1986 Corvette. I noticed today, when I let my windows up or down the voltage on the dash gauge drops to around 12.4v when the engine is at idle. I can actually feel and hear the rpm lower when I raise the windows. They go up and down fairly slow and the ribbon needs to be cleaned or motors upgraded anyway. I did not check the alternator output with a voltmeter, but wonder what the cause is? Too much draw on the electrical system?

Thanks Matt,

 

I would wonder if you have a battery/alternator connection that isn't quite up to snuff.

I agree, get the window regulators upgraded... that will improve overall reliability. It's just a pain to get the glass adjusted right.

 

Not totally unusual. Remember, the car runs off the battery, the alternator's job is to keep the battery charged. The amp draw from the window motors causes a volatage drop, the alternator then is trying to recharge the battery, but at idle it has very limited output. Rev it up to about 1000-1200 rpms and bet you won't see much of a difference. If there is a problem it is most likely that the battery is nearing the end of it's usefull life, not the alternator. Ever see a car's headlight's dim sitting at a traffic light idling in drive? Same basic principles apply.

 

hm... I always thought the alternator did all the work, and the battery was there to act as an equalizer and such.

 

As for the original post. It sounds like the window motors are drawing lots of current....obviously. Perhaps inspecting the wiring connection and window ribbon will shed light on it. I would go ahead and test the alternator output while someone lets the windows up and see if the voltage drops there.

The alternator runs all the electrical while the engine is running. I doesn't actually charge the battery. However on my 86, I have noticed when I disconnect the battery while the engine is running, the engine dies. A fellow forum member, whom is very knowledgable with the electrical in these cars, said that sounds normal. I haven't tried disconnecting the battery on my 95 with it running, but know that other vehicles...I could disconnect the battery and set it on the workbench and the engine keeps runnnig.

 

There is a lot of bad info in some of these threads. The alternator supplies all of the electricity for the car when the engine is running! It does this by maintaining 14.7 volts across the battery. This forces current into the battery because even fully charged, a lead acid battery has an output voltage of 13.2 volts, so until the battery voltage falls to below 13.2 volts, the alternator supplies power to the car AND charges the battery. At idle, the alternator is turning slowly and its current capacity is limited. If you draw enough current at idle to exceed the alternators capability, the battery voltage will drop and from your description with your windows, I believe that is what is happening. Rev the engine to 1500 rpm and try your windows, I am sure you will see that the battery voltage stays above 13 volts showing that the alternator is supplying current to the window motors. Maybe your idle speed is way too low!

 

Actually that is correct, as usual Andy. Now the rest is directed to other comments and their respective authors.

Check the lead from your alternator and where does it go??? Hopefully to the battery, via junctioing at the starter, and another lead to ground. Despite the comments from others I stand by my original statement until someone can prove it wrong. I mean proving it wrong with a verifiable reference from an accredited source. And to be perfectly clear I will restate my now disputed beliefs--The alternator's job is to supply power to the battery to keep it charged within the established perameters. All vehicle power comes from the battery, NOT directly from the alternator.

OK, Have at it and prove me wrong. I am man enough to take it with neither problems, nor hurt feelings. Go for it!

 

[QUOTE=jfb;1559898220]There is a lot of bad info in some of these threads. The alternator supplies all of the electricity for the car when the engine is running! It does this by maintaining 14.7 volts across the battery. This QUOTE]

Alternator output should be 13.8 to 14.2 volts. Output of 14.7 volts is out of spec and may damage the battery. Output of 14.8 volts or higher will 'cook-off' and destroy a battery. Go to the SAE or NIASE websites and if you are good in doing on line searches you will find verification of what I am saying, otherwise I would like a way to confirm your information. No offense meant, but I would like to keep current if things have changed since I last certified as an NIASE Master Automotive Technician. Quite a few years ago I was also an executive member of The Maintenance Council of the American Trucking Association and helped wirte or review many of the Recommended Maintenance Procedures used nationwide. Do NOT take this as bragging, I am really just trying to qualify my knowledge, or expertise as some like to say. Thanks!

 

Then what does charge your battery?

Long extension cord??

 

Quote from aminnich:
The alternator's job is to supply power to the battery to keep it charged within the established perameters. All vehicle power comes from the battery, NOT directly from the alternator.
This is wrong! The alternator charges the battery by applying a voltage to the battery that is higher than the battery which forces current into the battery. Any load connected to the battery (engine running), the alternator will be supplying the current. The battery cannot when current is being forced into the battery! You can prove this to yourself by solving for the battery current in the simple circuit with a battery, load resistance, and a voltage source higher than 13.2 volts all in parallel. Use the equivalent circuit of a car battery of a low (.05 ohms) resistance in series with a 13.2 v voltage source. The battery current will only be the charging current.
My GM service manuals for my 87 vette state that the alternator output must be between 13 and 16 volts, outside that, replace the alternator. My GM service manual, page 6D1-1 states, "The battery has three main functions in the electrical system. First, it is a source of energy for cranking the engine. Second, it acts as a voltage stabilizer for the electrical system. And third, it can, for a limited time, provide energy when the load used exceeds the output of the generator".
I would add a fourth function, it provides energy for a limited time in case the alternator fails so the car can be driven to a safe location.

C4's I have measured have alternator outputs of 14.7 volts cold and this drops to 13.7 volts hot. 14.7 volts will not, "cook off" modern car batteries. Modern car batteries are maintenance free (don't need water added periodically) and if you keep the charging current below 10 amps, the calcium added to the lead plates converts the hydrogen and oxygen gas produced back to water. Older batteries, not maintenance free, the charging voltage should be kept below 14.2 volts when the battery becomes fully charged to prevent gassing so that you do not lose water from the battery. I recommend you go to www.batteryuniversity.com for safe battery charging information.

 

This is from your source:
Car batteries and valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36V/cell. At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases. The exceptions are small sealed lead acid batteries (SLA), which can be charged to 2.50V/cell without adverse side effect.

Then we are in agreement that a sustained charge (I should have been clearer before as I did not say 'sustained') of 14.7V would 'cook off' and destroy a battery. Actually, I think we are both in agreement, but not speaking the same language. I, for one, will try to be more precise on issues such as this in the future. We could go on forever as there are factors we have not even touched on, but enough said.

 

I knew you meant sustained charging. The site I referenced doesn't say anything about maintenance free battery charging, their info is for lead acid batteries that have removeable caps and require the owner to periodically check the electrolyte level and keep the plates covered by adding distilled water. Non maintenance free lead acid batteries will, "outgas", and lose electrolyte when the battery becomes fully charged AND the charging voltage is kept above 14.2 volts. Maintenance free batteries are not perfect, but they lose electrolyte so much slower that they do not have removeable caps and no means to add distilled water and if charged properly will not need water added during their service life. I don't know of any car battery manufacturers that make car batteries that are not maintenance free!

Just found a site that talks about charging voltage and maintenance free batteries: http://www.landiss.com/battery.htm

 

I absolutely agree with you.


They put out 12.6v fully charged.

 

Whenever you have a low voltage system,poor connections,or an over taxed motor ,will cause a high amperage draw.Which is what the OP was asking.Check motor and switch leads for any corrosion,or poor connections.Also check ground connections.Verify alternator output with a meter.And yes,the alternator runs the system.Simply put the battery is like a sponge.It stores energy to fill the need for any high resistive or inductive loads,like starting for one.It's not a regulator.The alternator itself is technically a DC generator.Although it initially produces 3 phase AC ,the rectifier bridge coverts it to DC, which can be stored,unlike AC .Bottem line is 20 something car,20 something low voltage electrical system,be vigilant!

 

From www.catas1.org/eng/elect/edu/pt16.pdf

"The open circuit voltage (no current flowing) of a fully charged cell depends on its type, but will be 2.1 to 2.3 volts (12.6 to 13.8v for a 12v battery)".

 

Believe whatever you want to read. Put a meter on it, take a picture with the car OFF to show me a lead/acid battery putting out 13.8v. For that matter show me a lead/acid battery putting out 13.2.


DEPENDING ON TYPE AND YOU SAID LEAD/ACID.

 

Lead acid batteries come in many types. Pure lead plates, plates alloyed with antimony, plates alloyed with calcium (calcium is a metal), gel electrolyte, glass mat between the plates, vented cells.
I can show you a picture of a new Fluke 175 voltmeter measuring a good motorcycle battery that I have at work with 13.2 volts open circuit (I have already measured this before this thread ever existed). What do I get for this effort? I'm not jumping through hoops for free! I am an EE and I have seen 13.2 volts many times on new fully charged flooded cell (liquid electrolyte) lead-acid batteries.

 

Congrats, I've never seen that voltage on my batteries. I buy just the regular garbage batteries that 99.99999% of the other humans on this planet buy. Post the pic and I'll give you a thattaboy.

You're an EE? Lots of guys in OT would like to see your picture.

 

Partial Quote from hippy.
You're an EE? Lots of guys in OT would like to see your picture.
________________________________________ _________________

Really? You are the first that asked. I know a lot of EE's and I don't see anything in their looks that indicate they are an EE. Besides, I'm not anxious to see my photo photochopped for others amusement.

This past winter, my dad's Saturn wouldn't crank and had a Walfart battery in it that was only 2 month old. I charged it for a couple hours and it still wouldn't crank. I took it to Walmart and they tested it and told me that it measured 13.2 volts and it passed the full load test. Frozen snow and ice in the starter solenoid prevented it from moving, the battery nor the starter motor wasn't the problem. Also, I too was surprised to hear that it measured 13.2 volts!