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Old 03-05-2018, 01:02   #1
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Current Shunt - What size?

I want to install an ammeter on my Beneteau yacht to monitor amp usage.
I have 2 x 225ah house batteries.
Would it be correct to say I'd need a 500amp shunt?
The wires connecting the batteries are massive and when I view Shunts on ebay they look insufficient.
I assume the best place to put shunt will be where the batteries connect to the main isolation switch so that all current will be monitored?
Any suggestions?
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Old 03-05-2018, 01:43   #2
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Re: Current Shunt - What size?

500 amp should be fine. That's enough to handle a 6KW load. You should never get close to that. I would expect that a 200 amp shunt would also be fine. The important thing is to match your shunt to your ammeter.

It should be wired in line close to the negative terminal of your battery bank.
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Old 03-05-2018, 04:47   #3
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Re: Current Shunt - What size?

I have not seen many amp meters, and shunts to go with that, for 500 Amps. Such equipment could become nearly commercial/industrial gear, and prices to go with that.

Generally startermotor currents do NOT go through such shunt, and the likely highest current load could be inverter. 200 Amps would be good for 200x12volt=2400 Watt.
Or what you can do is:
* Put inverter on separate circuit, as the inverter panel often can show the amps used (in inverter mode) or produced (in charger mode) on its own system.
* That leaves you with a shunt of 200 amps for all other gear.

Generally that is more than enough, as several high amps users like (anchor)winches, furlers etc. are not used all at the same time.
If you have two times 225 Amp/hr batteries and no high demand users, you might decide that 100 Amp is good enough. Just add up all the things which could be running at the same time, and see what the ampere consumption would be. That would give you an indication of the value of the shunt and panel meter.

As Stum said, most of the time those shunts need to be inserted into the negative supply wire. Best is to buy the panelmeter with a matching shunt and there are literally thousands of choices, ranging from $20 to $200 for a matching set. Common values are 20, 50, 100 and 200 Amp. Just google "digital amp meter".
A set between $30 and $50 would suffice (in my opinion) as accuracy may not be vitally important, and such units could be still have an accuracy of +/- 1%.

Or to take it one step further, you can install one of those battery monitor systems, much more expensive but providing more information, including (calculated/estimated) SoC (state of charge) of the battery bank.
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Old 03-05-2018, 05:08   #4
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Re: Current Shunt - What size?

Every battery monitor I've used, can be calibrated to work with any capacity shunt.

500A is a minimum if the bank you're metering is used for cranking engines.

I've installed 1000A shunts, owner wanted to not worry using starter motors, winches etc.
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Old 03-05-2018, 05:32   #5
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Re: Current Shunt - What size?

I had to build a bus bar out of solid copper to connect all my negatives, so that nothing bypassed the shunt as Iím using it for an amp counter.
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Old 03-05-2018, 12:13   #6
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Re: Current Shunt - What size?

You may want to read that article:
https://marinehowto.com/installing-a-battery-monitor/
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Old 03-05-2018, 14:48   #7
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Re: Current Shunt - What size?

Quote:
Originally Posted by john61ct View Post
Every battery monitor I've used, can be calibrated to work with any capacity shunt.

500A is a minimum if the bank you're metering is used for cranking engines.

I've installed 1000A shunts, owner wanted to not worry using starter motors, winches etc.
I would think most people want to see the current from the house bank and not the starting bank. 100A shunt on house negative is usually OK for most of us.

It's actually pretty hard to get a stable reading when measuring cranking amps. By the time the reading stabilizes, your engine has already started.

Measurement of cranking amps is useful to check the health of your starter motor, but not something you need to do all the time. If you really insist on measuring cranking amps, get a clamp-on meter.

If you install a battery AH meter (really a good thing to do) you want it to use the shunt that is on your house battery only, not starting and house combined. Having starting and house on one monitor would be confusing.

If I had a powerboat, I would want a complete, independent, second battery shunt and monitor on my starting batteries only.
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Old 03-05-2018, 15:02   #8
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Re: Current Shunt - What size?

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I would think most people want to see the current from the house bank and not the starting bank.
I just meant those with self jump-starting setups. Very common feature on high-end combiners / VSRs.

> 100A shunt on house negative is usually OK for most of us.

Just giving fuller info for those interested, not pushing going higher where not needed.

Using too large a shunt loses resolution measuring lower currents too

> Measurement of cranking amps is useful to check the health of your starter motor, but not something you need to do all the time.

Never said anything about measuring cranking amps. But the shunt needs to be sized, as with CP, to withstand the highest currents anticipated on that circuit.

And with a BM, you don't want any circuits bypassing the shunt

> If you install a battery AH meter (really a good thing to do) you want it to use the shunt that is on your house battery only, not starting and house combined. Having starting and house on one monitor would be confusing.

The banks are separate in use, but usually combined while charging via ACR / VSR. This confuses nothing as long as the shunt(s) are placed correctly.

> If I had a powerboat, I would want a complete, independent, second battery shunt and monitor on my starting batteries only

As you say, pretty rare, I've never seen anyone care about AH monitoring a Starter batt, voltage only is enough.
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Old 04-05-2018, 22:37   #9
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Re: Current Shunt - What size?

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Originally Posted by john61ct View Post
I just meant those with self jump-starting setups. Very common feature on high-end combiners / VSRs.

> 100A shunt on house negative is usually OK for most of us.

Just giving fuller info for those interested, not pushing going higher where not needed.

Using too large a shunt loses resolution measuring lower currents too

> Measurement of cranking amps is useful to check the health of your starter motor, but not something you need to do all the time.

Never said anything about measuring cranking amps. But the shunt needs to be sized, as with CP, to withstand the highest currents anticipated on that circuit.

And with a BM, you don't want any circuits bypassing the shunt

> If you install a battery AH meter (really a good thing to do) you want it to use the shunt that is on your house battery only, not starting and house combined. Having starting and house on one monitor would be confusing.

The banks are separate in use, but usually combined while charging via ACR / VSR. This confuses nothing as long as the shunt(s) are placed correctly.

> If I had a powerboat, I would want a complete, independent, second battery shunt and monitor on my starting batteries only

As you say, pretty rare, I've never seen anyone care about AH monitoring a Starter batt, voltage only is enough.
Thanks for your thoughtful reply. Actually I think we are saying the same thing in different ways and you bring up a point I had not thought of. Just want to re-iterate that if you do want to use a battery monitor to keep track of house battery, which most of us do, then it is not a good idea to include either charging or discharging current from the starting battery into that information source. So, apply shunt to house battery only.

Even with automatic jump start, you still don't want the shunt of a house battery monitor to be placed where it can see any starting battery current.
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Old 05-05-2018, 02:28   #10
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Re: Current Shunt - What size?

Caroline to answer your question we really need to know what equipment you have that will be supplied by the battery bank you wish to measure. Also the voltage 12 or 24v?.

For example If the bank is supplying a bow thruster on a 50 foot yacht the requirements will be for large shunt especially for a 50 foot yacht.
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Old 05-05-2018, 10:52   #11
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Re: Current Shunt - What size?

Quote:
Originally Posted by waterman46 View Post
Even with automatic jump start, you still don't want the shunt of a house battery monitor to be placed where it can see any starting battery current.
The self-jumpstarting feature is manual, only for use when Starter batt fails.

The shunt must be placed between the BM target bank and the VSR. In fact nothing should be allowed to bypass the shunt if you want an accurate BM.

Those momentary cranking amp levels should be no problem with a shunt rated for 200+A, certainly not for 500A ones.
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Old 05-05-2018, 22:51   #12
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Re: Current Shunt - What size?

Hi, Other than Bow thruster & Windlass, its just Fridge autopilot etc which are fairly low amps. We don't have any flash toys on our 50ft yacht. Running 12v. Windlass 2100w & I think Bow thruster similar.
I won't be using Windlass & Bow thruster on long trips, so I'm really just interested in what amps everything else uses so I can plan my battery charging etc.
Have 420w solar & can run 1700w generator as top up.
Planning on running autopilot 24/7.
Sounds like 200amp would be ok?
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Old 05-05-2018, 23:15   #13
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Re: Current Shunt - What size?

Quote:
Sounds like 200amp would be ok?
IMO, certainly OK. Realistically, short term overcurrents will not hurt anything at all, so don't fret about it. Having the higher resolution measurements available with a lower range shunt on the other hand will be useful in tracking individual consumers.

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Old 05-05-2018, 23:53   #14
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Re: Current Shunt - What size?

Quote:
Originally Posted by Caroline Joan View Post
Windlass 2100w & I think Bow thruster similar.
I won't be using Windlass & Bow thruster on long trips, so I'm really just interested in what amps everything else uses so I can plan my battery charging etc.
The rating of the shunt is not the value at which it stops measuring, but the maximium current the shunt will handle before it starts to melt etc. So it is important if the shunt is inserted in the circuit that it can handle the maximium anticipated current even if you are not interested in measuring the current draw of the bow thruster.

200A is about 2.1kw @ 10.5v. It is not impossible that the bow thruster is this small, but it would be a small unit for a 50 foot yacht. For example the 2.2kw Side Power SE40 model is recommended for boats in the 26-34 foot size range. Larger units draw much more power.

One factor helping is the bow thruster will only be used for short periods and the current rating of the shunt is for continuous operation. If you can find the shunts intermittent rating (for 3-5min, not the 5, or 10 second rating), this would give you a better idea of its suitability. I would check the rating of the bow thruster before installing a 200A shunt. Bow thrusters are power hungry devices. A 200A may be OK (although I suspect it is too small) but at best it is marginal so you need to check the details carefully.

The other option is wire the system so the bow thruster current does not pass through the shunt. This will not work well for a battery monitor, but is OK for ammeter.
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Old 06-05-2018, 00:15   #15
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Re: Current Shunt - What size?

Most shunts include a short term overload current in their specs.
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