Volt meter settings

[Ripruss1]

Just purchased new digital oil pressure/temperature and volt meter gauge. I need to put in voltage parameters.  This is for 1976 m20f with 200 hp lycoming engine. I believe 14 volts would be mid normal. Anyone know the upper and lower limits. Any help appreciated 

[A64Pilot]

I would go plus or minus 1V, I don’t get that number from anywhere really but normal voltage for older automotive 12V systems is as you say 14V plus or minus .2V.

We essentially have an older automotive charging system.

A fully charged 12V battery is 13 ish V so if you set the lower limit at 13V it’s likely the low volt alarm will be on until the alternator comes online but isn’t that what you want?

Additionally at least a Concorde battery will accept 15V for a long time without damage and I believe that’s within Avionics limits as well.

Admittedly those limits are conservative but I like limits that alert me to an upcoming issue well before it becomes critical, but one that’s not so sensitive to give nuisance alerts, like alert every time the landing light or pitot heat is activated.

Of course if this doesn’t work you can adjust them 

[Vance Harral]

The service manual for the 1976 M20F says battery voltage in a normally operating system at 80F should be 13.8 to 14.8, see below.  This assumes you have the original alternator and voltage regulator, which you may not.  Another catch is that you're probably not going to wire your voltmeter directly to the battery or alternator output terminals, but somewhere else in the electrical system.  Depending on where and how it's tapped, there may be losses in the system that make the "normal" voltage seen on the meter slightly different than the spec'd voltage in the service manual.

If you want an in-the-field data point, we've had an AV-17 voice annunciator in our 1976 M20Ffor about a decade.  It has non-adjustable voltage warnings that the manual says trigger at 13.0 and 15.1V for low/high points, which is exactly what @A64Pilot suggests above.  We've never had an overvoltage warning, even during an ill-advised operation once when the battery was nearly dead and we accepted the risk of a very high charge rate once we got the engine started.  Low voltage warnings trigger as expected, basically any time we turn on the voice annunciator before engine start.

I don't think there is any perfect answer to your question, again because behavior varies depending on where you mount the meter.  But 13/15 for low/high seems very reasonable to me.

[EricJ]

Doube check that your POH doesn't have limits for the indicator markings in the Limitations section of the POH (it probably doesn't but I don't have an F model POH to confirm).

Other than that for "low voltage" it just needs to reliably know the difference between the alternator operating or not.    As mentioned, 13.0-13.5V is a pretty common threshold area for that.    A good new 12V battery might be 12.9V fully charged, the regulator should put out 13.8-14.2V or so when running.

 

[PT20J]

For reference, the low voltage annunciator on a M20J is set to flash at 26.5 volts for a 28v system, so 13.25 for a 14v. The overvoltage relay is set to trip at 16v.

 

[Ripruss1]

Thank you all for your responses. Much appreciated!