Join Date: Jun 2009
Location: Kingston, PA, USA
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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