Why do I get different volt readings on my monitors?

[DangerousPirate]

I had this on my other boat before, and even though it doesn't really mean anything, I wonder why that is. The built in, came with the boat, volt meter right now says 13.8 volts, the first PWM says 12.9, the second pwm (the renogy one) says 13.1, and the power converter says 12.6 but 20 minutes ago it was 12.9 (and before that higher).

It is night, so no solar power, all lights are off besides the power converter and the shore power is unplugged since at least 30 minutes by now. Why do my monitors not all show the same voltage altho they are all connected to the same batteries? Which one is accurate?

The batteries are lead acid batteries, dunno if that makes a difference

 

[kwb78]

Different calibrations and different circuit resistances are probably the reason. If you have a decent multimeter (ie one that is likely to be close to calibrated) you can probably get an idea. First measure the battery voltage directly, which should be the real value. If you then measure the voltage at the input terminals of each meter, you can compare their readings with the voltage after the cabling. If the multimeter and the other device agree, you know the difference is due to the cabling. If they disagree, then it’s probably the device calibration.

 

[William_H]

Op might be interested to buy a voltage calibration board or chip. 1X TL431 voltage reference board TL431 voltage source for calibrating multimeter | eBay is one example. Feed in a voltage of about 15 to 18v I use 2x 9v batteries. Gives a very stable quite accurate 10volts. (mine seems to actually 9.96 volts) This can used to check calibration of multimeter and possibly other volt meters. Usually multimeters are quite accurate but can have slight or large error. Gross over reading means meter battery is near flat.
Once you have an accurate multimeter check the voltages at the actual input to each device. As said it could be voltage drop in wiring. (not so easy for solar controller)

 

[Refueler]

To measure voltage requires resistance shunts in the units ... unlike amps that are read direct.

A small change in actual shunts resistance can easily alter the units accuracy ... the resistance difference can be factory based production.

 

[VicS]

To measure voltage requires resistance shunts in the units ... unlike amps that are read direct.

A small change in actual shunts resistance can easily alter the units accuracy ... the resistance difference can be factory based production.

Other way round surely.

You measure voltage directly but you can measure amps directly with an ammeter or by measuring the voltage across a shunt through which the current is flowing.
.

 

[Refueler]

Other way round surely.

You measure voltage directly but you can measure amps directly with an ammeter or by measuring the voltage across a shunt through which the current is flowing.
.

Been a long day ......

Was posting quick inbetween haggles on another boat ... setting ferry tickets ... organising guys to crew .... 4 ships in China ..... wife moaning she needs new shoes ..... tree cutters lefty a bloody mess at front of house ....

I need a f*******g holiday !!

 

[[194224]]

You measure voltage directly but you can measure amps directly with an ammeter or by measuring the voltage across a shunt through which the current is flowing.
.

Under most circumstances they are one and the same. Most digital multimeters for instance read current indirectly by measuring voltage across a shunt. A Hall effect sensor can measure current without a shunt but again it is an indirect reading.