When ever I have tested an alternator the output has always read about 14-15v and I remember my dads Lada always used to read 16v (the bulbs were always blowing on it) now either my meter must be well out or cars must run on more than 12v. Why is this? I realise the battery is a 12v DC battery so all the electrics are rated at 12v, so howcome the altnerators always tend to read more? I thought the output was supposed to be heavily regulated unlike the old dynamos.
I am right in thinking things like the ECU have further regulation to ensure it has exactly the correct voltage? I know on a PC if the voltage drops or increases only slighlty it can cause all sorts of problems yet on cars it seems to vary widly.
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a "12v" battery is six "2v" cells in series, giving 12V. However a fully charged cell is about 2.3V hence your battery may read 13.8V when fully charged. It will drop down a bit when loaded. You need up to 14V in order to charge the battery fully, ie. up to 13.8V.
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Well its mostly because 12v cars are not 12v cars. They are 13.8 volt cars.
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Just to be pedantic, a fully charged 12v lead acid battery will read 12.72 Volts, each cell being 6X1.2 Volts. It will read higher if you measured it after you had run the engine i.e. charged the battery. This is because an alternator runs typically at 14.2 Volts, and a certain amount of "surface" charge will still remain as the battery settles back down. The alternator needs to deliver over 14 Volts in order to overcome the battery voltage and "push" the current through the battery to charge it.
And just to be pedantic again a car could be said to be a 14.2 Volt vehicle since with the engine running the alternator essentially powers all the electrical equipment, the battery is simply a reservoir of stored energy whose most important job is starting the car, if starting was not required by the battery then it could be shrunk to a fraction of its usual size.
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A standard lead acid battery when in good condition and on charge will read 14.25 volts or so. A charged battery that has rested for 8/12 hours reads about 12.6 volts or so. Older worn out battery display a larger internal reistance and then on check the voltage can rise to 14.75 to 15.25 volts. Regards Peter
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It's not quite that simple - at 0C a battery will have a lower nominal voltage than at 20C.
www.xantrex.com/web/id/262/docserve.aspx
Alternators actually vary the charge voltage depending upon temperature - and in big industrial installations will have a thermometer on the side of the battery.
This is why it is better to measure a battery using a hydrometer. If my memory serves me right, a fully charged battery is about 25% concentration of sulphuric acid, a fully discharged battery closer to zero concentration - so discharged batteries can freeze.
Edited by Marlot on 16/02/2009 at 21:25
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The question is about the expected voltage not the state of charge of the battery. Regards Peter
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12v is just a nominal voltage. On most car's It varies between 10 and nearly 15 with no problems. If the voltage had to be absolutely correct all of the time, nothing would work !
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Some components are deliberately rated for a stabilised, lower voltage, supplied by a voltage regulator. Things like fuel and temperature gauges give erratic readings if fed directly on a voltage that varies from 11 to 14.
Some coils are likewise normally run on a lower voltage, but are supplied with the full battery voltage during starting and to counteract the effect of the heavy starting current.
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Alternators actually vary the charge voltage depending upon temperature
So how does the alternator running at 70 degC on my engine know that the battery in the boot is at 5 degC?
And no, there is no 'battery temperature sensor' :-(
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Here's all you ever needed to know about car batteries. tinyurl.com/c3k3w
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Here you go Rattle, download the application sheet for the L9407F
www.st.com/stonline/products/literature/ds/9235/l9...m
can you use your electronics knowledge let us know the solution to the conundrum of the cold battery in Oldsock's car boot?
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