is treated in a few different processes to make it work properly and more efficiently. The most important process is the anodizing of the foil. Anodizing is a process that forms a very thin layer of aluminum oxide on one or both sides of the foil when the foil is immersed in an acidic solution and direct current is applied to the foil (one lead of the DC power supply is connected to the foil and the other is connected to a conductive plate in the acidic solution). This layer of aluminum oxide is the dielectric (insulator) and serves to block the flow of direct current. To increase the surface area on the foil (and ultimately increase capacitance), the foil can be etched by a chemical process. This would be done before the anodizing.

Paper Element and Electrolyte:

The paper element serves to hold the electrolyte in place. The electrolytic solution (generally ethylene glycol and ammonium-borate) can vary in content but must generally serve a couple of purposes. First and foremost, it must be electrically conductive to help pass the electrons from one plate to the other (the glycol part of the solution does this). Secondly it helps to heal any areas of the dielectric that become damaged (the ammonium-borate does this). If the conductive properties of the electrolyte were absent, the capacitor's value would be drastically reduced. If the healing properties were absent and the anodized coating was scratched or otherwise damaged, the capacitor would leak DC from plate to plate. The healing properties greatly increases the useful life of the capacitor.

Reverse Voltage:

Electrolytic capacitors generally have a positive and a negative terminal. As we said earlier, the plates (foil) of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material (it deteremines which side of the plate is positive and which is negative). We also said that part of the electrolyte was to help heal a damaged plate. Since it has the properties to heal a damaged plate, it has the ability to reanodize the plate. Since anodizing process can be reversed, the electrolyte has the ability to remove the oxide coating from the foil. This would happen if the capacitor was connected with reverse polarity. Since the electrolyte can conduct electricity, if the aluminum oxide layer is removed, the capacitor would readily pass direct current from one plate to the other (it would basically be a short circuit from one plate to the other). This would, of course, render the cap useless.

Over Voltage:

All capacitors have a voltage rating. This tells you how much voltage the dielectric (insulator) can withstand before allowing DC to pass between its plates. Sometimes a capacitor has a working voltage (i.e. WVDC working voltage DC) and a surge voltage. The working voltage tells you how much voltage the capacitor can withstand long term (for the normal life of the capacitor). The surge voltage is the voltage is can withstand for short periods of time. Generally, if too much voltage is applied to a capacitor, it will fail. In electrolytic capacitors, the forming voltage (voltage used to anodize the plates) and the thickness of the paper element determine the working voltage of the cap. In film type capacitors, the insulating material (polyethylene, polypropylene...) will determine the maximum working voltage.

16 Volt Capacitors vs. 20 Volt Capacitors:

As you've likely noticed, large 1 farad (and 1/2 farad) capacitors are available in both 16v and 20v versions. As was said above, the voltage rating tells you how much voltage the capacitor can withstand. It does NOT tell you how much voltage it will have when connected to your system. If both a 16v and a 20v capacitor are conected to the electrical system (with a voltage of 14.4 volts), both the 16v capacitor AND the 20v capacitor will have exactly 14.4 volts. The voltage on the capacitor will be the same as the circuit to which it's connected. In this situation, both the 16v and the 20v capacitors (which have identical capacitance ratings) will hold precisely the same amount of energy. If (IF) the capacitors were charged to their maximum working voltage, the 20v capacitor would hold more energy because it can survive higher voltage. As you can see in the diagram below, all of the electrical components have the same level of water in them (they have the same voltage). If you continue this analogy, you'll be able to imagine that the lower voltage capacitor would 'overflow' if the voltage would go too high (above 16 volts). The 20 volt capacitor could accept a higher water level (voltage) before it overflows. You can also see that when the capacitors are fully filled, the 20 volt capacitor can hold more water (energy). But... in this situation (and in a car), they hold the same amount of energy.

As a side note... The volume of water that a cylinder can hold is equal to the surface area of the cross section of the cylinder (which is analogous to the surface area of the capacitor's plates) multiplied by the height of the cylinder (which is analogous to the voltage that the capacitor's dielectric (insulator) can withstand). Increasing the surface area and/or height of the cylinder (the maximum voltage rating) will increase the maximum volume (charge) the cylinder can hold.

Extra batteries:

Extra batteries are great if you want to listen to your system with the engine off. While the alternator is charging, the extra batteries will only draw current which could otherwise be going to your amplifiers. For proof, all you have to do is measure the voltage while the engine is running. It should be approximately 13.5-14.4 volts DC. Then turn the engine off and measure the battery voltage again. Now it'll be around 12-12.5 volts. Whenever the voltage at the battery is up around 14 volts, there is current flowing into the battery.

One Farad capacitors:

Large, one Farad, capacitors only help to maintain the charging voltage for a tiny fraction of a second under high current demand situations. They do a fine job of filling small dips in voltage and may help reduce your lights from dimming but they won't really solve your current supply problems if your alternator can't keep up.

NOTE: Capacitors DO NOT increase the charging system's voltage.

Battery Isolators:

Battery isolators only prevent draining your starting battery when playing your system with the engine off. Most of the time they will actually rob power from your system. Diode type isolators will usually have a small voltage drop across the diodes (approximately .4 - .7 volts). This loss of voltage will dissipate power in the form of heat and unless you're freezing to death, it doesn't help matters. Solenoid type isolators don't have as much voltage loss as the diode based isolators but the solenoid coil does pull current. Some coils may pull as much as 3 amps of current. Now 3 amps of current isn't much but if you're using 2 solenoids and you're already having trouble with a weak alternator, it'll just add to your problem.

Warning!

Some people tell you that you can check your alternator by disconnecting it from the battery to see if the alternator can produce enough current to keep the engine running. BAD IDEA! Disconnecting the battery will subject the voltage regulator (and computer and audio equipment...) to significant voltage spikes which may cause an otherwise good alternator to fail. Even if there were no damaging spikes, this test would not indicate whether or not the alternator was good because the engine will easily run with a weak or failing alternator.

Simple Test:

If you want to see if your alternator is producing current, turn on your headlights when you're parked and the engine idling with the headlights shining on a wall (at night). Notice how bright they are. Then turn the engine off. The lights should get dimmer when you turn the engine off. If the lights get brighter when you kill the engine, the alternator was not charging sufficiently. When doing this test, the lights should be the only load (turn the stereo, a/c and other accessories off). With a heavy load, an otherwise good alternator may not be able to produce sufficient amounts of current at idle.

BASIC

Battery Construction:

A standard 12 volt cranking battery has 6 individual cells. Each cell is designed to produce ~2.1 volts. The cells are connected in series for a total of about 12.5 volts. Each cell basically consists of 1 set of lead plates and 1 set of lead plates coated with lead dioxide submerged in a sulfuric acid electrolytic solution.

Electrolyte Levels:

The level of the electrolyte should be about 1/8" below the bottom of the filling wells. If the electrolyte is above the bottom of the well, it may be forced out when the battery is charged. If the electrolyte is allowed to fall to below the top of the plates, the battery will be damaged. If the level of the electrolyte is low, refill it with distilled water only. Regular tap water has minerals which may coat the plates and reduce the battery's capacity.

Distilled Water:

Distilled water is water that's been heated to cause it to evaporate into water vapor. The water vapor is then condensed back into liquid water. The distilled water is free of all impurities including minerals that would coat the plates of the battery and therefore reduce its capacity to produce electrical current.

Cranking Amps:

Cranking amps is the spec that tells you how much current a battery can produce for 30 seconds at a temperature of 32