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Old 22-11-2009, 19:01   #1
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Smaller Solar Panels

Our power plant on our new boat (Yay!) consists of two 12 volt batteries and the alternator from a 1980 Johnson Sailmaster 7.5. I am not sure what the alternator's output is, but I know that the PO had cut the wire that used to go to charge the batteries because he alleged its affect was nominal, I plan on hooking it back up but doubt I will ever know what it is rated at due to the engines vintage. The Johnson is pull start and so I won't need the batteries to start the engine. We use them for the usual light fare: running lights, depth sounder and - the kicker - my toughbook that we use as a chart plotter that draws about 4 amps ( I will be getting a 19 volt inverter, not an AC one). I am thinking of getting a smallish solar panel to keep my batteries topped up. Any recommendations?
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Old 22-11-2009, 19:04   #2
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We've got 3x10 watt panels that charge our two series 27 deep cycles. Works great. We killed them one night at anchor and I had left the start battery cross connected so it was dead too. An hour of sunlight and we had 'nuff juice to get the engine started.
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Old 22-11-2009, 19:24   #3
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The generator on a small outboard typically is designed with just enough power to replace what the starter used (if there was an electric start option) and that one white all-around light, or a bit more.

Cutting the wire sounds a bit foolish, I'd have to wonder if that wouldn't also have fried the generator or if it was already dead and this is his "excuse" for it not being available.

But either way...I think you'd want to plan on the solar panels providing ALL of the charge you need, so you need to add up the power draw for all the toys and then compare that to solar panels. In good summer sunlight outside of the trpoics, you can expect that a full day of solar panel production will put out power equal to about four hours at the full rated power, i.e. a 55Watt panel, producing a nominal 55W at 17V puts out about 3.2 Amps. So you'll see about 3.2*4, twelve or thirteen amp-hours worth of power from it after a full day of sunlight.

Depending on what toys you use and how you use them...And you might consider switching the lighting to LEDs if you are doing extensive night sailing, they wind up costing a lot less than panels and batteries. An extra battery for the Toughbook itself may also be a good option.
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Old 23-11-2009, 00:55   #4
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For about the cost of a solar panel, you can replace your toughbook with a netbook which only draws one amp.
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Old 23-11-2009, 05:01   #5
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Before hooking up the outboard, check that it has a stabilised output. Some versions of these were only set up for running lights, and the unstabilised provided a bit more power for less engine output, so was good enough.

If you hook the laptop up to that , you will fry it!
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Old 23-11-2009, 07:35   #6
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For about the cost of a solar panel, you can replace your toughbook with a netbook which only draws one amp.
Don't blaspheme please.

The Toughbook would have it's own breaker and everything I wouldn't be hooking it up to the alternator directly.
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Old 02-12-2009, 15:51   #7
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I said earlier in this post that my toughbook draws 4 amps but now I am starting to think that is grossly over exaggerated. That is just what my AC power supply is rated for. How can I tell how many amps my computer draws? How can I then convert that to DC amps?
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Old 02-12-2009, 16:47   #8
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Question What do you plan to do with the boat?

Your laptop will draw 4 amps at the 110vac that the power supply runs on. When you hook it into an inverter and then to your battery you will find a much higher current draw then that.

Even when we shifted down to an old P3 laptop, the current draw was significant enough it was our largest single draw onboard.

To answer your question on the solar panel you need an energy budget. You will also need a realistic idea of what you are going to do with the boat.

Are you cruising? Weekending? Live aboard? Day sailing? Each will have very different needs and loads.

What do you plan to do with the boat?
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Old 02-12-2009, 16:59   #9
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Your laptop will draw 4 amps at the 110vac that the power supply runs on. When you hook it into an inverter and then to your battery you will find a much higher current draw then that.
If true, that would translate to 440 watts!
The typical laptop with screen brightness at max draws in the neighborhood of 3 to 4 amps at BATTERY voltage (in my case 18 v.)
The AC adaptor for it states:
Input: 10-240 V AC 50/60 Hz 1.5A
Output: 19V DC 4.74 A

The current ratings are maximum input draw and available output maximum depending on demand (max screen brightness and how hard the guts are working).

That would be 19 x 4.74 or 83.6 Watts.
As a comparison, my boat's refrigerator draws about 60 watts when running.
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Old 02-12-2009, 17:27   #10
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Are you cruising? Weekending? Live aboard? Day sailing? Each will have very different needs and loads.

What do you plan to do with the boat?
Actively Cruising.

Isn't it Ohms Law? My spec sheet says it produces 26-40watts so that would be:

W/E=I

or 26W/12V=2.1Amps

or

40watts/12V=3.3Amps

Average those two and you would get
2.7Amps

I think I just solved my own problem. You guys are the best.
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Old 02-12-2009, 18:02   #11
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Originally Posted by unbusted67 View Post
1980 Johnson Sailmaster 7.5. I am not sure what the alternator's output is, but I know that the PO had cut the wire that used to go to charge the batteries because he alleged its affect was nominal, I plan on hooking it back up but doubt I will ever know what it is rated at due to the engines vintage.
Pull start outboards of this age and size generally produce very little juice, unless they were modified. By memory they produce ~0.1A, which was used mainly to drive a basic light while the engine was running ... if that applies, go the solar route and forget about connecting the outboard.

At least that was what the mechanic said about my Mariner 15 hp. Different story if an alternator was added.
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Old 02-12-2009, 20:40   #12
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Typically a laptop power supply is rated 75W to 90W and sometimes up to 130W for the large screen models using a desktop (rather than true laptop) CPU.

But you should bear in mind that rating is twice what the laptop needs. The power supply is designed to provide enough power to CHARGE A DEAD BATTERY and RUN THE COMPUTER at the same time. If you start out with a charged battery and a 90W power supply, and you are running the computer on AC, it probably will be drawing 45W not 90W. And that's with a bright screen, DVD drive running, hard drive running. (Three largest power consumers in most computers, in that order.)

You'd have to look at the numbers on your power supply, some are 17V some are 19V, etc. and the amperage and wattage are going to be tied into the individual specs for that power supply brick.

A direct "12volt" power supply for the computer, designed for use in cars and planes without the need for an invertor, may also save you another 10-15% over using the AC adapter plus a 110V invertor.
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Old 02-12-2009, 20:55   #13
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You'd have to look at the numbers on your power supply, some are 17V some are 19V, etc. and the amperage and wattage are going to be tied into the individual specs for that power supply brick.

A direct "12volt" power supply for the computer, designed for use in cars and planes without the need for an invertor, may also save you another 10-15% over using the AC adapter plus a 110V invertor.
...Oh yeah that's right I will be getting a Lindh Electronics DC/DC converter too so that is really going to change things. I can't keep any of this straight.

@100 VAC the brick draws 0.9 - 0.5 amps
after the brick converts to 16 VDC it will draw 2.5 Amps

How can I figure out what it will draw when I use the DC/DC converter? Is there an equation somewhere? Should I just assume it will be somewhere between 2 and 4 amps?
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Old 02-12-2009, 21:04   #14
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If you REALLY want to know what your laptop usually draws, plug it into on of these while simulating your normal use:

P3 - Kill A Watt

The efficiency of the AC power pack and a dc-to-dc converter will probably be similar. If your running through an inverter, account for its efficiency. Our library actually has one of these you can check out which is an awesome idea, but I don't expect a common one.

Cheers, Colin
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Old 02-12-2009, 21:07   #15
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Just calc the watts at ANY voltage:

Watts = Volts * Amps

Then convert back to 12V for your boat accounting:

BoatAmps = Watts / 12

Then modify it for any efficiency losses, e.g. running through an oversized inverter:

CorrectedBoatAmps = BoatAmps / 0.84 (or whatever the efficiency might be)
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