Determining the correct ammeters & shunts to buy

[Grace and Favour]

Hi chaps (gender neutral of course!)

 

May I seek the advice of those more experienced than me, please?

 

I'm of a mind to fit ammeters for both of my alternators, and downside of my inverter to measure power consumption - both to keep and eye on things, and learn about my usage.

 

If I have, (for example) a 175A alternator - should I be fitting (say) a 0~200A ammeter, and what size of shunt should I be fitting, please?

 

And how do I determine what size of ammeter/shunt I should fit on the output of my inverter (2.5Kw inverter - - 250A ammeter?)

 

Many thanks.

 

 

Dave

[Gibbo]

Like all engineering matters, never run anything close to its maximum rating. To do so is to invite disaster. Always build in a margin.

 

So your 175 amp alternator would be good with a 250 amp shunt.

 

Your 250 amp inverter would be good with a 500 amp shunt.

 

But it could also be argued that a 175 amp alternator is not going to be running at anything approaching that figure for most of the time so a 200 amp shunt will be ok. Not so the inverter. A 12 volt 2500 watt inverter will pull about 300 amps at full rating and will surge much higher than that. You need to be looking at a 500 amp shunt.

 

Then you have the other part, the bit that defines voltage drop at the rating. You'll see them marked as something like 50mV/500A. That means its rated up to 500amps and at that current it will drop 50mV. You need to match this figure with the rating for the meter you intend using.

 

Be careful, many digital ammeters only allow the shunt to be fitted in the negative so you've got a problem with the alternator (fine for the inverter). Others allow you to put it in the positive but only if you use an isolated power supply for the ammeter. You can buy these for next to nothing on ebay from China but they tend to be quite thirsty on power and something that is left on permanently (as most ammeters often are) really needs to be more efficient.

 

Some ammeters need an isolated power supply wherever the shunt is. These are best avoided.

Edited August 5, 2012 by Gibbo

[Grace and Favour]

Like all engineering matters, never run anything close to its maximum rating. To do so is to invite disaster. Always build in a margin.

 

So your 175 amp alternator would be good with a 250 amp shunt.

 

Your 250 amp inverter would be good with a 500 amp shunt.

 

But it could also be argued that a 175 amp alternator is not going to be running at anything approaching that figure for most of the time so a 200 amp shunt will be ok. Not so the inverter. A 12 volt 2500 watt inverter will pull about 300 amps at full rating and will surge much higher than that. You need to be looking at a 500 amp shunt.

 

Then you have the other part, the bit that defines voltage drop at the rating. You'll see them marked as something like 50mV/500A. That means its rated up to 500amps and at that current it will drop 50mV. You need to match this figure with the rating for the meter you intend using.

 

Be careful, many digital ammeters only allow the shunt to be fitted in the negative so you've got a problem with the alternator (fine for the inverter). Others allow you to put it in the positive but only if you use an isolated power supply for the ammeter. You can buy these for next to nothing on ebay from China but they tend to be quite thirsty on power and something that is left on permanently (as most ammeters often are) really needs to be more efficient.

 

Some ammeters need an isolated power supply wherever the shunt is. These are best avoided.

 

 

Gibbo,

 

Sincere thanks - - As ever, your advice was helpful, clear, precise, and even I understood it. (which - honestly, yours is an achievement my teachers never managed)

 

Thanks

[steve hayes]

Be careful, many digital ammeters only allow the shunt to be fitted in the negative so you've got a problem with the alternator (fine for the inverter).

 

Why does having a shunt in the negative feed to the domestic battery bank cause a problem with the alternator ? I am just about to do this..

[Tony Brooks]

Like all engineering matters, never run anything close to its maximum rating. To do so is to invite disaster. Always build in a margin.

 

So your 175 amp alternator would be good with a 250 amp shunt.

 

Your 250 amp inverter would be good with a 500 amp shunt.

 

But it could also be argued that a 175 amp alternator is not going to be running at anything approaching that figure for most of the time so a 200 amp shunt will be ok. Not so the inverter. A 12 volt 2500 watt inverter will pull about 300 amps at full rating and will surge much higher than that. You need to be looking at a 500 amp shunt.

 

Then you have the other part, the bit that defines voltage drop at the rating. You'll see them marked as something like 50mV/500A. That means its rated up to 500amps and at that current it will drop 50mV. You need to match this figure with the rating for the meter you intend using.

 

Be careful, many digital ammeters only allow the shunt to be fitted in the negative so you've got a problem with the alternator (fine for the inverter). Others allow you to put it in the positive but only if you use an isolated power supply for the ammeter. You can buy these for next to nothing on ebay from China but they tend to be quite thirsty on power and something that is left on permanently (as most ammeters often are) really needs to be more efficient.

 

Some ammeters need an isolated power supply wherever the shunt is. These are best avoided.

 

 

Just to point out that I think the OP specifically says he wishes to measure the OUTPUT (downside) of his inverter and that implies an AC ammeter. Now I have no idea why he wants to measure the output because the input seems far more relevant to me as that is the load on the battery.

 

I think he means the input current so what Gibbo says is correct, but he might want to measure the output.

[Chris Pink]

Why does having a shunt in the negative feed to the domestic battery bank cause a problem with the alternator ? I am just about to do this..

 

It doesn't, I think the implication was that the OP wanted to measure just the alternator output rather than derive it from the traffic in and out of the batteries. For this the alternator will require an isolated return (as opposed to via engine earth strap and other random connections)

[Gibbo]

Why does having a shunt in the negative feed to the domestic battery bank cause a problem with the alternator ? I am just about to do this..

 

I doesn't but as Pinky says, OP wants to measure just the alternator current, not the battery current. You can't get to the negative on most alternators.

[blackrose]

Why does having a shunt in the negative feed to the domestic battery bank cause a problem with the alternator ? I am just about to do this..

 

The shunt for my battery monitor is on the negative feed of the domestic battery bank and tells me amps in to he batteries from the alternator or battery charger and amps out to 12v systems and the inverter. It's all the information I need.

[Chalky]

 

Then you have the other part, the bit that defines voltage drop at the rating. You'll see them marked as something like 50mV/500A. That means its rated up to 500amps and at that current it will drop 50mV. You need to match this figure with the rating for the meter you intend using.

 

 

Something to consider is that 50mv is quite small so you'll only see 50mV when 500A is going through the shunt. At lower currents the voltage will be even smaller and there's a possibility that the electrical noise could be higher than the voltage produced by the current (google noise floor and Signal-to-noise ratio). When installing the cables between the shunt and meter be careful about the routing and keep the wires away from other cables if possible. Screened cable would be best (earth screen at 1 end only) or twisted pair.

[Gibbo]

Screened cable would be best (earth screen at 1 end only) or twisted pair.

 

Or both

[steve hayes]

I doesn't but as Pinky says, OP wants to measure just the alternator current, not the battery current. You can't get to the negative on most alternators.

 

Cheers Gibbo and Pinky , panic over, thought I was missing something but couldn't see what.

 

Can't see the point of measuring just the alternator current. Yes you might be able to count the Ah into the bank, but surely this is only half (or less in my case with solar panels) the story..... I want to understand the ins and outs of the bank.

[nicknorman]

Although I don't think the OP really wants to measure alternator current, it is feasible to measure it if the -ve lead from engine to batts is the only connection between bank and hull/engine. In our case with the Beta 43 twin alternators I think it would work since, even though there are two negatives from engine to banks, effectively connected together at the engine, if the bat banks do not have their -ves connected together then the current flowing in each -ve will reflect the current flowing in the respective positives, of which there is one from each alternator +ve to its respective batt bank +ve. I think!

 

Of course the starter current would also flow through the engine bank -ve so if you wanted to put a shunt in there (why would you?) it would have to be able to cope with starter peak current which is a lot!

 

Thoughts on this subject arose recently when I had to fix the "professional" install of a Mastervolt BM1 battery monitor on a friend's boat- it had been wired so the combi inverter/charger current was not going through it! Also since the battery bank -ves were connected together, current from the domestic alternator was sharing the two -ve leads. Not surprisingly it was indicating bollocks! Removed eng to domestic bank -ve link, moved Combi -ve and job's a goodun! All clearly shown in the installation manual of course!

[Chris Pink]

Although the engine -ve strap is the 'official' -ve return for the alternator, it is by no means certain it's the only one. Gear and throttle cables and prop shaft being other possibles even if your engine mounts are truly isolated.

[Grace and Favour]

Although I don't think the OP really wants to measure alternator current, it is feasible to measure it if the -ve lead from engine to batts is the only connection between bank and hull/engine.

 

My thought process (right or (probably) wrong) is a precautionary one, and that if I can easily monitor the output from the alternators (using gauges) then I should be able to spot an alternator problem early , rather than at the end of a day by which point the batteries are already depleted

(or have I got the matter completely wrong?)

[Chalky]

Although the engine -ve strap is the 'official' -ve return for the alternator, it is by no means certain it's the only one. Gear and throttle cables and prop shaft being other possibles even if your engine mounts are truly isolated.

 

Very true and ones that are often forgotten. I've seen several "interesting" faults many years ago caused by engines being cranked and trying to draw the entire cranking current through choke/throttle/gear selector cables after the engine earth cable has become disconnected.

Edited August 5, 2012 by Chalky

[Arthur Brown]

If you look in RS (rswww.com no www ) for 315-9537 you will find one of lots of direct reading meters that rely on a shunt in the circuit. No bother about getting extra supplies but it's an analog display. Fit the right one for the shunt.

[nicknorman]

Very true and ones that are often forgotten. I've seen several "interesting" faults many years ago caused by engines being cranked and trying to draw the entire cranking current through choke/throttle/gear selector cables after the engine earth cable has become disconnected.

It's obviously a possible scenario, but a well-designed system would have only 1 -ve thing connected to hull, and that might as well be the engine. If the engine is connected to hull and there is another connection, say a strap from bat -ve, that means that inevitably the hull will share some of the current which is a "bad thing". In our case as far as I am aware there are no other things connected to hull and even the throttle/gear control is mounted on wood, stop control is solenoid.

 

My thought process (right or (probably) wrong) is a precautionary one, and that if I can easily monitor the output from the alternators (using gauges) then I should be able to spot an alternator problem early , rather than at the end of a day by which point the batteries are already depleted

(or have I got the matter completely wrong?)

Measuring the current into / out of the battery bank is sufficient I would have thought. When charging in the morning you would expect to see a high current going into the batts, and the relatively slight loss into the domestic circuits eg the fridge will not really make much difference to a reading of many tens or hundreds of amps.

[by'eck]

I would suggest to the OP he forgets connecting a current monitoring system to the alternator exclusively and purchase a dedicated battery monitor with matching shunt. Victron, Mastervolt, Sterling & Nasa all do them. This will surely tell him all he needs to know of battery current flow in (alternator) or out (inverter). Fitting the shunt in the negative battery feed if necessary, won't be an issue either.

[Grace and Favour]

I would suggest to the OP he forgets connecting a current monitoring system to the alternator exclusively and purchase a dedicated battery monitor with matching shunt. Victron, Mastervolt, Sterling & Nasa all do them. This will surely tell him all he needs to know of battery current flow in (alternator) or out (inverter). Fitting the shunt in the negative battery feed if necessary, won't be an issue either.

 

Hmm - - it would appear that I've been trying top over-complicate things

 

(which is about par for the course with me)

 

Thanks

[smileypete]

Hmm - - it would appear that I've been trying top over-complicate things

 

(which is about par for the course with me)

 

Thanks

How often do you plan to go above say 2KW on the inverter? What sort of batt bank do you have?

 

cheers, Pete.

~smpt~

[Grace and Favour]

How often do you plan to go above say 2KW on the inverter? What sort of batt bank do you have?

 

cheers, Pete.

~smpt~

 

We'll go above 2Kw once a week for the washing machine (TravelPower driven)

 

The domestic bank is currently 660Ah, (6 x 110) (though will probably uprate to 800Ah whenever the current batteries expire)

Starter 100Ah

Throwbuster batts are 200Ah (2 x 100Ah)

[smileypete]

We'll go above 2Kw once a week for the washing machine (TravelPower driven)

 

The domestic bank is currently 660Ah, (6 x 110) (though will probably uprate to 800Ah whenever the current batteries expire)

Starter 100Ah

Throwbuster batts are 200Ah (2 x 100Ah)

OK so I take it when the Travelpower is running some power comes from that and some from the batts. A 200A shunt should take a reasonably small OR short term overload.

 

Nice thing about a 200A meter is that digital ammeters are usually 3 1/2 digits reading up to 1999, such an ammeter reading over 200A will only read to the nearest amp, less than that should give 0.1A resolution.

 

cheers, Pete.

~smpt~

[Chalky]

OK so I take it when the Travelpower is running some power comes from that and some from the batts. A 200A shunt should take a reasonably small OR short term overload.

 

Nice thing about a 200A meter is that digital ammeters are usually 3 1/2 digits reading up to 1999, such an ammeter reading over 200A will only read to the nearest amp, less than that should give 0.1A resolution.

 

cheers, Pete.

~smpt~

 

A lot depends on how accurate he wants it to be. If it's just a rough indication then you could use one of the hall effect current transformers. They work by measuring the size of the magnetic field generated by the current flowing in the wire. The advantage is that they work on DC but don't require a shunt. The disadvantage is that they're not as accurate as a shunt.

 

I've got a modified mega fuse that includes one of the Allegro Microsystems hall sensing devices in it that I use at work. It's isolated up to 5kV so its easy to use in lots of applications.

[nicknorman]

A lot depends on how accurate he wants it to be. If it's just a rough indication then you could use one of the hall effect current transformers. They work by measuring the size of the magnetic field generated by the current flowing in the wire. The advantage is that they work on DC but don't require a shunt. The disadvantage is that they're not as accurate as a shunt.

 

The better ones work by generating a null magnetic field on the hall effect sensor - the high current conductor is passed through a core, which has a secondary winding of many turns. The system then puts current through the secondary winding to counter balance the magnetic field until it is zero as detected by the hall effect sensor. The current in the secondary winding is less than in the primary winding by a factor according to the number of turns, but with lots of turns, it is a small current. It is pretty accurate since the non-lineararity of the hall effect device is not a factor. Of course you still have an analogue current (or volage if you pass it through a resitor) and so need an AtoD and display (aka digital voltmeter with a current scale). A quick google found this one - but I didn't look at the price.

 

LEM do a +- 200A one for £26. One advantage over a shunt is that there is no direct connection between the conductor being measured, and the output to the meter.

Edited August 6, 2012 by nicknorman

[Gibbo]

A 200A shunt should take a reasonably small OR short term overload.

 

No. Even running them at rated power wrecks them.

 

General opinion in the industry is never to run them past 66% rated power. Not even for a second.