Charging Systems And Protection - Back To The Basics, Part IIWritten by on October 15, 2010 Charging Systems ANd Protection
Hopefully you had a chance to read the first part I wrote on basic electrical theory. If not, don't worry; I am going to go over some of the basics all over again. Plus, it is easier for most people to learn by repeating things over and over. The more you teach yourself about electrical the more you can relax about putting some miles on that old rust bucket or pristine show truck. Regardless if you have a carbureted street rod or a new fuel-injected LS3 truck, there is an electrical component that can and most likely will fail. This time I am going to cover charging systems and how to protect starters, alternators, and batteries from premature failure with aftermarket parts.
Testing the charging system can be very misleading, especially if you have no clue about how the system works. I headed over to North County Rebuilders in Escondido, California, so the owner John Carrel could explain the inner workings of an alternator.
The battery supplies the voltage to the ignition coil allowing the car to run, but without a charging system the battery will drain very fast, leaving you with your thumb out on the side of the road. So what does an alternator do? The alternator provides the battery with a variable voltage from 13.5-14.8 when the vehicle demands it.
When you put electricity down a wire, the wire will have a magnetic field around it. If the wire moves through a magnetic field, small current electricity is created in the wire. The more wires you use or the longer the wire windings are the greater the strength of the magnetic field, the greater the current or charge will be. These two theories are the basis for electric motors, generators, alternators, and even relays.
Charging voltage is higher than battery voltage because it has to overcome the internal voltage inside the battery. Most people think the alternator is very similar to a battery charger, but this is not the case. An alternator is used to maintain the battery and not fully charge it from the dead. So now that you know what the alternator does, one question has to be asked: How does the alternator charge the battery? Alternators have several internal components that all do specific jobs.
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Rotor<br> First is a rotor or electro magnet witch has a positive and negative side and when applying power it becomes a strong magnet. The rotor spins inside the alternator housing via inner bearings and the outer pulley.Here is a view of the positive and negative poles on the rotor. These contacts are what the brushes push up against.Next, the stator is made up of individually insulated strands of wire that are wound neatly inside the alternator housing and coated with a final insulator. The stator and rotor are separated by an air gap, and when the rotor spins it generates AC.The AC is then sent to the rectifier bridge and it is located inside on the back wall of the alternator. The job of the rectifier bridge is to convert the stators AC voltage into DC voltage. Inside the rectifier bridge there are six diodes (three positive and three negative). Since the AC sine waves go negative and positive you need to convert both sides. So if you have a 75-amp alternator then the diodes in the rectifier would be rated at 25 amps each negative and positive. The larger the amperage of the alternator the bigger stator winding and the bigger the rectifier bridge needs to be to accommodate the power and not short out.Placed on top of the rectifier, but not a part of the rectifier is another set of three small diodes. These diodes convert AC to DC, but only a small amount (2 or 3 amps). The diodes are used to power the rotors' positive and negative side in DC in order to energize the electromagnetic field.Next, you have a voltage regulator also inside the back of the alternator. The voltage regulator controls the amount of power that the alternator can produce. The regulator measures the battery voltage drop under 14.2 V and has the ability to add and take away resistance, causing more or less charge to the battery. It can do this by using a variable rheostat inside that controls the available ground output on the rotor. If the regulator supplies 100 percent ground then the alternator will be at full output. So if the charging system only needs 25 amps then the ground will be reduced via the rheostat in the regulator producing a lower voltage.Brushes allow the current to travel through the stator from the rectifier bridge on the power side and the regulator supplies the variable ground on the negative side. Brushes wear out over a period of time due to normal use. Behind the brushes you have springs that push on the brushes and make sure there's solid contact with the stator.
Moving to the outside of the alternator housing, there is a fan that is used to cool the alternator during normal operation. When the vehicle is turned on the fan is on, and the faster the motor spins the faster the fan goes. Alternators can reach extremely high temperatures when not used properly.The alternator is beltdriven via a belt that is connected to the crankshaft pulley. Most of the early alternators were V-belt driven and require a tight belt that put a strain on the outer alternator bearing. The V-belt setup also needed a periodic tightening to make sure the belt was not slipping. Have you ever heard a car start and a nasty screeching noise come from the engine bay? Well, that noise was likely to be a loose V-belt on the power steering pump. That could also happen to the alternator with no screeching noise and the only way you would know is if you checked the belt tension.A better setup is a serpentine belt, because the increased surface area slipping is a lot less common plus the tension is also greatly reduced. Some serpentine belt systems have an automatic tensioner that requires no adjustment, and the less time underhood the more time you can drive. If you want a period-correct truck a V-belt still works great.A loose belt will charge normally at idle but once at idle the belt will most likely slip. Here is a neat tool to help diagnose a loose belt. It's called a photo tachometer and Carrel says he hooks it to a voltmeter, and it can read rpm on the front alternator pulley. It is necessary to put a mark on the front of the pulley. Measure the rpm at idle and then turn on the blower motor and high beams and raise the engine rpm. The meter reading should increase with the engine rpm and if not the belt is slipping. To fix this problem you could try to tighten the belt and check the belt for cracks. Also check the pulley for wear.This is where I go over a few things again. Check, check, and check your battery voltage again. If your battery is below the 11V level then the alternator will be charging harder than it needs to be and will possibly fry. Always charge your alternator on a fully charged battery.The first step when checking for alternator output is to check it with a voltmeter. Place the positive lead on the alternator's positive terminal, then ground the meter lead to the battery ground, and let the engine idle. The voltmeter should read 13.5-14.8 when at idle and increase when rpm goes higher.The alternator was only charging at 30 amps when the engine was revving. It was taken apart and the stator was shorted inside; notice the black wires and the melted insulator. Carrel said the customer was using the stock 100-amp alternator, and he had an aftermarket stereo system, airbags, electric fans, and a power inverter pulling 150 amps at idle. Carrel convinced the customer to use two alternators to reduce the risk of burning out.In order to test the alternator output it is necessary to use an amp meter. There are several different types for every budget. Before using the amp meter figure out what the max rating amperage on your alternator is; sometimes the rating is on the back. Let's say the alternator we are testing is rated at 100 amps max. Connect the amp meter around the positive side of the battery cable and let the engine idle with blower on high and the high beams on. The meter should read about 40-50 depending on how much current you're using. Now raise the idle and recheck, the amperage reading should have increased to about 70-80 amps. Again depending on how much amperage your vehicle wants it will vary between one vehicle to the next. Plus you need to take into consideration how many amps your alternator is. Carrel notes that he tries to set all charging systems to run at no more than 80 percent at the vehicle's greatest demand. Meaning if an alternator is capable of a 100 amps Carrel makes sure the truck only demands no
Speaking of connections here is an option for the battery terminals. Remember the starter, charging system, ignition, fuel pump, cooling fans, and stereo system all run off the battery, and it's a good practice to have great connections at the battery. I also like matching the ground side with the same size wire as the positive side cable. Just remember the easier the electricity flows the less stress on the starter and alternator.Starters can be subjected to tons of heat, especially when aftermarket headers are being used. Most headers run really close to the starter and after a long drive the starter can easily be overheated to the point it will not work. In this photo we used Heatsheild Products starter wrap, PN 500025, with double insulation. This can help in reducing starter failure by deflecting the headers heat.If you don't like to wrap your starter there is another option. I ordered Heatshield Products exhaust blanket (PN 175115). The blanket can be cut to fit and bent around the curves of the exhaust. In this case I cut a 3x16-inch strip and used metal wire ties to wrap around the closest tube near the starter. Notice I didn't wrap the blanket around the tube because I was simply trying to deflect the heat from the starter.