Battery Voltages

barra265t · 05-11-2009, 11:14 PM

What should the voltage across the battery terminals read on a brand new battery installed into a car? Is it meant to begin at the battery's current off-the-shelf voltage (12.5) and as it charges, climb up until it matches the car's electrical system (13.8)? These measurements are made when engine is running btw.

I've just installed a new battery, and the voltage at the terminals with engine on, began at 12.5. After a short 5 minute drive, it was climbing towards 12.8. Shouldn't the voltage at terminals be already at around 14v, as soon as a new battery is installed and engine is turned on, or is everything ok? From inside the car (cig lighter socket), there's a healthy 13.8v however, regardless of what the battery terminals are at.

Windsor220 · 05-11-2009, 11:18 PM

Yep. Will be around 12.5 when car is not running. Around 14 when running.

Windsor220 · 05-11-2009, 11:38 PM

It should read 14 almost straight away. Try putting the lead of the multimeter on the positive on the back of the alternator and see whats coming out of there.

barra265t · 06-11-2009, 12:09 AM

Quote:

Originally Posted by Windsor220

It should read 14 almost straight away. Try putting the lead of the multimeter on the positive on the back of the alternator and see whats coming out of there.

Where's the positive terminal on back of alternator? Bit hard to get to the alternator with the turbo/piping in the way

:

Mickxr8 · 06-11-2009, 12:51 AM

gday all . had this problem a few years back. turned out to be corroded connection on the alt. (main power feed to the battery). also the little wire going to the warning light was a bit loose. should read about 12.5 car off, 14.6 car running

Windsor220 · 06-11-2009, 01:09 AM

This pic is not of a Falcon its just an example.

The positive going to the battery is a thickish wire usually held on by a nut as in photo and covered by a rubber boot. Will be on the back of the alternator. If its accessible,(im not sure I cant remember on these engines) you can put the positive side of the multimeter on this to check if the alternator is putting out any voltage.

Warning - if this is attempted, make sure you dont touch any other metal and arc out the alterator. Will blow fuses

AUXRVIII · 06-11-2009, 10:57 PM

Quote:

Originally Posted by barra265t

What should the voltage across the battery terminals read on a brand new battery installed into a car? Is it meant to begin at the battery's current off-the-shelf voltage (12.5) and as it charges, climb up until it matches the car's electrical system (13.8)? These measurements are made when engine is running btw.

I've just installed a new battery, and the voltage at the terminals with engine on, began at 12.5. After a short 5 minute drive, it was climbing towards 12.8. Shouldn't the voltage at terminals be already at around 14v, as soon as a new battery is installed and engine is turned on, or is everything ok? From inside the car (cig lighter socket), there's a healthy 13.8v however, regardless of what the battery terminals are at.

It depends on what model of vehicle you have, some vehicles have a smart charge system and the charge rate will vary. If you have 13.8 at the ciggie socket and 12.8 at the battery, check the connections at the battery and make sure they are clean, also check the earth connection at the body /chassis to engine block. If the alternator cable passes through the fuse box, make sure the connections are tight and clean.