Can I use two shunts back to back?

[User 09]

Could I use two shunts back to back in the same negative line, probably connected with small bus bars so that two separate devices could monitor the battery bank independently?

[PippaB]

A genuine current shunt is just a high-precision resistor, and the measured value is the voltage drop across that resistance. Shunts are commonly designed for a 50mV drop at their rated current, although there are lots of options for shunt design. What that means is that if you have a 100A/50mV shunt you will lose 0.05V from your circuit when the current is 100A, and 0.005V when the current is 10A. You can put two shunts in series (or 5 or 10) but each one will contribute the same voltage drop. In the example above, with two shunts in series you would lose 2x0.05 = 0.1V at 100A.

For most boat installations I suspect this does not matter too much, I'd guess that in most installations the shunts we install run at less than 10 or 20% of their rating most of the time, so the voltage loss due to shunt(s) is in the 0.0xV range. How that applies to your scenario requires more information.

Most shunts can be connected to more than one measuring device without too much degradation, but again depends on details. Or you can install a single shunt with a signal conditioner that can provide outputs to two or more consumers.

[User 09]

Originally Posted by PippaB

A genuine current shunt is just a high-precision resistor, and the measured value is the voltage drop across that resistance. Shunts are commonly designed for a 50mV drop at their rated current, although there are lots of options for shunt design. What that means is that if you have a 100A/50mV shunt you will lose 0.05V from your circuit when the current is 100A, and 0.005V when the current is 10A. You can put two shunts in series (or 5 or 10) but each one will contribute the same voltage drop. In the example above, with two shunts in series you would lose 2x0.05 = 0.1V at 100A.

For most boat installations I suspect this does not matter too much, I'd guess that in most installations the shunts we install run at less than 10 or 20% of their rating most of the time, so the voltage loss due to shunt(s) is in the 0.0xV range. How that applies to your scenario requires more information.

Most shunts can be connected to more than one measuring device without too much degradation, but again depends on details. Or you can install a single shunt with a signal conditioner that can provide outputs to two or more consumers.

Thank you, good to know. This may be a minor issue but the batteries will be regularly charged to full and SOC reset so not a huge deal. Basically I want to use a second shunt just to conform the SOC of a new lithium battery bank used for an electric motor. The first shunt goes to the motor control head but i suspect the SOC drifts and is inaccurate. I want to add a second monitor (BMV-712), which im familiar with to measure the new lithium bank SOC more accurately. We will be upgrading the batteries soon so all of this wiring will be removed and allow me to add the second shunt. I did not install the motor or control head and do not want to tamper with them if I dont have to.