To shunt or not to shunt (ammeter)?

[KevinV]

I'm just fitting an ammeter to the switch panel, and it states that it can handle up to 20A without a shunt. My total load is well below this at the moment, but will go over this (I think) when I add in a tillerpilot and coolbox. I'm a bit averse to adding a shunt to the wiring if I don't really need it, but adding/ changing things later will be more bother. Am I silly for not wanting to put a resistor (shunt) in the system? And is it better to put in a bigger, or smaller one? The ammeter says a 200A/ 75mV shunt, but that's way beyond what I'll ever use - or am I misunderstanding? Will it misread if I use a different spec shunt?

Cost is not the issue (if it's better it'll be done), but physical size, placement and clutter are.

 

[[194224]]

You say "without a shunt" but it is quite likely that your 20A ammeter does has a shunt but an internal one rather than an external one. Do you have the specs for the 20A one? Does the 200A one you mention require an external 75mV shunt or is that self contained too?

Matching the wrong shunt to a specific ammeter is going to lead to incorrect readings.

 

[KevinV]

You say "without a shunt" but it is quite likely that your 20A ammeter does has a shunt but an internal one rather than an external one. Do you have the specs for the 20A one? Does the 200A one you mention require an external 75mV shunt or is that self contained too?

Matching the wrong shunt to a specific ammeter is going to lead to incorrect readings.

The ammeter will cope with 20A without an external shunt. Over 20A it requires an external shunt, specified at 200A/ 75mV

 

[[194224]]

OK thanks

Do you have a link to the meter or the specs?

 

[KevinV]

OK thanks

Do you have a link to the meter or the specs?

Just what I've given.

 

[[194224]]

OK sorry, just trying to help. I meant a link to the specific meter but never mind.

 

[B27]

An 'internal shunt' ammeter is OK if it's convenient to lead the high current cables through the meter on the panel.
An external shunt might allow shorter, neater lower resistance wiring, with just thin signal cables to the panel.

Does the shunt also see the charging current? in which case the alternator might be pushing more than 20A through it at times.

 

[KevinV]

An 'internal shunt' ammeter is OK if it's convenient to lead the high current cables through the meter on the panel.
An external shunt might allow shorter, neater lower resistance wiring, with just thin signal cables to the panel.

Does the shunt also see the charging current? in which case the alternator might be pushing more than 20A through it at times.

Ah, the wiring size on the panel is a good argument for using a shunt - I hadn't considered that

The only charging is a 50W solar panel and a tiny bit from the outboard, there's no big alternator or shore charger.

 

[[194224]]

If 20A is sufficient for your needs then you can, depending on the particular meter you have in mind, remove or disable the internal shunt and fit an equivalent replacement remotely and wire as B27 suggests.

 

[Alex_Blackwood]

I'm just fitting an ammeter to the switch panel, and it states that it can handle up to 20A without a shunt. My total load is well below this at the moment, but will go over this (I think) when I add in a tillerpilot and coolbox. I'm a bit averse to adding a shunt to the wiring if I don't really need it, but adding/ changing things later will be more bother. Am I silly for not wanting to put a resistor (shunt) in the system? And is it better to put in a bigger, or smaller one? The ammeter says a 200A/ 75mV shunt, but that's way beyond what I'll ever use - or am I misunderstanding? Will it misread if I use a different spec shunt?

Cost is not the issue (if it's better it'll be done), but physical size, placement and clutter are.

Ignore the waffle. Fit the shunt. Problem solved!

 

[[193211]]

I fitted a Victron Bluetooth shunt for some of the reasons you list. Very little wiring, tidy, same app for solar and mains charger.

 

[KevinV]

Right ho folks, I get the message! Thanks for the input

 

[rogerthebodger]

All meters are milli voltmeters that measure the voltage across a shunt.

Some shunts are internal to the meter which seems to be the one you have as it has a 20Amp shunt internal,

You can also get a milli volt meter with an external shunt it all depends what and how much current Amps you wish to measure

 

[john_morris_uk]

If you wanted to be really clever you could wire it to switch the shunt in and out and therefore have the best of both worlds.

200 A is a big jump and sound like a sop to the scale on the meter (0-20 or x10 = 0-200).

If you’re positive (pardon the pun) that you might only just go over the 20 A then the meter will probably survive. However if you wrap the needle round the end stop frequently then obviously you’ll damage it!

I’m concerned that if you’re charging as well as drawing current through the ammeter then it will be reversed in its polarity when charging current exceeds drain. In other words it’ll try and read backwards which at high currents won’t do it any good.

 

[roaringgirl]

What current does your starter pull?

 

[KevinV]

What current does your starter pull?

Well it takes me a few moments to recover if it takes more than three or four, but then I do smoke far too much

(there's no inboard engine)

 

[KevinV]

If you wanted to be really clever you could wire it to switch the shunt in and out and therefore have the best of both worlds.

200 A is a big jump and sound like a sop to the scale on the meter (0-20 or x10 = 0-200).

If you’re positive (pardon the pun) that you might only just go over the 20 A then the meter will probably survive. However if you wrap the needle round the end stop frequently then obviously you’ll damage it!

I’m concerned that if you’re charging as well as drawing current through the ammeter then it will be reversed in its polarity when charging current exceeds drain. In other words it’ll try and read backwards which at high currents won’t do it any good.

Interesting about the scale - I'll check that out when I have it in front of me. It wouldn't be hard to wire it both ways (with a switch to choose), and might be useful when measuring actual useage of things.

I'm not sure if it'll do negative draw (ie feed) - I'd planned on reversing the wiring if it wouldn't by way of a 1 0 2 switch next to it - though that does require guessing which it's doing before switching it.

Starting to think I should have spent the mental energy working out how to physically fit in a proper battery monitor, but they're just so expensive, and a bit deep for my panel.

 

[William_H]

All meters are milli voltmeters that measure the voltage across a shunt.

Some shunts are internal to the meter which seems to be the one you have as it has a 20Amp shunt internal,

You can also get a milli volt meter with an external shunt it all depends what and how much current Amps you wish to measure

Be careful here. One style of amp meter has a moving iron movement where the current goes through a heavy wire coil perhaps 6 turns which drag around an iron armature. Typical of old car amp meters reading + and - . Typically also non linear scale. It is conceivable that a shunt could be used with this type but it would mean a large current going through meter. So 200A 75mv shunt would not suit at all.
If OP has not bought the meter yet it is possible that it has a shunt bolted to the back of the (moving coil Mv) meter.In the form of a piece of sheet metal clamped under input bolts. This would have to be removed to use 75mv 200A shunt. But could in fact be used as the remote shunt by arranging a suitable insulating block for the shunt. use of a 200A shunt means at 200A 75mv will be dropped to drive meter. Which may mean 200A full scale so almost useless for OP typical current being measured. ie at 20 amps through 200A shunt you only get 7.5mv to drive meter to 20A mark FSD all very tricky ol'will

 

[Alex_Blackwood]

Ignore the waffle. Fit the shunt. Problem solved!

Apologies for a somewhat throwaway answer. On reflection I would suggest that the meter you have is not suitable for the application. There are many more if you look at some of the on-line retailers.

 

[john_morris_uk]

The Clipper ones are reasonably priced and give lots of information. I fitted one to our daughter’s boat and it seems to work well.

NASA Clipper Battery Monitor BM2