How to measure electrical consumption

[bbg]

It looks as if I may not be able to go sailing tomorrow - pilot still in for repairs.

In case I can't go out, I'd like to use my time productively, and perhaps measure the electrical consumption of everything on the boat. That will help on longer trips, as I'd like to know what are the big consumers and shut them off if not necessary.

I have a multimeter, but how do I actually measure consumption? Where do I put the probes and what should I be measuring?

I really need an idiot's guide to doing this. Step by step. The only thing you can take for granted is that I can read English.

 

[VicMallows]

With 2372 posts dating from 2005, and your profile, this looks like a wind-up. Convince me otherwise and I'll answer.

 

[bbg]

I know how to sail. Put me on deck and I'll make the boat go.

Put me next to the batteries and my best friend is a mobile phone and an electrician's number.

That's all I can tell you. No wind up.

Edit - I see you're in Chichester. I was going to invite you to come and visit the boat before I do the UK Mini Fastnet in May and prove to you in person what an electrical numpty I am, but the start is in Plymouth and that might be a bit of a hike for you.

 

[skyflyer]

You need to set the multimeter to read DC amps and then 'interrupt' the circuit so that the current flows in through one lead and out through the other.

Think of electrics as water running through a pipe and you are trying to measure the flow.

What you are doing is the equivalent of breaking the pipe and putting one end on the inlet of your flowmeter (ammeter) and one end on the outlet - thus the flow still happens but now it is running through the meter.

So, practically - remove just one of the connectors/leads to the item whose current you want to measure. Put the red and black leads of the ammeter across the 'gap' you have created. Operate the tem and see what the consumption is.

Do this for each item and add it all up.

OR

Disconnect the battery and run the ammeter between the 'gap'. Use some electrical tape or similar to keep the connections from coming apart.

Now, as you turn on each item of equipmenet (just one at a time) you can see how much current it draws.

It does get more complicated, but thats the gist of it!

 

[ostell]

Your meter should have a setting for current. You will need to set it to this. Beware though that if it is a cheap meter it may not be able to handle the current. You may have to move the leads to another socket,

If your meter will cope with several amps simply set the meter to current and put the probes across the switch for the device you want to measure. Easy if the switch has only two connections. If more than 2 connections, and especially if an illuminated switch, then think again.

 

[Playtime]

Before you try this, please make sure your meter can handle the largest current expected - otherwise you will blow the fuse in the meter or worse.

Ah - Ostell beat me to it

 

[sarabande]

the alternative is to use a clamp meter set to read amps. This way you don't need to cut any wires or remove any connectors.

e.g.

http://cgi.ebay.co.uk/Large-AEG-Cla..._Measurement_Equipment_ET&hash=item336237ae16

There are modern ones for about £60, and I reckon they are a good investment.

Or you could read off the wattage from the device's labels - though in general devices tend to use up quite a bit more current in the starting process, than simply running e.g. fridge.

It is probably one of those exercises where too much detail is not worth the bother. Precision vs accuracy.

 

[bbg]

skyflyer - thanks for that. It makes sense.

For most items I can't disconnect at the item itself - there is usually a plug in the back - so I take it I have to follow the wiring back to where it connects to the breaker panel? And remove one of the wires where it is connected at the breaker panel?

And am I correct in assuming that I measure on the item side of the breaker panel compared to the battery side? I don't even know how I'd measure on the battery side. I'll have to look at the wiring but I assume there is a single +ve and single -ve cable leading from the batteries to the breaker panel.

One other issue - the GPS and VHF seem to be wired direct to the batteries without going through the breaker panel. I guess I'll have to trace the wires back to the batteries and disconnect there?

In light of the later posts, what is the symbol for amps, and what kinds of currents should I expect?

Consumers are:
VHF; GPS (no plotter, just a Garmin 126 I think); NASA tricolour light; one interior light; radar dectector; speakers (but I didn't bring my ipod this weekend so can't test that); nav instruments and pilot (again can't test this as the control head for the pilot is in the shop); compasses with lights.

I expect the biggest draw is going to be the hydraulic ram for the pilot but I don't see how I can measure that, even once I get the control head back. At the dock it won't really give an accurate reading because it won't be experiencing much resistance, and in any event the draw is really going to be a function of the weather conditions at the time, which will vary.

 

[bbg]

It is probably one of those exercises where too much detail is not worth the bother. Precision vs accuracy.

It might be important for me. I'll be at sea for up to three weeks with just a solar panel and a fuel cell for power. If I can reduce my consumption enough I might be able to leave one or two 5-litre cans of methanol on the dock.

 

[sarabande]

why not stick an ammeter on to the battery/batteries, switch on or use all the systems one by one and check the reading ? Playing disconnect and reconnect has certain risks of unmaking a satisfactory connection.

 

[Amulet]

the alternative is to use a clamp meter set to read amps. This way you don't need to cut any wires or remove any connectors.

e.g.

http://cgi.ebay.co.uk/Large-AEG-Cla..._Measurement_Equipment_ET&hash=item336237ae16

There are modern ones for about £60, and I reckon they are a good investment.

Or you could read off the wattage from the device's labels - though in general devices tend to use up quite a bit more current in the starting process, than simply running e.g. fridge.

It is probably one of those exercises where too much detail is not worth the bother. Precision vs accuracy.

Am I dumb, or is it not the case that most clamp meters only do AC, which is easy to understand. I have no idea how they do DC, and I seem to think they are more expensive.

My multimeter can do up to 20 amps but warns that it's only to willing to have brief exposure to such currents. If you have an ammeter or multimeter which can handle all of your amps I'd stick it in the circuit at the battery terminal (+ or -, between the cable and the battery) and then switch things on one at a time and see what it reads.

I have NASA BM1's (one for each battery) and read what they say. They have got a bit of a bad press on the forums, but I think they are marvellous.

 

[bbg]

why not stick an ammeter on to the battery/batteries, switch on or use all the systems one by one and check the reading ? Playing disconnect and reconnect has certain risks of unmaking a satisfactory connection.

That is, in fact, a real concern given my (lack of) electrical skills.

How exactly would I do that? Would I disconnect the -ve and run the probes to the -ve terminal and the -ve attachment assembly?

I'm really sorry to be so dense about this but it is really something I don't know about.

 

[sarabande]

any ammeter which relies on a physical interruption is going to be a real pain if you do the connection business.

I have a Kyoritsu clamp meter (Kew Snap 2056R, which reads DC as well as AC. Almost instant readings as soon as you circle the wire.) It reads up to 600Amp - which is quite enough.

Any proper lecky should have one - or similar - you could borrow for a day for beer.

I would be very hesitant about using a small unsophisticated multimeter to check current by interrupting the flow for a lot of devices. It will takes ages.


Where are you based ?

 

[bbg]

Where are you based ?

Lorient. Want to pop over for a beer and lend me yours?

 

[sarabande]

ah ! Shuddav read the bio - oops !

 

[Ru88ell]

I was in a similar position, as in I wanted to know what was going on but had no experience. I bought a NASA BM1 Compact, and now I know what each draws and how long my battery will last. For < £100 a great investment for peace of mind. You could fit one in half an hour.

 

[bbg]

I was in a similar position, as in I wanted to know what was going on but had no experience. I bought a NASA BM1 Compact, and now I know what each draws and how long my battery will last. For < £100 a great investment for peace of mind. You could fit one in half an hour.

I seriously doubt that. You might be able to, but I suspect it would take me half a week, and still look a bodge.

And unless they are filled with hydrogen, they weigh something. Adding weight that doesn't make the boat go faster is a no-no. An electrcian has - quite seriously - advised me to remove the clamps for the battery terminals, drill down into the terminals a bit, then tap them. Then screw a bolt into the terminals and attach everything to that. Just to remove the weight of the clamps (and he says it will also make a better electrical connection).

Maybe when I'm feeling a bit more courageous with my electrical skills.

 

[DaveS]

I seriously doubt that. You might be able to, but I suspect it would take me half a week, and still look a bodge.

And unless they are filled with hydrogen, they weigh something. Adding weight that doesn't make the boat go faster is a no-no. An electrcian has - quite seriously - advised me to remove the clamps for the battery terminals, drill down into the terminals a bit, then tap them. Then screw a bolt into the terminals and attach everything to that. Just to remove the weight of the clamps (and he says it will also make a better electrical connection).

Maybe when I'm feeling a bit more courageous with my electrical skills.

Ah yes, saving weight... When I used to race 20 odd years ago there was one guy in the club who had a passion for cutting holes in his boat to make it lighter...

You say you want to measure your consumption accurately so that you can then calculate the minimum generation fuel you will need. Installing the smaller version of the NASA unit (which probably weighs no more than the battery clamps that you're thinking of binning) will give you a more accurate idea than testing with an ammeter since, as well as displaying V and A, it also records Ah. This is important where you have fluctuating loads, a fridge being the obvious example: with a direct reading of Ah you don't need to estimate a duty cycle etc. Now I'm sure you don't have a fridge, but the principle applies to other things: e.g. instruments which are backlit at night. Spending 24 hours at sea with the NASA recording your actual, as opposed to estimated, consumption will give you a far more accurate answer. You could then, if weight is so critical, remove it. I personally wouldn't, but each to his own.

If, instead, you insist on using an ammeter, then I would certainly agree with those advocating a clip on DC reading type. Clip it round all the cables leaving one battery terminal and you'll measure the total current being taken by everything switched on. Switch things on and off, remembering things like measuring instruments both with and without back lighting on, and write down the current drawn, in amps, for each load. You then need to estimate how many hours each will operate in a day and multiply that by the A figure to get Ah. Totalling these up gives your estimated daily requirement, to which you would be well advised to add a safety margin.

I really do not recommend poking about with a multimeter.

 

[VicS]

Put me next to the batteries and my best friend is a mobile phone and an electrician's number.

I strongly recommend that you read Tony Brooks' Boat Electrical Notes at http://www.tb-training.co.uk/.

Do so before you attempt to use a multimeter for measuring current.

If you can beg borrow or steal a clamp meter suitable for DC amps do so rather than use an multimeter.

For many items you will be able to use the rated power and calculate the current from amps = watts/volts.
For some items the current drain will be given in the specification of the operating manual.

 

[bbg]

In light of the various advice I probably won't be poking around with an ammeter today. I'll see if I can borrow a clamp-on one.

re the NASA BM-1 battery monitor. My set up is as follows: 2x105 Ah batteries with a 1-2-both-off switch for powering the consumers. Charging is via a 25 watt solar panel and a fuel cell. Both have a 1-2-off selector, so I can run on one battery while charging the other, or (for example) leave one battery fully charged while I run on the other and also run the solar panel to it so it doesn't discharge so quickly (or at all during peak sun hours).

For this setup would I need one or two BM1 units, if I chose to get one?