What size shunt?

[Blueboard]

I am designing a 12v system using a Renogy 170ah lithium battery. I'm trying to find out what size shunt is required for this small system and if you have any recommendations for a monitor. Thanks!

 

[John Frum]

Depends on the max load you will pull from the battery.
For a lifepo4 battery that should be somewhere in the vicinity of 1c or 170 amps.
You should get a shunt that has some headroom some multiply 170 * 1.33 = 226.1 amps
I suggest this one https://www.amazon.ca/AiLi-Battery-Monitor-Voltmeter-Motorhome/dp/B07FGFFHC6

 

[Blueboard]

Thanks, Joey.

1. So a larger amp shunt will require larger gauge wire, correct? I was planning on using 2/0 to handle the 175a battery (which I never plan on getting close to using that amount of draw, probably no more than 50a), but with the numbers you provided, I would be looking at 4/0, correct?
2. Selecting a 300a shunt far exceeds the calculated 226.1a, what type of protection does that actually provide? I would rather size wire based on battery sizing not shunt sizing. With that said, what is to stop me from getting a 500a shunt?

Thanks for the feedback!

 

[Zil]

Wire size is dependent on amperes expected and distance. Use a voltage drop table or calculator such as this by Blue Sea. http://circuitwizard.bluesea.com/ You need a shunt that covers the amperes you expect to flow. Example from my life. Large bank. 2000W inverter. 150 dc amps when microwave on high. Sized for inrush from microwave. 300 amp fuse on positive. 2/0 cables. 500 amp shunt on negative. The amperes determine shunt size. The shunt doesn't add to the ampere load.

 

[John Frum]

Thanks, Joey.

1. So a larger amp shunt will require larger gauge wire, correct? I was planning on using 2/0 to handle the 175a battery (which I never plan on getting close to using that amount of draw, probably no more than 50a), but with the numbers you provided, I would be looking at 4/0, correct?
2. Selecting a 300a shunt far exceeds the calculated 226.1a, what type of protection does that actually provide? I would rather size wire based on battery sizing not shunt sizing. With that said, what is to stop me from getting a 500a shunt?

Thanks for the feedback!

The wire size depends on the load.
If your are only ever going to draw 50 amps then you can get by with much less than 2/0.
2/0 awg pure copper wire with insulation rated to 105C can handle ~330 fuse amps.
Thats ~240 load amps.
That means a 2400 watt high frequency inverter at 12 volts.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

 

[Blueboard]

As of right now, no inverter. Loads will only be 12v which is why the low amperage. But I also don't want to have to redo everything in the future if I add an inverter and solar charger. Even then, I will definitely keep the load down. Right now it is shore power as my primary source. Battery is for running vent fan, lights and fridge while short term boondocking.

 

[Blueboard]

Wire size is dependent on amperes expected and distance. Use a voltage drop table or calculator such as this by Blue Sea. http://circuitwizard.bluesea.com/ You need a shunt that covers the amperes you expect to flow. Example from my life. Large bank. 2000W inverter. 150 dc amps when microwave on high. Sized for inrush from microwave. 300 amp fuse on positive. 2/0 cables. 500 amp shunt on negative. The amperes determine shunt size. The shunt doesn't add to the ampere load.

So back to the original question. What size shunt? I have a 250a main fuse run to main disconnect on the positive line. Why would I go way over size on shunt? You said you have a 500a shunt when only 400a is required, based on your 300a fuse x 1.33 = 399a. I know you aren't going to find a readily available shunt at 400a, but it just seems to me like it is pointless. Is it for lightning strike or something?

Just trying to learn. Thanks!

 

[John Frum]

So back to the original question. What size shunt? I have a 250a main fuse run to main disconnect on the positive line. Why would I go way over size on shunt? You said you have a 500a shunt when only 400a is required, based on your 300a fuse x 1.33 = 399a. I know you aren't going to find a readily available shunt at 400a, but it just seems to me like it is pointless. Is it for lightning strike or something?

Just trying to learn. Thanks!

my recommendation stands the shunt I speced is 350 amps.

 

[Zil]

The larger the shunt the lower the voltage drop? The larger the shunt the lower the temperature. I purchased a Victron Battery Monitor and they shipped it with a 500 amps shunt. I guess I could get an off market shunt and reprogram the Victron. But what the manufacturer recommended works very very well. My opinion only, I am too stupid to know it doesn't work. I am not acquainted with a Battery Monitor that uses a 350 amp shunt. Seems the standards are 50, 100, and 500. As the shunt is just a resistor in the negative line, it just needs be larger than your load. It doesn't effect the ampere unless it is too small and then it gets hot and could cause a fire. Bigger is better. Recommendation; Victron BMV with a 500 ampere shunt.

 

[Bob B]

I would disagree that bigger is better .... why use a 500A shunt when he only plans to use 50A at most and it sounds like most of the time he will be using much less than that.

If the shunt is sized to handle the largest sustained draw expected, it will provide better resolution at the low currents he is expecting most of the time. That's like putting a meter in a 500 ohm range when trying to read 1 ohm.

 

[Zil]

Not exactly. Victron BMS is a coulomb counter. OP stated he wants to have room for expansion.

 

[Bob B]

Not exactly. Victron BMS is a coulomb counter. OP stated he wants to have room for expansion.

I think what he said was that when he expanded he was still going to keep the loads down.

 

[Zil]

How low do you go with an inverter? OP plan already has 2/0 cables and 250 amp fuse. The Victron with a 500A shunt fits right into that plan.

 

[Bob B]

He stated that he never planned to go over 50 A .... and that would be rare. So why would you size the shunt so you are always operating less than 10% of the shunt rating .... and probably a lot less than that most of the time.

His cables are over-sized ... and the fuses are generally sized based on wire size, so I don't see where that has any relevance.

 

[Zil]

U win. OP will never plan on more than a 500W inverter.

 

[Luthj]

For accuracy, pick the smallest shunt that meets your needs and is compatible with your monitor/meter. You want some overhead, so take your largest load, and pick a a shunt thats slightly higher rating than that.