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Hugh Jorgan

Can I use the power supply on the NMEA 2000 backbone?

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I want to tap into the NMEA 2K 12v power supply (I'm assuming it's 12v?) wires and not use the data wires to charge a wireless B&G handheld vhf (H50).  This would be much easier than running all new power supply to the place where I'm putting the charging cradle.  Will this work?  Seems like it would but the 2k backbone can sometimes do funky things.

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You can take power from the backbone, but you need to be aware of how much power is available and how far your power tap is from the source of power on the bus.

The backbone power wires are AWG 22 and the voltage drops quickly as the current increases. The installation manual for the H50 says to use a 1A fuse for the wireless mic charging station. The details of how much current is available are dependent on the length of the backbone, the position of the supply and the position and size of all the loads - it's pretty difficult to say whether it will work. It's also the case that the spec for the voltage is, I believe 11…15VDC and so it's also important to know the supply voltage.

How long is your NMEA2000 backbone and how many devices are on it?

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Also, you can't make any assumptions about the current draw of that charger. I'd put the charger on the bench with a power supply and an ammeter in-line, run those handset batteries all the way down, and then charge that sucker up while monitoring for peak current draw. 

You should also short-circuit the charger contacts to see what happens should something like, say, saltwater gets on it.

Call me conservative but I wouldn't put the integrity of my entire instrument network at risk for this. Run some 18gauge duplex wire from the nearest decent power tap to the charger and sleep better at night. I know I sound like a broken record but it only hurts once to do it right.

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4 hours ago, Moonduster said:

You can take power from the backbone, but you need to be aware of how much power is available and how far your power tap is from the source of power on the bus.

The backbone power wires are AWG 22 and the voltage drops quickly as the current increases. The installation manual for the H50 says to use a 1A fuse for the wireless mic charging station. The details of how much current is available are dependent on the length of the backbone, the position of the supply and the position and size of all the loads - it's pretty difficult to say whether it will work. It's also the case that the spec for the voltage is, I believe 11…15VDC and so it's also important to know the supply voltage.

How long is your NMEA2000 backbone and how many devices are on it?

Thanks for the reply.

The backbone where I'd tap into is probably a 15-20ft  run from the H5 cpu (35.5' boat) and is at the end of the backbone giving supply to the H5 AP system.  It's a moderately sized instrument package, H5 CPU, 4 20/20's, 1 H5 graphic display, 3 AP controls, 1 triton, one Zeus plotter and one analog AWA.  Eventually I'll probably add a small Vulcan or Zeus to the cockpit and replace the analog AWA with a triton.

What would happen if I tried it and there wasn't enough voltage?  Would it cause any harm to the system or just make things run slow or incorrectly?  Could I turn everything on including backlights on the instruments and take a voltage reading where I'd tap in?  fwiw the charging cradle reads 12v 500mA

8 minutes ago, IStream said:

Also, you can't make any assumptions about the current draw of that charger. I'd put the charger on the bench with a power supply and an ammeter in-line, run those handset batteries all the way down, and then charge that sucker up while monitoring for peak current draw. 

You should also short-circuit the charger contacts to see what happens should something like, say, saltwater gets on it.

Call me conservative but I wouldn't put the integrity of my entire instrument network at risk for this. Run some 18gauge duplex wire from the nearest decent power tap to the charger and sleep better at night. I know I sound like a broken record but it only hurts once to do it right.

Good point and certainly something to think about also.  The charger is wireless with no contacts which I'm guessing was designed this way for the reason you mentioned.  Doesn't mean the wires supplying the charging cradle can't short out though.  The charger reads 12v 500mA 

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The problem with overloading your CAN bus is that, at best, something will stop working but probably something will just get flakey. Remember, power consumption isn't static, especially with your displays, it's a function of backlight intensity.

Maretron makes a tool called N2KBuilder that can be used to model lengths, loads and supply voltage and tell you what voltages to expect at each node on the network. It's got a horrible user interface but if you're happy drawing a messy diagram, then it's quick and accurate.

Also remember that if you're powering your N2K bus from some battery source, then you need to realize that the voltage will vary significantly as your batteries go from charging to nearly discharged - this is not a good plan. Instead, you should use a DCDC converter to source power onto the CAN bus and that DCDC converter should have the maximum allowed output, which is 15V. This will give you a much different result than if you run at, say, 12V. 

Finally, you say that your radio handset cradle will be near to the end of the bus at which you inject power. If so, why not just get power from that same source and forget using the CAN bus as the power source?

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To what istream mentions:

  1. In most cases, why be afraid to add another load to the bus when no one did any analysis in the first place. The right approach is to do the analysis regardless of what's going on and then be forewarned about what you have and whether it's actually likely to work despite outward appearances
  2. Why bother with 18 AWG wire? ABYC requires nothing smaller than 14 and you can run 1 Amp 5 meters each way with 22 AWG and only a 5% drop. 

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1 hour ago, Moonduster said:

Finally, you say that your radio handset cradle will be near to the end of the bus at which you inject power. If so, why not just get power from that same source and forget using the CAN bus as the power source?

I think ultimately this is what I'm going to do.  It's only a 15-20 ft run back to the panel but it's a hard fought 15'.  I've recently run some other wire through the conduit going back there and it was extremely tight with at least 2 90 degree bends.  I could probably get the line through but the fish tape to get the line through is going to be really tough, not even sure I'll be able to get it through any longer.  If I can I'm concerned about causing significant chafe to the lines inside the conduit.  

I figured if I could just tap into the N2k that's right there that would be the easiest and cleanest.  Sounds like I "might" or "probably" could do that but the best & right way is to bring it back to the panel and tap into the vhf or dc outlet circuit.  I'd hate for a fairly insignificant vhf charger to corrupt the data coming out of the B&G system.  I was hoping for a resounding "of course you can use the N2K" but I should have known better that nothing in boat electronics is ever that easy or straight forward.  Thanks for your input.

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You can usually float a mouse line through by tying a cotton ball to the end and sucking it through with a vacuum cleaner or shop vac.

But getting rid of the conduit is the best plan. It's just s pain in the ass, adds weight and hides problems. Cable tie bases and cable yours are the way to go.

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