Shunt Question

[Humblebumble7]

I watched Will’s videos showing shunt connections and that seems straight forward. When shopping for one it cautions to have amperage capacity greater than the battery. Is this correct? If so, I need to get a 300 amp shunt from the start so I can grow my battery over time. Thanks!

 

[RCinFLA]

Shunts are specified as max amps/mV, like 500A/50mV. This means the shunt produces 50 mV voltage drop at 500 amps through shunt.

That means the shunt resistance is 50 mV/500A = 0.1 milliohms.

Shunts can get pretty hot when there is sustained high current. Power dissipated as heat in shunt is current squared x resistance of shunt. For example, 250 amps through the 500A/50mV is 250A^2 x 0.0001 ohms = 6.25 watts of heating of shunt. That will get fairly hot so should allow some air circulation in any shunt enclosure.

Chinese shunts often are 75mV shunts. You have to be sure your battery monitor has ability to work with the associated shunt resistance. 75 mV shunt will have 50% greater heating than 50 mV shunts.

Lower current shunts, like 100A, are typically 100 mV, or 1 milliohm shunt resistance.

A quality shunt is made with an alloyed metal to provide close to a zero-temperature coefficient of the shunt resistance.

In addition to the actual shunt resistance, but not part of actual voltage drop across shunt resistance, is the terminal connections to the shunt. A good rule of thumb is about 0.05 milliohms for every compression terminal connection. In addition, there is a bit of series resistance in the actual terminal lug itself. Bottomline is the total heat generated is usually two or three times the heating of the actual shunt resistance. Beware of using vinyl insulated cable wires that melts at relatively low temperature.

Excelene welding cable is the best. Cost a bit more but you are paying for quality. Cheap vinyl insulated cables not only has the insulation melting issue but the percentage purity of copper is questionable, possibly having greater cable resistance.

 

[Rednecktek]

More importantly (and I think his primary point) is not to use a 100a rated shunt and then run 250a of load through it all the time. Magic smoke/fire/angry wife tends to happen.

 

[JoeHam]

Just get a 500 amp from the start.

Check around the forum for a ton of people happy with this one :

Amazon.com

Available from many different vendors.

 

[wopachop]

Shunts are specified as max amps/mV, like 500A/50mV.

Super informative. Do you by chance know what the 600a or 400a WonVon shunt is? Trying to decide between the Victron or WonVon for an RV and small 220ah 12v setup.

Would go with Victron, but reading you have to be close to the shunt for bluetooth to work. Would like to be able to check battery capacity at night. Without having to go outside and stand next to the batteries.

Amazon.com

 

[42OhmsPA]

Super informative. Do you by chance know what the 600a or 400a WonVon shunt is? Trying to decide between the Victron or WonVon for an RV and small 220ah 12v setup.

Would go with Victron, but reading you have to be close to the shunt for bluetooth to work. Would like to be able to check battery capacity at night. Without having to go outside and stand next to the batteries.

Amazon.com

I'm been running the 600A WonVon for months, it works great and seems to be very accurate. I use a cheap tablet for remote monitoring over Bluetooth.

 

[timselectric]

I use a 1000a Victron and Solar Assistant.
I can monitor my batteries from anywhere in the world.

 

[RCinFLA]

The higher the amperage shunt, the lower the shunt resistance. The lower the shunt resistance the less sensed output voltage. The less output voltage the tougher it is to get accuracy at low currents.

500A/50mV is 0.1 milliohm resistance. Detecting 1 amp is detecting 100 microvolt across shunt output. Battery monitors have a 1 to 2 Hz low past filter on their input from shunt sense output. It has two purposes. It reduces the noise bandwidth to get better sensitivity, and to average inverter DC current to remove 2x AC line frequency ripple current on sinewave inverter battery lines.

Because you have to detect positive and negative current flow, the DC offset on the monitor amplifier is also critical. Just a little DC offset will really screw up Columb count for battery capacity tracking. Quality monitors use a special op amp that has a continuously chopping background DC offset calibration, transparent to operation. Regular op amps have DC offset drift over temp.