What's a, "Shunt"?

[Marty1000]

What is a shunt, how is it wired in and why do I need one?
Thanks.

 

[Lt.Dan]

 

[Horsefly]

What is a shunt, how is it wired in and why do I need one?
Thanks.

Generically, a shunt is a hunk of metal that has a very precise/specific resistance and very small resistance, usually 50 milliohm or 75 milliohm. When current goes through the shunt the current creates a voltage drop across the resistance. So if 100A goes through a 50 milliohm shunt, the voltage drop across the shunt is (Ohm's law) 100 x 0.05 = 1V. By measuring the voltage drop across a shunt, you know what amount of current is going through the shunt.

Although that is the actual definition of a shunt, most people on this board use one of many "smart shunt" products that include a circuit that measures and reports the current, doing the conversion for you.

The main purpose of shunts is to provide arguably the best measure of the state-of-charge of a battery. By measuring the current going into and out of the battery over time you can tell how many amp-hours have been taken from the battery.

Almost all "battery monitors" that people use in solar have a shunt at the heart of it. Most go between the negative terminal of the battery and the rest of the system (although there are some that operate on the positive side).

 

[JoeHam]

The main purpose of shunts is to provide arguably the best measure of the state-of-charge of a battery. By measuring the current going into and out of the battery over time you can tell how many amp-hours have been taken from the battery.

Also known as Coulomb Counting if you see people using that term.

We use this with the LiFePO4 chemistry since that chemistry has a flat discharge curve which means a simple Voltage reading is a terrible estimate of state of charge.

More information here :

SoC Estimation by Coulomb Counting - Battery Design

SoC Estimation by Coulomb Counting is based on the measurement of the current and integration of that current over time.

www.batterydesign.net

 

[42OhmsPA]

Now that the experts have explained it better than I could, it's one of those things that you should have in your system... If you start without one and add one later you'll say to yourself, shint I should have bought one sooner.

The favorites here seem to be victron and wonvon/junctek. I'm very happy with my wonvon.

 

[Mattb4]

While most use a shunt to monitor their batteries, shunts can also be used to monitor volts and amps on your incoming PV or any DC circuit. I have taken to using a shunt and meter on my PV negative wire because my AIO's are not very accurate in measuring PV wattage. Some meters for the shunts can also maintain watt-hours if the AIO does not which can be handy for figuring how the system is performing.

 

[Madcodger]

I have an all-Victron equipment + LiFePO4 Amperetime batteries system that has been operating since last fall ('22). I did not have a shunt until about a week ago, using the inverter's estimate to gauge battery depth of discharge. It was way off. I actually bought an additional (third) $700 battery because I thought I needed it to power my AC. After installing a Victron Smart Shunt at the same time I installed the new battery, I now see that the additional battery was unnecessary. I wish I had installed that shunt on day one, even though I like having the additional spare capacity. Winter's coming and all that.

 

[pollenface]

You don't specifically NEED one, it's personal preference.

 

[Bossrox]

I don't see how you could go without 1 unless your gear has it already in it. A most valuable tool!

 

[pollenface]

I don't see how you could go without 1 unless your gear has it already in it. A most valuable tool!

Each to their own