Ammeter technical question

[AOWYN]

I want to fit two ammeters into the charging circuits from a solar panel and from a wind generator so that I can see instantly what they are each producing. I am interested to learn how much power is actually lost, consumed, within the ammeters as over a week that could be a fair amount of power lost just for the pleasure of me being able to satisfy my curiosity while I am on board. The question is, does anybody on the form have information about the power losses within the types of meters available, digital versus analogue, etc?

 

[john_morris_uk]

I haven't got specific figures for the various ammeters, but I can reassure you that the losses on most digital ammeters (and most of the analogue ones) are very low.

They work by measuring the voltage across a shunt in the circuit.

If 50mV across the shunt were equal to 50 amps (Not an unreasonable ball park figure for such a device) then the resistance is R=.05/50 =0.001 Ohms and the power loss though having the device in circuit when 50 amps are flowing though it will be 2.5 Watts

 

[john_morris_uk]

I haven't got specific figures for the various ammeters, but I can reassure you that the losses on most digital ammeters (and most of the analogue ones) are very low.

They work by measuring the voltage across a shunt in the circuit.

If 50mV across the shunt were equal to 50 amps (Not an unreasonable ball park figure for such a device) then the resistance is R=.05/50 =0.001 Ohms and the power loss though having the device in circuit when 50 amps are flowing though it will be 2.5 Watts

PS - the 2.5 watts would be out of a total power going into the batteries of 50 amps times 13.8 volts = 690 Watts so the 2.5 watts lost in measuring is insignificant.

 

[SHUG]

If the meter drops 50mV across it when a current of 50mA is passing through it then the power loss is 2.5mW which is indeed insignificant. The appropriate shunt would be 0.001 Ohms for full scale deflection at 50A since the meter has a resistance of 1Ohm. The dissipation in the shunt would be 50mW which is still a very low figure.
So, total dissipation is 52.5mW (ie 52.5 thousandths of a Watt)

 

[pvb]

If the meter drops 50mV across it when a current of 50mA is passing through it then the power loss is 2.5mW which is indeed insignificant. The appropriate shunt would be 0.001 Ohms for full scale deflection at 50A since the meter has a resistance of 1Ohm. The dissipation in the shunt would be 50mW which is still a very low figure.
So, total dissipation is 52.5mW (ie 52.5 thousandths of a Watt)

Even though you've edited it, it's still wrong! Shunts are usually designed to produce a 50mV drop when their rated current is flowing. john_morris_uk correctly said that a 50mV shunt rated at 50A would only lose 2.5w.

 

[VicS]

john_morris_uk correctly said that a 50mV shunt rated at 50A would only lose 2.5w

.But it would only lose 2.5watts when the full 50amps was flowing.

if only 25 amps is flowing the loss will be 0.625 watts

If 10 amps is flowing it will be 0.1 watts

if only 1 amp is flowing it will be 0.001 watts

All above assuming a 50mV /50 amp shunt is used but ignoring the power consumption of a digital meter

Ask what you thought was simple question .........!

Forget the fecking ammeters hoist the sails... go sailing!

.

 

[noelex]

The consumption of the meter needs to be taken into account. This will be small but does vary a lot and can be more significant than the shunt. Particularly if the boat is winterized and the meter is left on..
50mV 100A shunts are used by the popular NASSA bat monitor, but many others use a 50mv 500A shunt. Reducing the power consumption of the shunt even further.

I applaud the goal of reducing the power consumption but shunts consume very little, but be careful of the battery monitors, or even simple digital ampmeters, if the boat is left with no charging source for a long periods. Battery self discharge will be more significant, but its often worth powering down the battery monitor.

 

[VicS]

To put some perspective on the power consumed by battery monitors.
The Nasa BM1 takes only 1.5mA

In 1000 hours thats only 1.5Ah. Next to nothing which is they are normally left on

 

[Billjratt]

I have the same disease as you. It's comforting to know that your charging appliances are doing what they say on their relative tins....
However, I HATE SHUNTS ..... whether they are inside cheap ammeters or connected to your wiring somewhere. They are added connections and resistance, no matter how small and precisely calculated.
The fact is, anything with a resistance is a shunt - a voltage drop(proportional to the current flowing) will be measured across it.
So measure across the existing wiring - I moved all the negative feeds to the boat domestics etc. from the battery negative to the starter negative (maybe engine block in some cases?) and then put a center-zero meter across that feed. A preset potentiometer at the meter allows it to be calibrated to a known load like the cabin lights. There's about 40 mA drop, but the 50uA meter only wants about 5, so it can be tweeked to suit.
Having just had a seawater invasion, I am very aware of the problems caused by extra connections involving shunts, among other things.

 

[noelex]

The NASA monitor has low power consumption, but they do vary a bit as I indicated in my post. the Link 20 has for example a typical consumption of 90mA . The blue seas digital ampmeter 0.6 to 1 watt both some 50X greater than the NASA.

 

[pvb]

To put some perspective on the power consumed by battery monitors.
The Nasa BM1 takes only 1.5mA

That's if you believe Nasa's website. If you look at their BM1 manual, it says 3mA, and I'd imagine that's without illumination, if it's true at all.

 

[VicMallows]

That's if you believe Nasa's website. If you look at their BM1 manual, it says 3mA, and I'd imagine that's without illumination, if it's true at all.

It's significant enough that I disconnect mine when I expect to be away for more than a few weeks.

And (general comment) no! ... it doesn't store any 'learned' information about your batteries. You will simply need to fully charge your batteries when you return and reset the BM-1 AHr counter to zero.

Vic

 

[SHUG]

Even though you've edited it, it's still wrong! Shunts are usually designed to produce a 50mV drop when their rated current is flowing. john_morris_uk correctly said that a 50mV shunt rated at 50A would only lose 2.5w.

OOPS!!! you are of course correct!

 

[noelex]

I don’t want anyone to get paranoid about there battery monitors. The amount of power they consume is small.
A display showing the amount of power consumed is a powerful reminder to turn off unnecessary loads. By providing accurate information they will help you make sensible decisions about when to start and stop recharging. They will keep your batteries in much better shape and are easily worth the small electrical cost.
Most boats should fit one.

If however you are leaving the boat for long periods without a charge source its worth considering disconnecting them. Many are wired to be permanently connected even with the battery switches off. This would concern me more than the extremely small power loss in the shunts.
If you have any doubts the handbook will list the power consumption There is a great deal of variation between models. Batteries have higher self discharge rates than many people realize and if you leave them without a charge input small power drains add to the self discharge rate and can drop the charge levels to damaging levels.

 

[john_morris_uk]

Golly, there's a lot of hot air in some of these posts.

1. The OP said he was interested in the power lost by his ammeters whilst they were in circuit.

2. We don't leave our battery monitors connected all the time - when the batteries are isolated then they are ISOLATED - nothing on at all.

3. Those who don't like shunts I think are a little paranoid about the extra connections. In Serendipity, when I turn the batteries on, the first thing that comes on is the battery monitor; it has no separate switch and is 'on all the time' through its own fuse. Everything in and out of the banks is monitored and for that peace of mind and knowledge I put up with a shunt on each bank and the handful of extra connections. The connections are five years old and look like new...

 

[pvb]

Golly, there's a lot of hot air in some of these posts.

1. The OP said he was interested in the power lost by his ammeters whilst they were in circuit.

2. We don't leave our battery monitors connected all the time - when the batteries are isolated then they are ISOLATED - nothing on at all.

3. Those who don't like shunts I think are a little paranoid about the extra connections. In Serendipity, when I turn the batteries on, the first thing that comes on is the battery monitor; it has no separate switch and is 'on all the time' through its own fuse. Everything in and out of the banks is monitored and for that peace of mind and knowledge I put up with a shunt on each bank and the handful of extra connections. The connections are five years old and look like new...

Hot air and confusion! In (2) above, you say your battery monitor is not connected all the time. In (3) above, you say it is on all the time, through its own fuse. Is one of those correct?

 

[VicS]

Hot air and confusion! In (2) above, you say your battery monitor is not connected all the time. In (3) above, you say it is on all the time, through its own fuse. Is one of those correct?

Maybe I'm wrong but I took it to mean that the monitor is on all the while the battery isolator is on ( it has no power switch) but that when the isolator is off the monitor is off as well.

That seems reasonable if there are no permanently on circuits such as a auto bilge pump, solar or wind charger.

 

[pvb]

Maybe I'm wrong but I took it to mean that the monitor is on all the while the battery isolator is on ( it has no power switch) but that when the isolator is off the monitor is off as well.

That rather defeats the purpose of a battery monitor, as most of them will default to factory values and lose their stored data when disconnected from the battery. In fact, the monitor would just become a fancy voltmeter/ammeter.

 

[William_H]

Ampmeters

The OP asked about ampmeters and the discussion then went to battery monitors. I am assuming he really meant Amp meters.
The OP has given no indication of just how big his solar array is. ie are we talking 1/4 amp or 10 amps?

An analogue amp meter will have a moving coil meter probably taking about 1ma at full scale deflection at about 1 millivolt. It would be normal to select an amp meter with a full scale of something just a little more than the max capability of the panel. The analogue meter will then drop about 1 millivolt at full charge current. In others words nothing!

Now a digital amp meter will usually require 200mv to reach max readout (actually 199.9mv) So if you set up the shunt to give max panel current near to the 200 reading at max current you will lose 200mv in the shunt.NB if the reading is only 100 then it is 100mv lost if the reading is only 10 then 10mv lost. Even that is not really significant considering the internal resistance of the panel. Or put another way the panel usually provides 20 volts and you only need 14v so 6v is lost in the panel or regulator so the 200mv lost will mean less loss in the panel or regulator. The digital meter will require also some small current to run the meter but this can be left off until you actually look at the meter.

So the bottom line is don't worry about power lost in the amp meter. Even for a small power panel.

A friend fitted a 10w panel to his boat. He bought a little 20v analogue volt meter which he connected up.
he then discovered that his depth sounder actually had a volt meter built in. I was able to pull the volt meter apart and found the resistor connected internally to the terminal. Known as the multiplier resistor typically about 20000 ohms this makes it a volt meter. I removed the resistor and fitted a shunt resistor on the terminals at the back. This made it into an amp meter. he needed .5 amp for full scale being a 10w panel so I relabelled the dial. The shunt resistor was made up of small value 1/4 watt resistors in parallel.
I forget the precise value but if the meter were 1000 ohm internal resistance 1 mv and 1ma for FSD then
.5 ohm shunt resistor is needed.
If you want to try it you can try a .5 ohm resistor then using a digital multimeter on amp scale check the calibration and change the value of the shunt accordingly. Note in this example if 5 amps was expected for a 100w panel then a shunt resistor of .05 ohm is needed. This can be made from 5 .1 ohm resistors in parallel.

I think analogue meters are just fine for current. You don't need great precision or accuracy.
But reiterating don't worry about power lost in the shunt in a solar circuit. On the other hand in a normal power circuit the power lost in the shunt for a digital amp meter can be significant if the current approaches the max for the meter/shunt combination. (not usual) good luck olewill

 

[john_morris_uk]

Maybe I'm wrong but I took it to mean that the monitor is on all the while the battery isolator is on ( it has no power switch) but that when the isolator is off the monitor is off as well.

That seems reasonable if there are no permanently on circuits such as a auto bilge pump, solar or wind charger.

That's exactly how the boat is wired. When I switch the isolator switch off - that is exactly what happens - the batteries are completely isolated except for the wind generator. I don't think that there's anything contradictory in my previous posts - and my apologies if I didn't explain myself clearly enough.

In reply to pvb who suggests that this makes our monitor little more than a fancy ammeter and voltmeter, I don't think that this is true. Certainly the Ah meter resets to zero when the batteries are isolated, but I don't find this a problem. Because I always leave the batteries charged when I leave the boat, and there's a wind generator (with a clever charging controller to top up any self discharge) for the battery monitor to start at zero discharge, which is its default position at switch on, is reasonable. When we live on board for extended periods, the battery monitor tells me how much deficit we are in on the domestic banks in Amp-Hours. It simultaneously tells voltage on the bank and current going in or out.

Although we have a electric bilge pump, its not 'permanently on'. The bilges on our boat aren't dry - we get some rain water down the anchor chain etc, but even after months without attention its never a problem. I have a quick look in the bilge when I first step on board, but after that they get mostly ignored.

The only things that come on (through their own fuses - and not through the circuit breakers on the panel) when the battery isolator is switched to 'on' are the Battery monitor, the Eberspacher, the Navtex and the SSB Radio and I am still trying to decide whether to have a 50 amp circuit breaker to separately isolate the HF radio or not - at the moment its just fused.