syoo8

Hi everyone,

Why does the voltage on my (86.5) shark read almost 14 volts first thing in the morning, right after I start it up? Then, when I start the A/C or the HVAC system or radio it drops, and when I turn those systems off the voltage never recovers. The car's voltage seems to be higher also in damp weather.

I realize that the voltmeter in the dash is not accurate, but it is accurate relative to itself, yes?

Is this a ground issue? (I've cleaned most of the ground points though...)

danglerb

I've noticed this in most cars, my guess is that right after starting the car the battery needs a bit of recharging and something related to that makes the meter show higher. Dash meter is pretty worthless though, the real test would be to use a DMM at the jump post or the battery.

Landseer

Think of it as 928 morning wood.

AO

nc_growler

Droplink

Pitchin' a tent in the engine bay...

LT Texan

im sure you guys love the visualisations you've inspired.

But I thought it was simply heat making the alternator less efficient. Obviously not woody's.

So my answer is the old girl's in heat.

GlenL

Bad grounds.

The alternator is pumping juice to charge the battery and the electrons are flowing through the car differently than after the battery is re-charged. I've noticed this on several cars. Several old cars. Clean your grounds or ignore it.

The alternator doesn't raise the voltage to match demand. It raises current to meet that demand while keeping the voltage constant....more or less. It ain't perfect. Put a real voltmeter across the charging post and body to see what's really going on. I suspect you'll find ~13.5V.

syoo8

Thank you- Glen- for your response!

Alan

It is pretty much just to do with temperature - the alternator has no idea what it is supplying - so the only thing it knows when the battery needs extra recharging - is that its current draw is higher. This is the same as extra accessories being turned on.

Higher current draws lead to lower output voltage (not higher). There is a load range for which it makes very little difference to voltage at idle and above that load its a pretty direct correlation of reduced voltage. These ranges vary with temperature. Battery recharge current on a healthy battery is initially high but drops quite quickly. At much above idle (say 1.5K - 2K engine RPM) the alternator can regulate pretty well at all spec loads (say to 100A or more) - though still its regulated voltage may go down slightly with temperaure - really we mostly see problematic issues at idle.

When the alternator is cold it is more efficient. Given its location it gets quite a transferred heat load - quite asside from its own self heating - so it warms up quite fast. At idle especially after a high speed run it reheats fast from residual engine bay heat. When running at speed it tends to get cooled OK from airflow... Wet weather provides a lot more cooling for the low mount alternator.

If your voltmeter bounces around a lot you shoudl check with a DMM - could indeed be ground or connection issues elsewhere.

Alan

Relaxed90

It is natural. During start-ups, the battery needs to be charged from sitting. The voltage regulator compensates from low charge - once the battery is charged, it then equalizes. It is also load dependent.

Put a volt meter on it right after start up and once you have driven after a while using the same accessories. There should be a difference.

Alan

Many people think the alternator is smart enough to know when its charging the battery - unfortunately it cannot possibly be that smart without additional connections.

In fact our alternators are really quite dumb - since they have only a simple connection scheme (initiation and power out) - they cannot do remote sensing & can only regulate to their set point voltage as well as possible, they can't possibly have any idea of the battery condition.

The things that affect how well they can do this this are alternator RPM, current draw and temperature - thats pretty much it... they don't know anything else.

Over time they tend to get worse at regulating due to wear-out mechanisms and connection quality issues.

Keeping the shroud & air feeder tube on helps keep the regulator cooler

On initial cold start-up the RPM's will be higher than at hot idle and the alternator will be cold - good for higher voltage, this will hold true (incl for high RPM) until it warms up.

The 928 alternators are very sensitive to RPM around the idle point - the lower idle on later cars creates quite margiinal supply issues at hot idle with a depleted battery and lots of accessories on...

So - what I said before.

Alan

syoo8

Thank you Alan. I appreciate your responses-- a lot!

smiffypr

Lead acid batteries need more voltage to charge when they are cold, so alternators regulate to a higher voltage when cold.
This is great when the battery and alternator are both in the engine compartment, but not when the battery is at the back of the car.
Smiffy

smiffypr

For examples of charging voltage needing to be higher at lower temps, see the tables on this page
http://www.powerstream.com/SLA.htm