Why a Shunt ?

45North · Apr 3, 2021

Al Slitter said:
I hope that I can get clarification on why I need a shunt on my system?
I understand the basic functions of a shunt and in my mind that is to work with a Amp meter to determine the amperage/current being used.
My system has 4-200Ah cells placed together to form a 12Volt battery.
On the positive side I have a 150Amp flat fuse on the end of a 35mm welding cable which is connected to the battery.
The positive battery cable then runs to a DC on off switch and to a 150Amp Amp/Volt meter then to a Positive Bus Bar.

On the negative side the battery is connected to a 100Amp BMS then runs to a negative Bus Bar.

I hope I have this right my mind does faid out from time to time.

Thanks in advance.

At risk of stating the obvious, this is the way I think about shunts and why the method used for measuring voltage is different from measuring amps:

Voltage is measured Across the circuit, so you just put your meter's probes directly on + and - to measure, whereas
Amps are measured flowing Through the circuit ie. in-line with the circuit. This means that you need to either
- break the circuit and put a shunt in-line with it which you then measure the current flow through, or
- put a ring around one of the wires (such as by a clamp meter) where the current if flowing and measure it indirectly by inference

Hope this helps with the general concept. The rest is details!

Al Slitter · Apr 4, 2021

Thanks everyone for your input.
This has turned out to be a very active discussion.

GSXR1000 · May 13, 2021

my bms gives me great information and battery level is very accurate.. no history like my smart shunt...
my bms is Bluetooth

Gadgetnerd · May 13, 2021

JeepDaddy said:
You Fluke might measure up to 600V at 0.1V resolution also. Imagine a shunt/meter that reads from 0 to 50mV with the same resolution.

At lower volt ranges a DMM will read much higher resolution than that, even a cheap BMS will read millivolts and balance cells to a threshold of 0-2mv. My point being to the OP is if you already have a bms why bother with a shunt.

Horsefly · May 13, 2021

What a BMS can provide is a less accurate approximation of the state of charge of a battery pack. The shunt does actual Ah / Coulomb counting, going in and out, and so should always be able to give a better indication of the status of the battery. A BMS may be good enough for some folks in some situations.

Gadgetnerd · May 13, 2021

Horsefly said:
What a BMS can provide is a less accurate approximation of the state of charge of a battery pack. The shunt does actual Ah / Coulomb counting, going in and out, and so should always be able to give a better indication of the status of the battery. A BMS may be good enough for some folks in some situations.

My BMS has coloumb counter, but that's over the top in almost all situations IMO.

GSXR1000 · May 13, 2021

the nerd side agrees.

but in reality it what situation would you need more then 5% accuracy

Gadgetnerd · May 13, 2021

GSXR1000 said:
the nerd side agrees.

but in reality it what situation would you need more then 5% accuracy

For a battery meter? Sure, but a BMS generally tracks each cell in a pack to a millivolt for balancing. If one cell gets out of balance it affects the overall pack voltage the charging voltage sensor sees and reduces pack capacity at both end of the SOC spectrum.

GSXR1000 · May 13, 2021

Gadgetnerd said:
For a battery meter? Sure, but a BMS generally tracks each cell in a pack to a millivolt for balancing. If one cell gets out of balance it affects the overall pack voltage the charging voltage sensor sees and reduces pack capacity at both end of the SOC spectrum.

the post is about do your need both.... yes a bms will be required

Gadgetnerd · May 13, 2021

GSXR1000 said:
the post is about do your need both.... yes a bms will be required.

Can't see the point of having both. They do the same thing.

JeepDaddy · May 13, 2021

Gadgetnerd said:
At lower volt ranges a DMM will read much higher resolution than that, even a cheap BMS will read millivolts and balance cells to a threshold of 0-2mv. My point being to the OP is if you already have a bms why bother with a shunt.

It's the same resolution, just a different scale.

You don't need to balance to anything less than 10mV, IMHO. You should only balance cells that are above 3.50 volts.

Bob B · May 13, 2021

Gadgetnerd said:
Can't see the point of having both. They do the same thing.

They do the same thing .... but not necessarily equally well.

Many of the BMS that have SOC tracking don't measure low current draw .... so, in a situation where you have a small draw over a long period of time .... the SOC readings can be off by a large margin.

Gadgetnerd · May 13, 2021

Bob B said:
They do the same thing .... but not necessarily equally well.

Many of the BMS that have SOC tracking don't measure low current draw .... so, in a situation where you have a small draw over a long period of time .... the SOC readings can be off by a large margin.

Gadgetnerd · May 13, 2021

JeepDaddy said:
It's the same resolution, just a different scale.

You don't need to balance to anything less than 10mV, IMHO. You should only balance cells that are above 3.50 volts.

The factory setting for my BMS was .02 volt But yeah people tend to get crazy obsessed.

Gadgetnerd · May 13, 2021

Bob B said:
They do the same thing .... but not necessarily equally well.

Many of the BMS that have SOC tracking don't measure low current draw .... so, in a situation where you have a small draw over a long period of time .... the SOC readings can be off by a large margin.

Practically every BMS will detect to the watt so at 48V that's .02 amps.

Bob B · May 13, 2021

Gadgetnerd said:
Practically every BMS will detect to the watt so at 48V that's .02 amps.

Sorry .... but that's just not true. Some don't detect less than an amp and some don't detect less than .5 A.

They aren't measuring watts, they are measuring amps .... in some cases they also calculate the watts or WH

JeepDaddy · May 13, 2021

Gadgetnerd said:
Practically every BMS will detect to the watt so at 48V that's .02 amps.

It may show you that reading, but it will not be accurate. Thus the quality shunt.

Gadgetnerd · May 13, 2021

Bob B said:
Sorry .... but that's just not true. Some don't detect less than an amp and some don't detect less than .5 A.

They aren't measuring watts, they are measuring amps .... in some cases they also calculate the watts or WH

Gadgetnerd · May 13, 2021

Bob B said:
Sorry .... but that's just not true. Some don't detect less than an amp and some don't detect less than .5 A.

They aren't measuring watts, they are measuring amps .... in some cases they also calculate the watts or WH

Agree to disagree, my charger/inverter also detects in watts. You need volts and current to make that calc so how does it show these increments?

Bob B · May 13, 2021

Gadgetnerd said:
Agree to disagree, my charger/inverter also detects in watts. You need volts and current to make that calc so how does it show these increments?

You just made my point for me .... a shunt or hall effect sensor measures current. It you take that current reading and multiply it times a voltage reading you will have a calculated watts .....

Watts aren't MEASURED .... they are calculated by using the measured amps and volts.

Is there a specific BMS your are referencing?

Why a Shunt ?

Let it shine!

Trying to learn something new every day!

Solar Enthusiast

New Member

Solar Wizard

New Member

Solar Enthusiast

New Member

Solar Enthusiast

New Member

Solar Enthusiast

Emperor Of Solar

New Member

New Member

New Member

Emperor Of Solar

Solar Enthusiast

New Member

New Member

Emperor Of Solar

Similar threads