Ammeter versus Voltmeter

[Traveller]

Our boat has an ammeter that shows the amps being pumped out by the alternator. Usually it starts off around 25 amps and drops very quickly to 10 or under. I seem to recall a boat electrician saying to me some years ago now that the ammeter is not really worth the space and a voltmeter does a far better job in showing the state of the battery/batteries. On the boat I had then I did in-fact switch out the ammeter in favour of a voltmeter. Since then I have seen several boats with a voltmeter but no ammeter.

 

So, my question is what is really best? I rather suppose both is the answer but what does an ammeter really bring to the party?

Edited July 27, 2014 by Traveller

[Ex- Member]

Neither are best, both are useful.

[Paul C]

If they're dial-type gauges then all they can do is give a vague visual indication, you need to be able to read the instrument very accurately to do anything useful with the numbers, for example trying to do a power audit by measuring current draw using an analogue ammeter would lead to very inaccurate end results. And trying to determine state of charge from voltage, similar. Once you get decent digital instruments then their results can be interpreted better and lead to useful info etc. Ammeter on its own is only useful if you're looking at it all the time. Combine ammeter and amp-counting; and allow for the inaccuracy in simply interpreting the results without considering charging efficiency and Peukert's law, and it becomes genuinely useful.

 

At the end of the day devices need to "see" a voltage to work, so a voltmeter on its own is the more useful for diagnosis.

Edited July 27, 2014 by Paul C

[Traveller]

Thanks PC

[Sir Nibble]

I seem to recall a boat electrician saying to me some years ago now that the ammeter is not really worth the space and a voltmeter does a far better job in showing the state of the battery/batteries.

Your recollection may be at fault. Neither would be of any use in determining the state of your batteries but rather to determine that the charging system is working properly. I would say a voltmeter would be best for that but the little red light can tell you an awful lot.

[nicknorman]

... and Peukert's law ...

Myth Alert!

[Chalky]

Combine ammeter and amp-counting; .

 

 

A dice can be more accurate than amp counting. There are so many unknowns and variables that they very quickly increase the errors so render the results meaningless. Even the so called constants change after a few battery cycles. More accurate are methods using extended Kalman filters that include accurate models of battery chemistry. I'm not aware of this method being used in boat SOC meters.

 

I have a volt meter and an ammeter. When the battery is low the voltage drops. As the battery charges the current starts off high (>40A) and drops quite quickly (could be an hour or so). Once the current is down to 10A ish the voltage starts to rise. After a couple of hours the voltage gets to about 14.4V and the current is down to about 1A ish. Then the batteries are charged. If the boat has stood for a long time this last section could take 2 days cruising. I tend to think of it as a 3 dimensional surface - current and voltage are X & Y, SOC is Z.

Edited July 27, 2014 by Chalky

[Arthur Brown]

It's not the meter -either of them, but what you do with the readings.

[Traveller]

Chalky, thanks for that it confirms what I thought (or what I was once told).

[Paul C]

 

A dice can be more accurate than amp counting. There are so many unknowns and variables that they very quickly increase the errors so render the results meaningless. Even the so called constants change after a few battery cycles. More accurate are methods using extended Kalman filters that include accurate models of battery chemistry. I'm not aware of this method being used in boat SOC meters.

 

I have a volt meter and an ammeter. When the battery is low the voltage drops. As the battery charges the current starts off high (>40A) and drops quite quickly (could be an hour or so). Once the current is down to 10A ish the voltage starts to rise. After a couple of hours the voltage gets to about 14.4V and the current is down to about 1A ish. Then the batteries are charged. If the boat has stood for a long time this last section could take 2 days cruising. I tend to think of it as a 3 dimensional surface - current and voltage are X & Y, SOC is Z.

 

Dice must be way better than ammeter + voltmeter then! Amp-hour counting is useful if you interpret the results properly! For example, unless you stayed up all night, you'll never know how often the fridge comes on and off - an easy job for an amp-hour counter to answer.

[dmr]

 

A dice can be more accurate than amp counting. There are so many unknowns and variables that they very quickly increase the errors so render the results meaningless. Even the so called constants change after a few battery cycles. More accurate are methods using extended Kalman filters that include accurate models of battery chemistry. I'm not aware of this method being used in boat SOC meters.

 

I have a volt meter and an ammeter. When the battery is low the voltage drops. As the battery charges the current starts off high (>40A) and drops quite quickly (could be an hour or so). Once the current is down to 10A ish the voltage starts to rise. After a couple of hours the voltage gets to about 14.4V and the current is down to about 1A ish. Then the batteries are charged. If the boat has stood for a long time this last section could take 2 days cruising. I tend to think of it as a 3 dimensional surface - current and voltage are X & Y, SOC is Z.

 

We've got one of those Karbon filters on the breather pipe on the holding tank and it does not work too well, it all gets a bit whiffy sometimes. Maybe I should use it for counting amps instead????

 

but seriously, why not just get a Smarguage?????.

 

...........Dave

 

perfe

[Sir Nibble]

My dashboard voltmeter under reads considerably and is only an indicator anyway. I have no other instrument aboard. By this forum's standard my batteries may have been knackered after 6 months but I noticed no fall off in performance for 10 years! Perhaps if we stopped obsessively monitoring the damned things life may be a little easier?

[Chalky]

 

Dice must be way better than ammeter + voltmeter then! Amp-hour counting is useful if you interpret the results properly! For example, unless you stayed up all night, you'll never know how often the fridge comes on and off - an easy job for an amp-hour counter to answer.

 

Provided you know the correct values to use in your equations and you can design the circuit correctly to cancel out noise and quantization error. Most ADCs are 10 or 12 bit (unless you pay a lot for one). If you've got to cover say 100A you end up with 1 bit per 250 ma ish (10 bit adc) or 62 ma for a 12 bit adc. Add quantisation noise of 1 bit and you're either 500ma or 125ma out. In 250ma you could loose some LED lights and low power devices so you need to make it more accurate at the lower end. You could increase the accuracy of the ADC which will cost a lot or add a log amplifier that gives greater sensitivity at the lower end. This will solve your led light problem but will then mean that the top end looses accuracy so you loose again. One way round it is to add an integrator which would help catch fast transients however they're prone to noise and thermal drift which will add errors. These systems can be made to work - they're used in mobile phones however they can get out of sync. The fix is to reset them when you leave them on charge over night so you at least know where full is. If it goes flat during the day you then know where empty is. Unlike a boat a phone tends to have a definite charge / discharge cycle. Boats don't and its this along with system noise and errors that mean that amp counting isn't very accurate.

 

An alternative is to model the battery chemistry and performance and base the SOC off that. If you use a Kalman filter which will interpolate based on known data then resync when it gets an update then you've got the basis for a gauge that can work in a chaotic environment. Kalman filters were first used to pilot space craft based on limited and noisy data. They have been used in several different battery monitoring systems and I suspect that Smart Gauge may be based on this technology (or something similar).

[George94]

Back in the real world, a Smartgauge and a clamp-ammeter that can be used anywhere seems to be the best bet.

[Hairy Animal]

The eBay seller elite.element have a very useful range of combined digital ammeter/voltmeter digital displays, but if fitting an ammeter the current shunt needs to be carefully specified to handle the absolute maximum currents involved, as well as being fitted in the right place. If in doubt consult a qualified electrician, fires can easily be started by getting it wrong!

 

The combined meters are very useful if you know what you're looking at/for, and some of them even have logging/integrating functions so that during discharge for instance, they can show how many amp-hours have been taken from the batteries over a given period, or put in during charging, but you need to take into account that the charging current from the alternator will be partly feeding the boat's electrics as well as charging the batteries.

[Paul C]

The eBay seller elite.element have a very useful range of combined digital ammeter/voltmeter digital displays, but if fitting an ammeter the current shunt needs to be carefully specified to handle the absolute maximum currents involved, as well as being fitted in the right place. If in doubt consult a qualified electrician, fires can easily be started by getting it wrong!

 

The combined meters are very useful if you know what you're looking at/for, and some of them even have logging/integrating functions so that during discharge for instance, they can show how many amp-hours have been taken from the batteries over a given period, or put in during charging, but you need to take into account that the charging current from the alternator will be partly feeding the boat's electrics as well as charging the batteries.

 

Trust me, elite.element's stuff is junk, avoid. There's better ammeters out there, but you won't find them from China, they have recognised/reputable manufacturers brands and (unfortunetely) a price to match. Trust me, I've been through 4 of them trying to find a decent ammeter for the boat.

 

Where is the right place then?

 

What's the qualifications for working on boat electrics?

 

If the ammeter shunt's been fitted in the "right place" then why do you need to take into account the alternator charge feeding the boats electrics too?

 

 

 

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The "right place" is between the domestic bank negative terminal and ALL other negative connections, including the engine earth strap, hull grounding strap, engine/start battery negative, inverter negative, negative from battery charger, negative from solar MPPT controller, main boat electrics negative to fusebox/distribution circuit breakers, etc etc. If its fitted (properly) here, then it will monitor the electricity going into and out of the domestic bank. If the alternator is generating say 30A, but 10A is being used for boat electrics (eg 5x 2A lights or whatever) then 20A will go into the domestic bank (charging it) and the shunt will only 'see' 20A, so no need to do sums in the head just because the engine's running. And this holds true for any charging method, and any discharge.

[Hairy Animal]

 

Trust me, elite.element's stuff is junk, avoid. There's better ammeters out there, but you won't find them from China, they have recognised/reputable manufacturers brands and (unfortunetely) a price to match. Trust me, I've been through 4 of them trying to find a decent ammeter for the boat.

 

Where is the right place then?

 

What's the qualifications for working on boat electrics?

 

If the ammeter shunt's been fitted in the "right place" then why do you need to take into account the alternator charge feeding the boats electrics too?

 

 

 

----------------

 

The "right place" is between the domestic bank negative terminal and ALL other negative connections, including the engine earth strap, hull grounding strap, engine/start battery negative, inverter negative, negative from battery charger, negative from solar MPPT controller, main boat electrics negative to fusebox/distribution circuit breakers, etc etc. If its fitted (properly) here, then it will monitor the electricity going into and out of the domestic bank. If the alternator is generating say 30A, but 10A is being used for boat electrics (eg 5x 2A lights or whatever) then 20A will go into the domestic bank (charging it) and the shunt will only 'see' 20A, so no need to do sums in the head just because the engine's running. And this holds true for any charging method, and any discharge.

Ah, interesting that you've found these meters to be disappointing. I've bought several of them but not fitted them yet (the restoration/electric conversion hasn't got to that stage yet) so I have yet to discover how well they work, or not as the case may be!

 

Indeed, quite right about the 'right place'. I was only thinking about monitoring the alternator current.