BATTERY OVERCHARGING


Q:

I seem to have a very unusual problem with my 1959 Corvette. I think the battery has been overcharged. Often, I have to add water to the battery. Also, after driving for a half hour or so, I smell sulpher. The top of the battery is always wet with acid which causes the terminals and the battery hold down to corrode. I know what to do if the battery goes dead, but, what do you do when it is charging too much?




A:

Your problem isn’t as rare as you think. As you know, your Corvette has a generator which is intended to charge the battery and keep the accessories supplied with electricity while the car is in operation. Because a generator is capable of unlimited voltage and current, it is paired with a device called a voltage regulator. A generator will generate higher voltage, and is capable of providing higher amounts of current flow, as the RPM is increased. If left uncontrolled, the generator is capable of supplying so much current and voltage that it will melt itself down. It will also damage other electrical devices at the same time because the excess voltage produced by the generator will over tax those devices, and may burn them out as well. A voltage regulator is incorporated into the system in order to help control the “run away charging” that a generator is capable of producing.  Inside the voltage regulator, you will find three coils.


One coil is voltage sensitive, one is current sensitive, and the third is called a Cut out relay. In that order, I will explain: The voltage sensitive coil is able to sense charging voltage, and, when the charging voltage level goes up to a pre set number, it will safely control that voltage by switching the field circuit on and off. The field circuit is the circuit inside the generator that is used to control the generator output and keep it at a safe level. When I say a safe level, I mean a level that is safe for both the generator and for all the electrical accessories in the car. The current sensitive coil inside the regulator controls the output current because it is wound in such a way as to be sensitive to current flow. Similar to the voltage coil, it can control current by switching the field circuit on and off in an appropriate manner.


The Cutout Relay has the job of switching the entire charging circuit on and off as the speed of the armature changes. As you start the engine, the battery provides all of the electrical energy necessary for the ignition system, the starting motor, and any other electrical devices that may be in use at the time. After the engine is running, the battery will continue to serve all of the electrical needs of the vehicle while the engine is idling. This is because, at idle, the generator is turning at a relatively slow speed, and at that speed it will not provide a voltage level that equals the voltage level of the battery. It is not until the speed of the engine increases, that the speed of the generator becomes high enough so that its output voltage is capable of overcoming the voltage of the battery. At that time, the cut out relay will close and the generator will start providing the electrical power for all of the electrical circuits. Additionally, because the generator voltage is now higher than battery voltage, the generator will begin recharging the battery. It is in this way that the cut out relay controls the charging circuit alternately between the battery and the generator. If the car is idling, the speed of the generator is low, and the generator isn’t capable of producing the level of voltage needed to keep everything running. At that time, the cut out relay opens, and the battery is used to operate all electrical devices. If this happens to be at night, while you have the headlights on at idle, you will see them dim out considerably due to the drop in voltage.


This happens because the battery supplies a voltage of about 12.2 volts, which is lower by about one and one half volts than regulated generator charging voltage. When the engine speed increases to a point where the generator is capable of supplying the needed power, the cut out relay closes, and the generator is brought into service. At that time, the lights will brighten up because the voltage jumps from battery voltage, about 12.2 volts, to about 13.8 to14.3 volts, the governed charging voltage. The cut out relay is therefore voltage sensitive, and senses the balance in voltages between the battery and generator, and switches service between them, as appropriate.


If it were possible, we could place a smaller pulley on the generator and operate is at a higher RPM level. Generator speed could then be high enough to run the electrical devices, even when the engine was idling. That is not possible because the generator has such a heavy rotating member, the armature. Because of its mass, the armature would fly apart due to centrifugal force when operated at high engine speed.  It is because of this factor that the generator has a larger pulley, keeping its speed down, and needs the voltage regulator with the cut out relay for it to function satisfactorily in automotive service. Corvettes with the high lift cam shaft have a higher red line on the tachometer, allowing for higher maximum rotational speed. If you notice, those Corvettes have a larger diameter generator pulley than the pulleys on the standard engine. This is designed to save the generator from explosion at those high revolutions per minute.


This brings us to one of the major advantages of an alternator. Alternators can be operated at much higher rotational speeds due to the light weight and design characteristics of the rotor. Notice the smaller diameters of the belt pulleys on alternators as compared to the pulleys on generators.  With an alternator, the smaller pulley allows for higher rotational speed at idle, so the system is able to charge while the engine is idling. The lights stay bright, and the battery is not over taxed. Alternators also have limited maximum rotational speeds, so you will also recall that on alternator equipped high performance Corvette engines, those with a 7,000 RPM red line, the pulleys are a little larger as well. Additionally, alternators have built in maximum outputs, so the big, complicated voltage regulators are not required. However, this does not explain why your battery is overcharging.You should know that the coils in your voltage regulator are adjustable. They are very sensitive and delicate to adjust, but, they can be adjusted, and it seems that you may need to explore that. Primarily, the voltage sensitive coil will probably need the adjustment, as it is probably responsible for the high voltage that is causing your battery to boil over and yield the smell of sulpher.


I do not have the space here to cover the process for adjusting the charging voltage, but it is covered in most of the automotive manuals of that era. It takes a special volt meter with an expanded scale, reading in tenths of a volt, to properly adjust your voltage regulator. Also, because the settings are critical to tenths of a volt, and because these regulators are temperature compensated, so there are certain procedures pertaining to the temperature under the hood which have to be followed in order to be accurate with the settings. An easier fix would be to simply replace the voltage regulator with a new one. Many people do that because the art of adjusting the voltage regulator is almost a lost science. Additionally, there is a chance that your high charging voltage is caused by a defective ground path.


If your engine ground straps are not attached and operating properly, the generator could overcharge the battery and burn out electrical devices due to spiking voltages caused by the poor ground path. I would first check that issue because, even if you have a perfectly operating charging circuit, poor grounds will cause problems in any charging circuit, especially in a Corvette. Back in the 1950s and 1960s, I remember an accessory that was sold and used often, relating to this same issue. It was a voltage regulator with a small knob on top of the cover. That knob allowed for adjustment of the charging voltage. If you were using your car mostly in town driving, you can see how the long periods of idling could cause the battery to become low on charge. The knob could be turned to allow for a higher charging voltage. If, however, you were going to be using your car in sustained highway driving, the knob could be turned to hold the charging voltage on the low side of the scale. With the low setting dialed in, you would not boil the water out of the battery, corrode the terminals and hold down bracket, or cause the smell of sulpher. It was not a bad idea.


Thank you for writing