Shunt wiring

[transmitterdan]

Originally Posted by fritzdfk

I have a little bit unrelated question. Is a shunt generic other than amp rating? Can a various amp meters be connected to a shunt? Only one connected at a time of course.

The resistance of the shunt has to match what the meter expects. Some meters can be programmed for different resistance shunts and some are fixed programming.

It is not a problem to connect multiple meters from a single shunt all at the same time. The meters must be compatible with the shunt in terms of resistance and where the shunt is relative to the battery (connected to minus or plus). The shunt is unaffected by how many meters are connected.

[Frankly]

A meter connected to a shunt is normally a voltmeter not an ammeter. You are reading the voltage drop across a small and precise resistance and the relationship to current is based on Ohms Law I=V/R. The meter is calibrated to make this computation automatic and yes you can connect several high impedance voltmeters to read a single shunt with little reduction in accuracy.

[Paul L]

Originally Posted by transmitterdan

This will not work. There are 4 connections to a shunt. 2 are high current connections. 2 are just for measuring the voltage which is related to current. The big connections can never give the right voltage and the current measured will always be wrong and and it will vary too much with temperature.

Either fix the small broken connections or get a new shunt.

Why would the voltage drop between the two large connectors be any different than between the two small ones? They are only a few cm further apart in the solid shunt, if that.

[Paul L]

Originally Posted by fritzdfk

I have a little bit unrelated question. Is a shunt generic other than amp rating? Can a various amp meters be connected to a shunt? Only one connected at a time of course.

I have Link10 and BlueSky meters connected to a single shunt. Works fine.

[Zil]

You can connect the meter wires to the battery wires at the shunt. The polarity must be correct. My Victron has a special connecting device for various functions, but it will read amps or volts connected to the cables.

[Wotname]

Originally Posted by fritzdfk

I have a little bit unrelated question. Is a shunt generic other than amp rating? Can a various amp meters be connected to a shunt? Only one connected at a time of course.

No, there is no generic shunt. There are some industry standards but no requirements to be a set standard.

Some common numbers are 50, 75 and 100 mV.

Thus for a certain amp rating, say 100 amps, the shunt might be designed to drop (develop) 75 mV when 100 amps is flowing though the shunt. Another 100 Amp shunt might drop 100 mV and so on. You can do the maths to find the resistance of the shunt (by using ohms law).

The "amp meter" is really a voltmeter designed to full scale deflection (FSD) at 50 or 75 or 100 mV (as appropriate) and then is scaled in Amps.

An example - 50 Amp / 75 mV shunt will drop 75 mV across it when 50 Amps are flowing so the actual meter will read 75 mV (FSD) and the scale will show 50 Amps. If there is only 25 Amps flowing, then the voltage across the shunt will be 37.5 mV and the scale will show 25 Amps.

Further, the shunt has to be designed have it's designed resistance very stable across a wide temperature range.

[senormechanico]

As previously posted, the small twisted pair goes to a VOLTmeter, not an ammeter.
Further, if the small connectors on the shunt are damaged, you can (HORRORS !) simply solder the twisted pair in place.

[transmitterdan]

Originally Posted by Paul L

Why would the voltage drop between the two large connectors be any different than between the two small ones? They are only a few cm further apart in the solid shunt, if that.

Shunts measure their resistance in milliohms. The big bolts will be tens of milliohms and thus make the reading inaccurate. Errors of 100% are likely. So if you are drawing 25A your meter might read 50A. And tomorrow it might read 40 or 60A.

[Paul L]

Originally Posted by transmitterdan

Shunts measure their resistance in milliohms. The big bolts will be tens of milliohms and thus make the reading inaccurate. Errors of 100% are likely. So if you are drawing 25A your meter might read 50A. And tomorrow it might read 40 or 60A.

I'll take your word for it, but it still surprises me on a shunt like this that it would matter.

[transmitterdan]

It will matter a lot. Shunts are built the way they are for a good reason. If your shunt looks like this you should be able to repair it.

[Paul L]

Originally Posted by transmitterdan

It will matter a lot. Shunts are built the way they are for a good reason. If your shunt looks like this you should be able to repair it.

It's not mine. The OP has the issue.

[daletournier]

It's working fine attached to the ends. No different than before. I turned everything off last night, nothing in nothing out and it was out by -. 4, I'm happy with that.

I was approx 50ah down this morning which is consistent with previous readings.

[Wotname]

Originally Posted by Paul L

I'll take your word for it, but it still surprises me on a shunt like this that it would matter.

Paul L, I am sure you are correct and I'm sure Txdan is wrong (in this instance).

The issue isn't the resistance of the various connections, rather it is the voltage drop across the resistance which is dependent on the current flow in the various legs of the circuits. The current flow in the voltmeter circuit is very small, microamps.

Consider a 100 amp / 100 millivolt shunt; the shunt resistance is 1 milliohm.
The voltmeter current draw will in the order of microamps (say 200 microamps max). Should the resistance of the connections to the voltmeter be say 20 milliohms, then the voltage drop across the connections will be 4 millivolts (200 microamps times 20 milliohms). So at 100 amps through the shunt, there will 96 millivolts to the voltmeter and 4 millivolts across the connections. Using these numbers, this shows a 4% error - i.e. nothing really. If the current flow to the voltmeter is only say 20 microamps, then the error will be 0.4%.

[alctel]

I actually have a spare 50mv shunt if you want

[daletournier]

Thanks mate but I'm ordering a new battery monitor which enables me to program my regs, it comes with a shunt.