Voltmeter indicating 15 Volts

[Marco Polo]

Just noticed that the voltmeter on the dash is indicating 15 v with the engine running, are any of you electrical gurus able to point me in the right direction to fix this please?
regards,
Raoul

[trunt]

First step would be to get a proper multimeter on the battery to see what the voltage really is. Those meters are notoriously inaccurate.

terry

[Phil S]

a fully charged healthy battery will read 14.7v

[richardthestag]

a fully charged healthy battery will read 14.7v

you mean 12.7v

the purpose of any of the electric gauges on the dash is simply to inform you that the gauge is working A vomiter across the battery terminals with the engine running should show 13.7 or thereabouts. If it is higher get the battery condition checked out before rushing to buy a new alternator.

[marshman]

Just to clarify what Richard has said:

A fully charged battery in good condition will have a terminal voltage of 12.7 V - this is with no load and with the engine off - wait 10 mins after switching off before checking to let the battery "voltage settle" Note this voltage is at 21 deg C. The voltage may vary by a small amount if the battery is colder or hotter than this.

The terminal voltage on a fully charged, or near fully charged battery, should be between 13.7V and 14.3V with the engine running at a fast idle (approx 1500 RPM). No lower than 13.7V as it will never fully charge. Much more than 14.3V and there is a risk of overcharging on a long journey. This assumes you have a standard alternator.


As others have said, first check the voltmeter is reading correctly, is the voltage really 14.7V ? - check with a Digital Voltmeter directly on the battery terminals. If so it is a tad high and risks over charging the battery on a long run. The culprit is normally the regulator or it could be down to poor connections, though this would normally lead to a lower terminal voltage. There is also the outside possibility of a failing battery. (Failing battery often has a higher internal resistance resulting in a higher terminal voltage for the same charging current - the converse of this is that when you "drag current out " to start the car the voltage drops further, giving less current and sluggish starting).

Roger

[022Dave]

you mean 12.7v

the purpose of any of the electric gauges on the dash is simply to inform you that the gauge is working A vomiter across the battery terminals with the engine running should show 13.7 or thereabouts. If it is higher get the battery condition checked out before rushing to buy a new alternator.

Does a vomiter show if the battery is sick??

[tvrgeek]

14.5 to 14.7 is the alternator output if set aggressively. Older Lucas alternators were closer to 13.7. A fully charged battery is 12.6V, not to be confused with the small capacitive charge that may read a little higher until the first small load gets on it. ( one of the reasons I like the Delco CS130 alternator is the 14.7V setting so a stronger ignition. ) Lucas are set lower as the older regulator technology had more ripple and they wanted to be sure it did not exceed 14.7. Newer alternators have tighter regulation so they can push it closer.

A true 15V, if accurate is considered the damage threshold and will possibly overheat the battery, outgas, sulfate, and all kinds of nasty things. If it is in fact true, then the fault is in the alternator regulator.

A dirty connection on the reference line would logically lead to higher than optimum voltage. Not lower.

A voltmeter does not show if the battery is faulty. You need a load tester for that. Best you can do is test If you drop below about 10V on cranking, the battery is bad but the thermal strip Smiths meters are not quick enough to tell you. The auto parts stores will have the load tester.

One can search the WEB and find lots more information. There is nothing unique about a Stag battery and every single other car on the road ever. Same chemistry.