Help with an ammeter

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rascacio

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Good morning all. I am installing a new 110 amp alternator on my boat in a few weeks, however the current ammeter is only 70 amp. I am trying to find the best/cheapest solution to replacing it.

I've been all over the internet and to all the usual suppliers ie RS and Howard Butler etc but am having some difficulties.

RS seem to only have up to 100 amp available then a big jump to 300 amp. Would a 100 amp suffice as the alternator is connected to a Sterling AtoB digital charger and will put out a lot of juice ?

HB can supply a 200 amp induction ammeter but as it's a made to order it's expensive with vat and p&p.

Can the 100 amp one be rescaled easily enough ? For instance by buying a new scale face and recalibrating somehow ?

Any help will be much appreciated.
 
I suspect that the 100A one will be fine, that's a lot of amps and you should only get close to that sort of current when you run the starter motor. An ammeter is basically a low value, accurately measured resistor (shunt) and a meter (e.g. a moving coil) that measures the Voltage drop across that resistor.

Changing the shunt will change the full scale deflection of the meter, but that assumes that you can get inside to do it, and as you say the scale will have to be changed as well, and you'd probably want to, at least roughly, recalibrate it.

Putting more than the rated power through a component can burn it out, your 100A Ammeter can obviously take 100 x 12 = 1.2kW, putting 1.32kW through it (110A) is unlikely to exceed the safety factor, especially if it is only for a short time, I would say.
 
There are other practicalities.
When upsizing to bigger and bigger alternators you need to change the belt system as well.
A single 'V' belt is fine on a 70 Amp alternator but above 100 Amps and you need to start looking at different belts which will require you to change the pulleys as well.
 
What do you want from this ammeter?
Is it a simple indication of lots of charge/bit of charge/ discharge, or is it a numerical measurement you want?

I suspect the 70A ammeter will do fine, the 110A alternator will rarely actually give 110A into the battery, as the batteries will not charge that fast.
Your starter motor will not draw through the ammeter.
Neither should intermittent heavy loads such as bow thruster, anchor windlass.
Where the big alternator will score is that it will give the batteries as much charge as they will accept, at the same time as running domestics such as fridge, inverter etc. But these domestic currents will not be going through the ammeter.

If your current (sic) alternator is getting to voltage limit when charging the batteries, the new one will be regulated at the same voltage and hence give the same current. But it might do it at lower engine rpm.

TBH, a good voltmeter is often more use than an ammeter, but the battery monitors which do both and process the results are worth having if you have a heavy domestic load.
Have you looked at the NASA battery monitor?

Hope that helps?
 
Evadne said:
Changing the shunt will change the full scale deflection of the meter, but that assumes that you can get inside to do it,
Click to expand...

You can stick a link across the exterior terminals;- a bit of copper wire and tune the length/gauge to, say, halve the reading when within the range of the existing. Then you need to double the number indicated thereafter.

Of more concern is the drop on the route to and from the meter.

It is sobering to measure the volts out at the end of a cable run compared with the volts in. Even with my silver plated quite substantial saloon wiring driving only LEDs it is a surprise.

A better general solution seems to be to site the shunt remotely to minimise the extra sable run and use a low current meter.
 
china could be your friend:

http://www.ebay.co.uk/itm/3-DC-150A...t_Equipment&hash=item3a58d83fed#ht_3905wt_918

A couple of the narrowboaters have these and seem happy. I have a voltmeter and an ammeter waiting to be fitted. They look OK, and we're delivered in about a week.

Worth a punt at £13 or so.

The NASA monitors only accept up to 100A. Not sure what they would do if they saw 110A ... Probably not a problem, but be aware.
 
Unless you are happy with an ammeter located close to the point of measurement the way forward is without doubt a meter at some convenient location and a shunt located in the wiring at the point of mesurement.

Only light wiring is then required from the shunt to the meter.

Otherwise you will need heavy wiring, capable of carrying the full current without excessive volts drop, from the point of measurement to the meter and back again.

On the point Richard raises about currents in excess of 100 amps and the NASA battery monitor. Its been asked before and the answer is that the monitor will operate and record higher currents but will not display more than the specified maximum. I cannot tell you what the limit is though.
 
Evadne said:
Putting more than the rated power through a component can burn it out, your 100A Ammeter can obviously take 100 x 12 = 1.2kW, putting 1.32kW through it (110A) is unlikely to exceed the safety factor, especially if it is only for a short time, I would say.
Click to expand...

There is neither 12V across your ammeter nor 100A going through it. There certainly isn't 1.2kW being dissipated by it!
 
BM-2

Richard10002 said:
china could be your friend:

http://www.ebay.co.uk/itm/3-DC-150A...t_Equipment&hash=item3a58d83fed#ht_3905wt_918

A couple of the narrowboaters have these and seem happy. I have a voltmeter and an ammeter waiting to be fitted. They look OK, and we're delivered in about a week.

Worth a punt at £13 or so.

The NASA monitors only accept up to 100A. Not sure what they would do if they saw 110A ... Probably not a problem, but be aware.
Click to expand...

BM-2 goes up to 200A and can look at Start battery voltage as well as Service bank.
 
Thanks for all your replies. All of them have good information I'll just have to decide what to do.

My original ammeter is an old style straight through type with no external shunt and when I upgraded from a 50 amp to a 70 amp alternator 2 years ago in conjunction with the Sterling AtoB charger it was putting out close to 70 amps for a significant time when the domestic batteries were a bit low and I noticed that the gauge was a bit on the 'warm' side. It never seemed to be too hot though and the charge rate did drop to about 50 amps fairly quickly.

All the cable has been upgraded to carry at least 200 amp as I was going to add a second alternator at the time but have since changed my mind.

I think the 100 amp meter with shunt will probably be the answer but the digital ones pointed out seem quite attractive especially from the price point of view. It's just a question of recutting the control panel to accept it I suppose.

I'll have a think about it. Thanks to everyone who has replied.
 
Evadne said:
I suspect that the 100A one will be fine, that's a lot of amps and you should only get close to that sort of current when you run the starter motor. An ammeter is basically a low value, accurately measured resistor (shunt) and a meter (e.g. a moving coil) that measures the Voltage drop across that resistor.

Changing the shunt will change the full scale deflection of the meter, but that assumes that you can get inside to do it, and as you say the scale will have to be changed as well, and you'd probably want to, at least roughly, recalibrate it.

Putting more than the rated power through a component can burn it out, your 100A Ammeter can obviously take 100 x 12 = 1.2kW, putting 1.32kW through it (110A) is unlikely to exceed the safety factor, especially if it is only for a short time, I would say.
Click to expand...

CreakyDecks said:
There is neither 12V across your ammeter nor 100A going through it. There certainly isn't 1.2kW being dissipated by it!
Click to expand...

maxi77 said:
He was talking about the shunt not the meter!
Click to expand...

There's not 1.2kw being dissipated in the shunt either ... that's more than a single bar electric fire!

Typically shunts produce a potential difference of 50mV at the specified current.

Eg 50mV at 100amps

The power dissipated in the shunt at 100 amps is 100x50/1000 = 5 watts.

Enough to make the shunt no more than warm.
 
VicS said:
There's not 1.2kw being dissipated in the shunt either ... that's more than a single bar electric fire!

Typically shunts produce a potential difference of 50mV at the specified current.

Eg 50mV at 100amps

The power dissipated in the shunt at 100 amps is 100x50/1000 = 5 watts.

Enough to make the shunt no more than warm.
Click to expand...

But you do agree the 110 amps will go through the shunt, which is what he said, you were the one who started talking about power disipation.
 
maxi77 said:
But you do agree the 110 amps will go through the shunt, which is what he said, you were the one who started talking about power disipation.
Click to expand...

Yes practically the full current passes through the shunt... thats the point of a shunt it takes the full current. Next to no current then passes through the meter. (A few milliamps perhaps in the case of an analogue meter probably even less with a digital meter.) This means that the meter itself can easily located some distance from the measurement point and connected with light wiring.

A 100 amp shunt will happily withstand more than 100 amps, even if the meter does not read any higher. Cannot tell you what the maximum safe current will be though.

BUT 'twas not me who started talking about power dissipation. 'twas Evadne in #2, misleadingly perhaps
 
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VicS said:
Yes practically the full current passes through the shunt... thats the point of a shunt it takes the full current. Next to no current then passes through the meter. (A few milliamps perhaps in the case of an analogue meter probably even less with a digital meter.) This means that the meter itself can easily located some distance from the measurement point and connected with light wiring.

A 100 amp shunt will happily withstand more than 100 amps, even if the meter does not read any higher. Cannot tell you what the maximum safe current will be though.

BUT 'twas not me who started talking about power dissipation. 'twas Evadne in #2, misleadingly perhaps
Click to expand...

At my age I am easily mislead
 
rascacio said:
I think the 100 amp meter with shunt will probably be the answer but the digital ones pointed out seem quite attractive especially from the price point of view. It's just a question of recutting the control panel to accept it I suppose.
Click to expand...

Despite having bought separate digital ammeter and volt meter, I'll almost certainly be getting a NASA BM-2. The facility to have a reasonably good estimate of nett amps in and out makes life incredibly simple when it comes to knowing where you're at with your batteries.
 
Richard10002 said:
Despite having bought separate digital ammeter and volt meter, I'll almost certainly be getting a NASA BM-2. The facility to have a reasonably good estimate of nett amps in and out makes life incredibly simple when it comes to knowing where you're at with your batteries.
Click to expand...

I'd agree that a proper battery monitor is the only practical solution. An ammeter showing alternator output really doesn't tell you much.
 
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