Alternator voltage check - BMW Luxury Touring Community
 
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post #1 of 4 Old Oct 5th, 2011, 3:43 pm Thread Starter
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Alternator voltage check

Hi All,

I have question on the alternator output voltage.

If I measure the voltage on the battery, what voltage should I expect to see at idle?

Is there a range?

Will the voltage increase as RPM's increase?

Thanks,

Erick
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post #2 of 4 Old Oct 5th, 2011, 3:53 pm
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Re: Alternator voltage check

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Originally Posted by ehaughn
Hi All,

I have question on the alternator output voltage.

If I measure the voltage on the battery, what voltage should I expect to see at idle?

Is there a range?

Will the voltage increase as RPM's increase?

Thanks,

Erick
Engine and ignition off, across the battery should read 12.5 volts plus.

Engine running, at idle, across the battery should read 13.5 to 14.5 volts. Revving the motor will show a slight increase but still should not exceed 14.5 volts.
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post #3 of 4 Old Oct 5th, 2011, 4:24 pm
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Re: Alternator voltage check

It will depend on the charge state of your battery. Right after you start the bike the voltage will be higher as the voltage regulator monitors the battery and adjusts accordingly. With the battery fully charged after a good ride you should see about 13.5 give or take. Anything over 14.0 is cause for concern if the battery is fully charged.


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post #4 of 4 Old Oct 5th, 2011, 6:23 pm
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Re: Alternator voltage check

I'll tell you as it was taught to me.

A fully charged 6 cell 12V battery will read 12.72V when fully charged at 12.3 - 12.4 you're only half way there.
It's calculated at .2V voltage drop per cell (internal resistance) X 6 cells is 1.2V
Add this to 12.72 and you get 13.92, this is the voltage you would need to see to fully charge your battery.
A normal charge voltage reading is 13.9 - 14.2, less than 13.9 and it won't be fully charged, more than 14.2 and you will start to cook it.
It doesn't matter how many amps you have if the regulator voltage (and voltage being pressure) is low it will only charge the battery to the regulator voltage -1.2
The alternator test is usually 2k - 4k RPM's with an applied load.

As a side note, your battery can read higher than 12.7 and this is due to a surface charge.

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