Another ammeter question

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Storyline

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Am thinking of getting an ammeter. There seems to be a lot of criticism of the cheap ebay ones with shunts (some of which also show voltage). The main complaint seems to be the need to calibrate them and even then they are not very accurate.

There seems to be a huge price difference between these ebay ones which go for a tenner or so and then something that does the job properly like the Nasa bm1/2 which are over a hundred.

The question is, are there any options in the £25 - £50 range that might not have all the functionality of the Nasa models but are at least accurate ? Think I would probably prefer digital but would not rule out an analogue dial type.

Another thought, would it be possible/feasible to buy additional shunts and then wire the meter with a switch/s so it could measure different current flows such as current in from wind generator and also current current (!) usage and maybe a third shunt to measure charging current from the alternator ?

Thanks in advance for any info :)
 
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Storyline said:
Another thought, would it be possible/feasible to buy additional shunts and then wire the meter with a switch/s so it could measure different current flows such as current in from wind generator and also current current (!) usage and maybe a third shunt to measure charging current from the alternator ?

Thanks in advance for any info :)
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Thats how I wired my 4 solar panel banks and my 3 battery banks.

BTW I used hall effect current transducers from another fora mamber for my battery banks to avoid the volt drop of the shunts when powering windlass and bow thruster
 
Rogershaw said:
Thats how I wired my 4 solar panel banks and my 3 battery banks.

BTW I used hall effect current transducers from another fora mamber for my battery banks to avoid the volt drop of the shunts when powering windlass and bow thruster
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Do the shunts have a significant effect on the voltage ? (sorry for what maybe a dumb question but not very good with electrics !)
 
Storyline said:
Do the shunts have a significant effect on the voltage ? (sorry for what maybe a dumb question but not very good with electrics !)
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When you are drawing 600 amps through a 100 amp shunt yes it does have a significant effect with a 12 vdc system.

On my solar panels may be max 20 amps, not much.
 
Storyline said:
Do the shunts have a significant effect on the voltage ? (sorry for what maybe a dumb question but not very good with electrics !)
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You have a number of problems, depending on installation, volt drop in shunt due to high current from engine start and overload to meter or shunt from this high current. A 100 amp shunt that can carry engine start current could be 500 amp may burn out the shunt, or the high voltage generated could damage the display, .075v @ 100a becomes 0.375v. With Hall effect shunts the power cable just passes through a sensor ring, with no cutting or fitting terminals and provides infinate overload with output overload limit.

The one thing with normal shunts is the low operating voltage can be influenced by nearby power cables ( 50mV 100 amp shunt, 1 amp + 0.0005 V ) so you should us twisted pairs to connect up the meter.

Brian
 
I think it's a mistake to dismiss the Ebay cheapies as being inherently inaccurate. I've used a few for different projects and, without exception, they have been accurate, reliable and excellent value for money. They are not precision instruments but, for that matter, I doubt that the NASA product could be described like that either. Unlike the NASA ones they are produced in quantity; hence the price advantage.

There are other versions of the Ebay meters which are similar to the NASA BM1....http://www.ebay.co.uk/itm/170869014075?_trksid=p2055119.m1438.l2648&ssPageName=STRK:MEBIDX:IT
There are several different models so it would be necessary to see what is best for any particular setup, but they do offer estimates of capacity and percent discharge.I've not tried one though, but it is on my todo list.

You could switch from one shunt to another to take readings from different sources but you would need to be careful not to damage alternators by open circuiting them whilst switching and, if you did it, I don't know what it would do to the capacity estimates. It might make them meaningless. Personally I would use one main meter which would cover voltage, current and capacity, with simple ammeters for secondary sources of power.

If you don't want to calibrate ammeter/voltage meter just buy one that doesn't need calibration. There are lots to choose from on Ebay and elsewhere.
 
Rigger Mortice said:
You could switch from one shunt to another to take readings from different sources but you would need to be careful not to damage alternators by open circuiting them whilst switching and, if you did it, I don't know what it would do to the capacity estimates. It might make them meaningless.
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You would not switch the cables carrying the current being measured you would just switch the shunt signal wires to the meter. A shunt for each current to be measured with a single displaying the current in the selected shunt. As you are not switching the current from say the alternator you are in no danger od blowing the alternator diodes.

If you switched shunts external to the NASA battery monitor the battery capacity display would be totally meaningless.
 
nigelmercier said:
Agree. You want to know:

[1] Am I charging
[2] Am I charging at 10A or 50A
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Sounds reasonable to me! Actually, I find a volt meter on the domestic side more useful than an ammeter. If the voltage is over about 13V then the answer to 1 is yes. If it's less than about 12.2 unless I have something drawing a heavy current, I know the battery's getting low and I need to start turning things off or run the engine for a bit.

For question 2, I'm not that bothered. If the engine's running, I'm chucking in pretty much as much as the batteries can handle, if not, it's up to the solar panels, which, in the summer, keep up with everything except the cold box and the voltmeter soon tells me if I've overdone it with that.
 
Rigger Mortice said:
I think it's a mistake to dismiss the Ebay cheapies as being inherently inaccurate. I've used a few for different projects and, without exception, they have been accurate, reliable and excellent value for money. They are not precision instruments but, for that matter, I doubt that the NASA product could be described like that either. Unlike the NASA ones they are produced in quantity; hence the price advantage.

There are other versions of the Ebay meters which are similar to the NASA BM1....http://www.ebay.co.uk/itm/170869014075?_trksid=p2055119.m1438.l2648&ssPageName=STRK:MEBIDX:IT
There are several different models so it would be necessary to see what is best for any particular setup, but they do offer estimates of capacity and percent discharge.I've not tried one though, but it is on my todo list.

You could switch from one shunt to another to take readings from different sources but you would need to be careful not to damage alternators by open circuiting them whilst switching and, if you did it, I don't know what it would do to the capacity estimates. It might make them meaningless. Personally I would use one main meter which would cover voltage, current and capacity, with simple ammeters for secondary sources of power.

If you don't want to calibrate ammeter/voltage meter just buy one that doesn't need calibration. There are lots to choose from on Ebay and elsewhere.
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I bought one of these amp meter/voltmeter/cumulative AH meters. Sure it was cheap enough and volt meter was fine. However the amp meter was very difficult to calibrate and unstable at zero set. I discussed this with another forum member who also had similar problems. It should be a really good gadget although instructions and set up can be confusing regarding battery %. Anyone else bought one of these? olewill
 
Yes, I have one. It needed calibration for the voltage to be accurate and I found a pot that did that, but the amp readings didn't seem right and, as mentioned above, I've come to the conclusion that the voltmeter tells me all I need to know, so I haven't bothered yet. I probably will one day, when I get to the the end of the to do list - If I live that long!
 
he NASA BM-1 is quite complicated to fit, and IMHO doesn't offer much more than a two decimal place VM from EBAY. I fitted a BM1 to my last boat and it gave up the ghost within two seasons, and what it didn't do was put any extra power back in the batteries after other boat's services had used it up !

I now have a 2 decimal point VM on each battery bank, which I regard as reasonably accurate to the first decimal place. They still don't re-charge the batteries, which often is the real problem for those requiring to know where their batteries are at every minute.

So turn off the standby battery, relax, and enjoy the boating !
 
Rigger Mortice said:
.....

Personally I would use one main meter which would cover voltage, current and capacity, with simple ammeters for secondary sources of power.
.....
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That sounds the best idea. I understand the point made by various posters that an important function is that you know your batteries are being charged but I have a couple of voltmeters which give a rough indication and I sort of instinctively know all the time where we are up to regarding battery capacity/usage. I also have a controller for the wind genny which gives a constant readout of the amps being generated but that is buried away in the battery lockers. I acutally enjoy looking at the all the free energy we are getting so want a meter I can mount at the nav station just for fun - would this be suitable ?
http://www.ebay.co.uk/itm/Auto-Mari..._Measurement_Equipment_ET&hash=item19b96aa642
 
Storyline said:
- would this be suitable ?
http://www.ebay.co.uk/itm/Auto-Mari..._Measurement_Equipment_ET&hash=item19b96aa642
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If you want an analogue one it would be Ok, with one proviso. If your genny is producing up to 10amps it would be fine. If your genny only produces, say, 2 amps you will only ever get readings over a small part of the dial. The difference between, say, half output and full output will result in only a small deflection on the dial. Likewise at the lower end of the scale (where the genny is likely to be spending a lot of its time!) the deflection will be very small indeed.
Personally, I prefer digital meters for this kind of application.
 
Actual current need not be measured accurately in most situations. So what Rigger Mortice says is perhaps not so much a concern. The meter does need to be able to cope with max current it will see. Actually anlogue moving coil meters have much to commend them as an amp meter. A movement like 50 microamp FSD can mean you need a very low resitance shunt. Hence very small volt drop. Indeed if you are measuring alternator output you may find the existing feed wire of sufficient resistance to give a suitable reading.
You would need a calibrated amp meter to use to calibrate your new meter. It is fairly easy to open up the meter and carefully relable the meter to what you want.
The probelm with most digital meters is that they are a 200mv volt meter. So any shunt will drop 200mv for max current reading unless it has an amplifier or magnetic pick up.
good luck olewill
 
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