Accuracy of Voltmeter in Multi-function Display

Electrical and electronic topics for small boats
jimh
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Accuracy of Voltmeter in Multi-function Display

Postby jimh » Fri May 12, 2017 2:56 pm

I compared the voltmeter in my Lowrance HDS-8 when reading its SUPPLY VOLTAGE to a FLUKE Model 73 Series II DVM. The Fluke is rated for 0.3-percent basic accuracy on measurement of DC voltage. The results:

--FLUKE Model 73 read 12.63-Volts
--LOWRANCE HDS read 12.6-Volts, with occasional flicker to 12.7-Volts

The LOWRANCE was connected to a power distribution panel with the OEM power cord, about 2-feet long. The FLUKE was also connected to the same power distribution panel with its test leads. With the FLUKE rated for 0.3-percent accuracy, a reading of 12.63 would have a range of

12.63 × 0.003 = 0.0378-Volt

Thus the Fluke reads the voltage as somewhere between 12.6678-Volts and 12.5922-Volts. The Lowrance HDS reads 12.6-Volts. I grabbed this image to preserve this measurement data:

voltMeterComparison813x526.jpg
voltMeterComparison813x526.jpg (24.76 KiB) Viewed 3168 times


I am impressed with the accuracy of the voltmeter in the Lowrance HDS. When it reads the supply voltage as 12.6-Volts, I am quite confident that is very close to the actual supply voltage.

OTHER VOLTAGE READINGS ON MULTI-FUNCTION DISPLAYS

In most multi-function displays the BATTERY VOLTAGE can be shown. In this case the device measuring the battery voltage is not the display device, but rather some other device that is on a NMEA-2000 network and is sending the battery voltage data to the display. The typical source for the instrument doing the measurement is an electronic controller in an outboard engine. An outboard engine with NMEA-2000 will usually send the BATTERY VOLTAGE associated with the battery connected to that engine, or the outboard will send the ALTERNATOR VOLTAGE of its own alternator. Again, these voltages are not being measured by the display device; they get measured in the outboard engine.

If you put a voltmeter across the terminals of the battery connected to the engine, you might get a different voltage. The engine is reading the voltage at some point in the circuit farther away from the battery terminals, so there could be voltage drop in the circuit. Or, the engine could be reading the battery voltage right at the point where the alternator current is being applied, which could be higher than the terminal post voltage at the battery, as the current flow is toward the battery.