Battery Power

I noticed today, while driving, that my battery light would come on and the voltage meter would read low for a minute or two and then everything would go back to normal. During the low spots, the car still runs fine except when at a stop it idles a little rough. I have disconnected the negative cable to the battery while it was running to test the alternator, it ran fine. I have cleaned both battery stems with a metal brush. I have checked the alternator belts to make sure that they are tight and not slipping. And I had attempted to find the ground wire to make sure it was well grounded, but was unable to find it. (Don't know exactly what that looks like or where it would be grounded to).
Do you
have the same problem?
Tuesday, April 30th, 2013 AT 5:34 PM

1 Reply



Every year I did a demonstration on the generator test bench for my students to show what can happen when you do that. It was real easy for the voltage to reach over 35 volts. That WILL destroy any computer on the vehicle, the generator's internal diodes and built-in voltage regulator, and any light bulbs that are turned on.

The thinking is that if you disconnect either cable and the engine stays running, the generator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good generator appear to be bad so that test is not valid.

If a mechanic is caught pulling this stunt he will typically get one verbal warning. For the second offense he will be fired. It's that big a deal.

Some generators respond to the high points in the ripple. That momentary higher voltage goes right back to the field winding and creates a stronger magnetic field. That stronger electromagnet creates a higher output voltage which again creates a stronger electromagnet. It's a vicious circle and voltage can keep on rising until something gives out. The main thing that smooths out that ripple so it doesn't affect the voltage regulator or the generator is the battery.

Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, ... As long as you don't raise engine speed.

One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the generator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.

Another common generator problem is one defective diode out of the six. You will lose exactly two thirds of the generator's capacity but system voltage will remain normal or it could even be just a little high from the voltage regulator responding to the greatly increased dips in the ripple voltage.

It's always a good idea to wear safety glasses when working around car batteries, but if you still insist on removing a cable while the engine is running, a face shield makes more sense, and have plenty of water on hand to wash any acid off the vehicle's paint.

Ford had a really nice generator design that allowed testing right on the back of the unit. Only Chrysler alternators are easier to diagnose. Unfortunately the engineers don't really care about ease of service on GMs and many other brands.

The way you tell if the charging system is working is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. There still could be a bad diode though. You need a professional load tester to test for that. Ripple will be very high and the most output current you will get will be one third of the generator's design value. That is not enough to meet the demands of the electrical system under all conditions so the battery will have to make up the difference, until it runs down.

You still most likely have a generator problem. First of all, you know it's an intermittent problem because the warning light turned off at times. Any testing has to be done while the problem is occurring and the light is on, otherwise of course everything will appear to be okay. The common intermittent problem is caused by worn brushes in the generator. This can go on for weeks or months before the problem becomes permanent. In less-common instances the voltage regulator can be intermittent. The way to identify that is by grounding the "Ground Here To Test" screw head on the back of the generator. If the system voltage rises. Lights get bright, and you can hear the generator strain, that indicates the voltage regulator is open, (defective), and the rest of the system is working because you just bypassed it. It can be replaced separately. If there's no change, and measuring shows no voltage on that test screw, that proves the brushes are worn. They are attached to the voltage regulator and can also be replaced separately.
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Tuesday, April 30th, 2013 AT 6:17 PM

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