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Best Current sense option on a boat?

Discussion in 'General Electronics Chat' started by Little Ghostman, Apr 19, 2017.

  1. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    I am finally getting around to sorting out my Norman 20 cruiser, its originaly a river cruiser. However I have used it for inshore fishing and it has been partly adapted for this, I intend to finish the adaptions and so far its been really stable at sea.

    It has a very old but good suzuki 65 out board on it, I upgraded the battery to a heavy duty leisure battery, the bilge pumps have also been upgraded but the main cabin one I want to refit and upgrade to a bigger one. the electrics on a whole are a real mess. It does have a boat switch panel, but this could do with replacing and I would like waterproof switches, the main awning and top screen was removed as part of the sea conversion (it helped stability alot removing it).

    The lights are being replaced with high power leds instead of the bulbs that are currently fitted. I am looking at putting 3W Leds in nav lights etc, But maybe 10W leds in for the deck lights and cabin lights. I havnt decided what to use for the spot lights yet, doing a few tests the leds seem alot brighter than the current bulbs and use alot less power.

    While working on the boat I had to pump alot of rain water out as the boat has stood alot over winter, the main cover is shot!! I noticed the battery although fully charged at the start had a fair drain after a few hours, I finally tracked it down to a bilge pump that the cable had been chewed a bit. So being under water it was shorting a bit.

    The boat also has hydraulic electric trim, and I would like to add in some kind of main cabin heater. Probably something really simple like a large industrial fridge compressor radiator, heated with a couple of peatier cells and a 1Amp 12V fan (might change all that). The Battery is used both for starting and all electrics, the engine charges the battery when the engine is one. When I use the boat I have periods of upto 6-10 hours with the engine off, I would like a energy efficient way to sense the current load on the battery and the Battery voltage and discharge rate.

    My reasoning being If something goes a bit wrong like the bilge pump did, I can isolate or decide to switch off anything I need too if the battery starts to get low. The engine does have rope start as well, But using that is a long way from funny!!

    So Any suggestions on what kind of circuit would be best for current sensing? I have looked at some linear technology devices and circuits, but I cant decide on high or low side, or if I want to do both. So any suggestions on what would be the best way? I am also likely to add in small solar panel, I dont have a 240V inverter on the boat and I dont at present intend getting one.

    The Boat is a blank canvas pretty much, needs alot of work doing but is basically sound and I have been using it. I did have a stand by out board but had to sell it, I know This is risky but in all honesty it will be a while before I get another.

    I do have a fish finder, but I use paper charts, I would like to get GPS charts at some point. I am also looking at buying another project boat, mainly because it comes with alot of extras I could use, although its unlikely I would use the Hull itself. It is however a cheaper way to upgrade mine!! sailing boats in general are going really cheap, so a good opportunity to use one to upgrade mine.

    Any thoughts and suggestions? On doing the electrics? Oh it dosnt currently have a marine radio, but I dont normally go out of phone signal, I will be getting a marine radio in the next few weeks.
     
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  2. JonSea

    JonSea Well-Known Member

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    It seems like low-side current sensing could lead to problems. The low connection point of everything (I would say "ground" but I'm not sure what terminology you would use) should be at the same potential.

    I thought I had seen some sealed Hall effect DC current sensors on ebay but I must be picturing AC sensors. This picture from ebay sort of shows what I was thinking, but it's not sealed. With a Hall effect sensor, there's also the need to zero out any magnetic fields - I don't know how often you'd have to do this.

    $_1.JPG(3).jpg
     
  3. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Thanks for that Jonsea. Is it low side or hide side that cant sense shorts well on the load? I cant remember having just read loads and loads on it :D. I would be cautious with hall sensors on the Boat, space is tight and no guarantee something wouldnt interfere with it.

    I might post up some diagrams of what solutions I have been looking at. Low side to me is also ground.
     
  4. dave

    Dave New Member

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  5. AnalogKid

    AnalogKid Well-Known Member

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    When growing current sensing from scratch, low side is easier because there are many opamps with a common mode range that includes the negative rail. However, most of the commercial parts are high side. If there is no firm reason *not* to put the measurement shunt in series with the neg, I'd do that. Hall effect sensors do not have this problem, but they are neither precise nor accurate, and drift with temperature. What are your peak charge and discharge currents, and how low a current do you want to detect?

    Switchcraft makes waterproof connectors and switches for marine use.

    ak
     
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  6. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Good question on how low is low.... When the boat is moored up both Battery isolator switches are off, so technically there should be zero draw. When at sea I want to measure charge and load. How low would likely not be lower than 100mA.

    I have been looking at this app note

    http://cds.linear.com/docs/en/application-note/an105fa.pdf
     
  7. JonSea

    JonSea Well-Known Member

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    With automotive circuits, low side current measurements are a problem because "ground":is everywhere. If the load shorts to ground, a low side sensor won't see it.
     
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  8. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    In the app note it lists a couple of disadvantages of low side, The two that concern me is the load can be activate with a low side short. And the inability to detect high current shorts. As its a saltwater environment, and I have already experience a short with a pump in water, this is actually exactly what I want a current monitor for. It isnt always easy to know if something like a pump is shorting out, especially as they are always in some water as they sit at the lowest points.
     
  9. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    We wrote at the same time, so that is mainly a car problem then? And not something that should effect me as its a GRP boat :D
     
  10. JonSea

    JonSea Well-Known Member

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    Well, like you said, water is everywhere too, and that's where shorts are likely.
     
  11. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    The most likely places for shorts are the pumps,trim,main switch panel. MAYBE the transducer but that should auto shutdown on a short. I think the pumps bother me most as they are really difficult to monitor visually and to wire in a water tight way. They are also on 3 switches, 1 auto level switch to pump automatically and one main switch on the panel and finally a remote switch near the pump itself.

    Two switches per pump are by default close to water normally, one switch (the auto one, is actually always in water.
     
  12. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Perhaps I'm overly cautious, but I am a firm believer in redundancy at sea.

    Get a second battery and and wire both to a marine change-over switch (if you don't already have one).

    As for load monitoring, I used the main cable between the battery switch(s) and all boat electrics (EXCEPT starter motor) as the "shunt". I attached leads across the cable to a simple DVM using DC voltage range selections. I didn't bother to calibrate the readings to the actual current flowing because it was irrelevant: I was only concerned with any errant current flow when moored.

    <Edit> changed "amp" range to "voltage". 4/19.2017, 1925hrs.
     
    Last edited: Apr 19, 2017
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  13. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    I posted a while back about using two batteries, you advised against doing this?? I am more than happy to have two though. So you didnt have them in parallel as such? One battery 'ON' at a time.

    The way the wiring is at the moment, the starter is on the same battery switches as everything else. I will change this though, The negative battery switch seems to have been by passed! So this is something else I will sort out. At the moment most the wiring is hidden, I intend to use trunking instead with clip on tops. I intend to go a bit further out this year, opinion on the boat is they are not great at sea. I personally think this is down to the wind catcher type cockpit it had and awning, I got rid of both. Yes you get wet when steering and on deck, but that is what waterproofs are for :D.

    Not been out in really bad weather but did get caught if a hefty swell, it bobs around a bit but I was pretty impressed with how it handled. The Transom will be beefed up and part of the well at the back is missing. So I will put marine ply in and cover with GRP. Front windows are a joke, badly degraded thin acrylic. I doubt they would take much to break through, so I have got some tougher, thicker stuff to replace them with. If I had sense I would use the cheap doner boat to use, but the Norman my dad got as a project for us, so I am kind of attached to it.

    I doubt I will ever take it around the world, but France wouldnt be out the question one day lol :D. Nah I am not that stupid, the only reason for beefing up is where I live we have a point around Loch Ryan, where I launch from I cant go past this, I have to go the other way. The currents and water around that area can be pretty bad even when it looks flat calm. I did try it once, but TBH I was stupid considering the boat wasnt water tight around the Transom etc. Obviously I will add superfluous add ons, like GPS TXT for last resort trouble.....Dosnt need a heater but I made one for the small shed, works great and dosnt weigh much.

    Whats your opinion on LEDS as lights? The main downside is the circuits for bulb out, isnt always easy to see the Top deck Light even at night. I got a really High powered search light from a tractor, but its juicy. Then again its only needed rarely isnt it. I like plenty of light on a boat at night, especially when fishing.

    Any more advice? Some numpty painted it with Dulux house paint!!:mad::mad::mad:Thats coming off and if I can get hold of some Oxalic acid I will bring the old Gell coat back up to standard, its a bit tatty looking but I think its surface discolouration. The actual Gel coat is really thick for a boat.
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    There's a very easy and simple way - assuming there's a single 'earth' wire from the negative of the battery?.

    Simply use an opamp to amplify the voltage drop across it, and display the resulting voltage however you wish. This also measures both charging and discharging.

    I posted a suitable circuit on these forums a long time ago, it came from a very old issue of Practical Electronics.
     
  15. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    I dont know the answer to that at the moment. At the moment there is a single one, But there would be two if I take the starter out of it. But kind of more confusing with doing that is, a boat lives in sea water. So is the negative ever actually isolated from the rest? Seeing as part of the engine is in the water, and although the boat is GRP other bits come into contact with the water.

    I will wait to see what CB has to say, looks like a simple boat rewire isnt so simple :D. I will go look for the opamp circuit. But that is still low side yes? So it would give me a problem if one of the pumps were shorted. Not sure why I dont sell the boat and make a balsa wood glider :D, much simpler :D
     
  16. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You wouldn't need to take the starter out of it, you could read the starting current as well (so extra functionality :D)

    Instead of monitoring with one opamp use two, one with high gain (for normal) and one with low gain (for starter) - feed the outputs to two PIC A2D inputs. Then simply read the high gain one, if the reading exceeds the maximum you choose, you then switch to the low gain one to read the starter current - and update your display accordingly. Then when the reading drops below a specified minimum, switch back to the high gain opamp.

    Or you could use two separate digital voltmeters (they are dead cheap to buy), one on each at all times.
     
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  18. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    The multi would be ok, as long as I remembered to change the batteries etc. So I would probably go with a coil meter as I have some, or LCD. I will also look at mikes suggestion.
    I want to think it through carefully, there is a chance I might get brave (read stupid), and take the boat further out than I intend at the moment.

    Slightly off topic, but at the moment sailing boats are going stupid cheap. Especially compared to other types, but then again mooring fees are horrific! I am lucky in not having to pay for mooring, I am 2 miles ish from one place to launch and 4 from the other. I could also moor near the harbour for free, but I kind of like taking the boat home after each trip.
     
  19. JimB

    JimB Super Moderator Most Helpful Member

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    From what you say, I think that you will be making friends with these guys quite soon!
    https://rnli.org/find-my-nearest/lifeboat-stations/stranraer-lifeboat-station

    JimB
     
  20. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Lol, nah I aint that stupid yet. The boat stays in the cove for now, or just off the point from the light house. I dont know the NRLI guys, but I know some of the coast guard. When I was at Stranraer Academy, my mate Neil has a dad who is Coast guard. they have two stations, One by the old ferry terminal and one on the little industrial estate in stranraer. We used to go up and see his dad at the industrial estate one, I am too chicken at the moment to go far. And once the boat is fixed up enough to go further out, I will likely be a fair bit older.

    Besides I have 'limits', set by my mum. These were set in conjunction with Neil's dad, as Neil is the one I go fishing with most the time. We did go too far on Loch Ryan last year, near the end of the Loch is where the point rips start. Scared the hell out of me going full chat and not moving much. Part of the reason I want to do the boat up, is so we can out further. But seriously I am really not as stupid as I sound :D. Plus we got the Ferries to contend with, PITA especially in the summer.

    I got some of that Toughened Plexi glass to go in, but I will need to alter the windows to fit it. Not worth doing them until I get the wiring sorted out.
     
  21. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    No. Two batteries, with a switch like this:
    upload_2017-4-19_19-26-6.png
    and (after thinking about it for a bit) using the battery to switch cabling as the high side shunt and a DVM monitor for ALL juice in either direction. Simplicity is my mantra.
     

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