Amp reader..

[Mickkeee]

Hi..
I have a swift conqueror 630 2010. It's got the 100w truma solar kit built in. So far after 2 years I've never needed to plug in.. I run a tv at 33w (around 2.5 amps) and only run the wet system and lights to which I've worked out to about on average 20w.
However I do like to see the amps I use live.. so I bought a digital display amp reader and I've just crudely wired it up between a cigarette ext lead so I no how much my tv was actually using.. but.. I would now like to wire it to the hole of the 12v system in the caravan. However I'm a little confused on "shunts" the one I have witch is matched to the digital reader is 50a 75mV.. is this ok to use safely?or do I need to match it to the leisure battery ampage?? (100a)
Thanks in advance.
Ollie

 

[Mark31415]

Not entirely sure what it is you want to measure. Problem is if you put it in line with the battery wiring you'll get a mixed reading of draw and charge. Not sure quite what it will make of that. For the max draw, the 100A figure you have is probably amp/hours rather than max amps. In reality apart from the motor-mover you are likely to be under 50a unless you have some sort of 12v water heater which is also unlikely.

Knowing your amps in reality is useful if your battery is draining while left alone or if you are trying to see what you can plug in without tripping the site electrics. For drain issues you'll probably want to use a multi-meter so you can move it around the system. For the site mains you'll need a 240v device so the 12v one you have won't work.

I suspect you are setting yourself up for some fun wiring for no real benefit. Simplest way would be to disconnect from site mains, put your ammeter in line with one of the battery terminals and see what happens when you turn your 12v devices on and off. I doubt you'll have any meaningful info from leaving it permanently wired up and may well cause some sort of issues with charging depending on how it copes ( or doesn't) with reverse current flow. ie, what will it do when battery is charging rather than draining.

 

[VicMallows]

Firstly, the shunt you have is fine for everything except the motor-mover. It is usual to leave the mover directly connected to the battery, and connect EVERY thing else via the shunt. The mover will of cause use some power which will not be recorded, but the actual Ah typically consumed is surprisingly small.

I consider a monitor which can show instantaneous power consumption (Amps) and cumulative charge/discharge of the battery (Amp hours) an absolutely vital item for good battery management, especially if you are frequently 'off-grid'. Yes, the instantaneous current with it wired as above will be the net charge into/out off the battery, but you can always switch off the PV and mains charger if you want to measure the consumption of a specific item.

A long established and simple to use monitor is the NASA BM1 (and BM2) at around £100. However very good (and in some respects more versatile) monitors can be sourced from China at around half the price.
(I've had a NASA on my boat for over 15 years; and a Chinese version in the caravan for 3 years).

 

[Mickkeee]

Thank you to you both.. that has answerd my question..
Apologies.. this is only for 12v.. I will never use 240v
I'm measuring the amps being drawn from the battery.. the wire diagram shows you can wire it up either with amps in from the charger or amps out.. just by changing the one wire on the shunt.
Regarding the monitor.. I've got this.. I've got the truma monitor that plugs into the solar charger witch gives me very good info of what going into the battery.. hence now I want to see what's comming out for that good battery management.
Regarding draining my battery down.. I've lived in my caravan since owning it so it very difficult.. specially this time of year when the solar panel is producing more than I need.
Don't have a motor mover but I think for safety I'll get a 100 amp shunt just because I'm a worrier.. don't want my home up in flames
Again thanks to you both..

 

[ProfJohnL]

Hello mick,

As someone else pointed out, the caravan mover is best to connect direct to the battery terminals, becasue of the peak currents it may draw. any thing even a A shunt will impede the current and might be counter productive.

The other disadvantage of going for such a large current shunt, is you will loose resolution on the typical currents normally drawn by the caravan. I would suggest leaving the mover connected directly to the battery, and only measure the current on the other cable that feeds the caravan. You should only need about a 20A shunt, Or consider using a DC current clamp ( yes they do exist) This will remove the need to break any wires, and they will handle both positive and negative current flows.

 

[paws]

Just for information,
Mickkeee ( The OP) mentioned previously that he doesn't have a "motor mover"
Regards to all and as always
Happy Caravanning
paws

 

[BaileyNiggles]

Might someone humour me by explaining what these "shunts" are?

 

[Anseo]

Might someone humour me by explaining what these "shunts" are?

# tag me too, we deserve an explanation of what shunts are

 

[Woodentop]

Might someone humour me by explaining what these "shunts" are?

A moving coil meter - the sort used in older analogue meters - has an internal resistance and requires a certain current to show as full scale. Typically the meter might have a resistance of 1000 ohms (1000R) and a current requirement of 50 microamps (50uA). By Ohms Law the voltage across the meter at full scale will be 1000x0.00005=50mV.
A shunt is a resistor of very low value (and in this case high power) that will cause a 50mV drop across it at the maximum current you want to measure.
Therefore if you want to measure a maximum current of 50A and achieve a voltage drop of 50mV, again by Ohms Law
R = V/I or 0.05/50 = 0.001R or 1milliOhm
Power = VxI so 50x0.050 = 2.5W. To make sure the resistor doesn't get too hot you should get a 20W resistor and bolt it to a chunk of metal to dissipate the heat.

As Prof(?) said a 50A meter won't show low currents as drawn by lights or whatever so the way to do it is to make the meter read different voltages. To save going into a lot of description and maths to give you a meter that can read, say, 5A, 15A, and 50A full scale but using the same meter movement as above (1000R and 50uA) you will need the following:
5A: a 0.01R shunt will match the meter perfectly
15A: a 0.01R shunt with a 2000R resistor in series with the meter
50A: a 0.01R shunt with 9000R in series with the meter.
Putting a resistor in series with the meter increases the voltage required to give a full scale reading.
2000R is achieved by putting 1K8 and 180R in series - these are preferred resistor values
9000R will be 8K2 with 820R in series
The slight overall resistor value errors are insignificant in terms of readings
However, the shunt resistor of 0.01R with 50A flowing through it will need to be 25W rated!

 

[ProfJohnL]

Might someone humour me by explaining what these "shunts" are?

When measuring the current in a a circuit one of the common methods is to use a meter that simply connects in series in the circuit. But most meters only need a few milli-Amps to fully show full scale current.so that would be far too sensitive for the sizes of current that can flow in caravan wiring.

To reduce the sensitivity of the meter so it can show bigger currents a low value resistor is connected in parallel to the meter which provides a partial bypass or shunt route for most of the current leaving just enough flowing through the meter. The value of the" shunt" is picked so the meter will still show the correct numbers , but the reading will need to be multiplied by the ratio of the shunt resistance to the value of the meters inherent resistance to scale the value displayed. The shunt values chosen usually give actual current capacities in multiples of 10 so the numeric value displayed still remains true.

These days there are non invasive methods of measuring currents reasonably accurately. With AC its simplhy a clamp transformer, but there also now methods of detecting DC currents, but these systems are relatively expensive and are not likely to be fitted in a caravan.

 

[BaileyNiggles]

Hmmm, yes, understood the shunt principle as regards shifting a meter's scale and measuring current by way of a voltage drop across a known resistance. I was ... and I confess I might need re-read some of the foregoing posts ... not really getting that meaning coming across from what was being said.

 

[Guzzilazz]

I am a newbie with caravan 12v systems, but have many years of boat 12v experience. You rarely need to have need to continually monitor current, but frequently it's useful when troubleshooting. A few years back I spent about £70 on a DC clamp meter, where you place a cable (battery positive is a good one). Turn all circuits off, then switch them on, one at a time and the meter will display the current draw for that circuit. I have a compressor cold box and could see that compressor running was drawing 5A but that it would run only 5 minutes, and then switch off for 25 minutes. So the average draw was 5A for 10 minutes in an hour so in that hour the fridge draws 5A÷(60 minutes ÷10 minutes) =0.83Amp Hrs, or 20A per day.

 

[Icaru5]

Most modern caravans, especially those built during the last 3 years or so have permanent current monitoring as standard, even on entry level systems.

Mine has the Sargent EC600 PSU/Consumer unit and the EC620 control panel which displays the current loading in real time. With yours being a 2010 model OP, would it be worth investigating if the EC620 can be retrofitted ? -It does of course depend on the type of PSU/Cubsumer unit fitted, as I would imagine that upgrading to both the EC600 and EC620 would be beyond the scope of your project in terms of budget ?

 

[Woodentop]

Most modern caravans, especially those built during the last 3 years or so have permanent current monitoring as standard, even on entry level systems.

Mine has the Sargent EC600 PSU/Consumer unit and the EC620 control panel which displays the current loading in real time. With yours being a 2010 model OP, would it be worth investigating if the EC620 can be retrofitted ? -It does of course depend on the type of PSU/Cubsumer unit fitted, as I would imagine that upgrading to both the EC600 and EC620 would be beyond the scope of your project in terms of budget ?

Er 'most' modern caravans? We have just bought a new Bailey and looked at Lunars, Coachman, and Swift and I have never seen one yet. Voltmeter yes but never an ammeter.

 

[ProfJohnL]

Hmmm, yes, understood the shunt principle as regards shifting a meter's scale and measuring current by way of a voltage drop across a known resistance. I was ... and I confess I might need re-read some of the foregoing posts ... not really getting that meaning coming across from what was being said.

The OP first mentioned he had a 50A shunt for his meter, which implies it was a sensing load resister to produce a PD across it to be measured probably by a voltmeter. This is how most of the cheaper Ebay units seem to work.

 

[ericmark]

As said hall effect meters are expensive, and to measure amps the whole of the power goes through the meter, so you would need very heavy cable to meter and back, so instead we measure volt drop, so wires to meter can be thin.

CAV used a shunt to measure alternator output, and it was nothing more than a bit of steel with saw cuts in it. My car multi-meter came with something like a hacksaw blade to measure high current it was nothing more than a strip of steel, if you have a means to calibrate it could be the cable connecting battery to fuse box.

So a 200 mV meter to measure 20 amp using ohms law needs a resistance of 0.01 ohm, and 2.5 mm sq cable is around 0.035 ohms per meter when running at 20A so around 0.3 meters of cable would have around the right resistance to let a 200 mV meter measure 0-20 amp.

The problem arises that copper cable resistance changes with temperature, So for accurate measurement you would not use copper, and the shunt needs to be kept cool, but the shunt could be simply a length of wire and you measure the volt drop across it.

 

[JTQ]

If it is actually Amps current, rather than Amp hours that the OP wants to monitor in the 12 volt system, then this little cheap device is well suited for use in circuits up to 20 Amps:

https://cpc.farnell.com/tenma/ten01046/current-tester-automotive-std/dp/IN04874
In use, it plugs into the system's flat fuse holders, with the removed fuse refitted into its inbuilt fuse holder. Mounted as such the system functions as normal whether the tester is switched on or not so can be left there if wanted, the reading being seen by switching it on. They are immune to the orientation inserted, readily reading positive or negative as appropriate.
If the van features the mini fuses rather than the normal size, an alternative model for this is available from the same source.
IMO a very useful tool to have in one's collection for its modest cost.

For years I had one in my solar feed into the battery; I now though have an Ah meter a feature of my Morningstar solar controller.

 

[Icaru5]

Most modern caravans, especially those built during the last 3 years or so have permanent current monitoring as standard, even on entry level systems.

Mine has the Sargent EC600 PSU/Consumer unit and the EC620 control panel which displays the current loading in real time. With yours being a 2010 model OP, would it be worth investigating if the EC620 can be retrofitted ? -It does of course depend on the type of PSU/Cubsumer unit fitted, as I would imagine that upgrading to both the EC600 and EC620 would be beyond the scope of your project in terms of budget ?

Er 'most' modern caravans? We have just bought a new Bailey and looked at Lunars, Coachman, and Swift and I have never seen one yet. Voltmeter yes but never an ammeter.

My 2017 Swift has an ammeter function, accessible from the EC620 control panel above the door. That system was only fitted to the high end models prior to 2017, but from 2017 became standard across the range. For 2019 the control panel is a new, even more modern design but I very much doubt they would remove this useful feature.

Are you sure that Bailey, Lunar and the others don’t have an ammeter ?

On Sprite and Swift models you can monitor current draw for the 240AC system, the leisure battery and even the vehicle battery.

I’ve just been out to the van and taken some pictures to demonstrate ...

 

[Craigyoung]

Same here , but you have to search through the control panel functions to find this bit of feature .

 

[Icaru5]

Ammeters also displayed in mobile app so you can monitor your power consumption from the comfort of whichever pub takes your fancy !

 

[Craigyoung]

Lc ,
You beat me to it ! I was just going to pop out to my van and put the display on as it's on the drive but since reading your last post you've added the pictures

 

[Icaru5]

Look what Swift are now fitting to the 2019 vans Craig ...

This also includes a DAB and FM tuner, which would render the standard fit £7.50 JVC unit surplus to requirements !

I’ve just emailed Sargent enquiring as to the feasibility of an ‘upgrade’

Will keep you posted !