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Old 21-01-2011, 09:53   #1
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Slowly Going Insane Thinking About Batteries !

Itís time to replace batteries and I will be using AGMís as the boat may be sitting unattended for months at a time. The boat is a 1981 36 Pearson Cutter with a W40 diesel. The alternator is the stock unit and will upgrade to a 100-110 amp alternator. I think that as large as I would like to run on a half inch belt. I donít know what my daily amp hour usage will be at this time because we are still dockside and have not installed any electronics. But my best guess would be around 150 amp hours a day. From what I have read I would need a 600 amp hour battery plant. Or is this over kill? Would a 400 amp hr plant suffice?
My main concern is keeping a 400 to 600 amp hour battery plant properly charged from the 100 to 110 alternator I plan to install.
Or do I add solar and or wind generation to the boat and go with the larger plant or smaller plant?
I have replaced all the interior lights to led or ccfl and led nav lights. The refrigerator is a Frigoboat keel cooler.
I know Iím probably going about this backwards as I donít really know what my requirements will be. But I need to upgrade now so Iím trying to get it right the first time.

Hope my ramblings makes some sense and thanks in advance for the help.
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Old 21-01-2011, 10:23   #2
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Hi Lennie,
At the top of the Electrical: Batteries, Generators & Solar forum is a study hall on electrical that has some information well worth reading including this post which answers many of your questions Electrical Study Hall:

which is not to short circuit any discussion but it might save you some time

For those of you who went "keel cooler?" like me check this out:
Frigoboat Home Page
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Old 21-01-2011, 19:29   #3
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Lennie, part of "how much" depends on your wallet and philosophy. All the battery makers disagree about specifics but most generally agree that if you discharge lead batteries some 30-50% before you recharge them, you get the most watt-hours for your bucks, in terms of getting more cycles out of the batteries overall.

So start by tallying up all the equipment ratings with a pencil, how many amps (or milliamps) each device take. Or watts, then divide out by 12.4 to get amps based on reasonably charged battery power rather than what they'd take from a 14.4v alternator.

If your daily needs are 100AH, and you plan to cycle the batteries 50%, you'd need a 200AH bank. If you want better battery longevity and a 33% charge cycle, you'd need 300AH capacity--for the same loads. The bottom line is often going to be "What can I afford? What can I physically lift? What will fit in the boat?!" and there will be a hard limit somewhere in there.<G>

With AGM batteries, as you may know, charging care is critical to battery life. Get a good external regulator, make sure it is programmed to the settings that your particular battery maker wants, and get fancier from there if and as you want to.
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Old 22-01-2011, 08:29   #4
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150 amps per day is a large draw for a 36 footer isn't it?
Freezer? etc is still well under 100, isn't it?

With solar panels trickle charginging happily (forever) you are only drawing down on your batteries at night so I would assume you can use Hellosailors figers and save yourself a few hundred amphours of batts....

I just got 2 x 165 amp hour non-agm batts, sealed, maintenance free lead acid. They seem happy so far....
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Old 22-01-2011, 08:43   #5
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I have 3 X 105 ah house batteries. 2 X 50 watt solar panels with BlueSky MPPT (panels are on a SolarStik so I can re-position the panel angle a couple times a day to get max sunshine.) All house, Nav lights, and anchor light are LED's. This system runs all electronics (autopilot, 2 x GPS, wind, depth, knotlog, stereo, stove solinoid), all lighting, and refrigeration nicely 24 hours a day.
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Old 22-01-2011, 09:57   #6
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Liam,
What kind of autopilot do you use? When you mention GPS, is this a chartplotter that you run while underway?

The reason I ask is that I am surprised that your 100W of solar panels can keep up with your 24/7 power consumption when at sea. At anchor, perhaps (depends on the refrigerator). I know that if I run my chartplotter (2A), below-deck autopilot (3A), refrigerator (2A), and LED tricolor (0.1A) it adds up to about 170Ah / day.

Your panels might deliver 35Ah / day. If you are using a tillerpilot or hand-steer most of the time, and turn off the chartplotter, and the refrigerator is extremely low draw, I might understand. How do you do it?

Lennie,
you really ought to put together a power budget spreadsheet, or at least a list. At this stage you can be pretty generic, and it will still be close enough to be useful. You will probably find that your at-anchor consumption is significantly less than your at-sea consumption.
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Old 22-01-2011, 10:21   #7
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I have a RayMarine wheel pilot. I have learned to trim my boat pretty well (after about 10k miles under the keel) so that it is mostly on standby. My guess is that the pilot only cycles about 5-10% of the time.
The GPS's on board are a Garnin 76C mounted at the helm and a Garmin 440 at the nav station. Other than that the knotlog, depth sounder, and windspeed/direction are always on but set to the lowest illumination setting.
I usually run the referigeration for a couple of days on shorepower to get the box good and cold before I head off-shore. Once the box gets cold it doesn't take much to keep it that way. My guess is that the refer compressor runs about 25% of the time. All lights are LED's and I often use a kerosine lamp in the salon.
The longest that I have gone without running the diesel is two weeks. At that point everthing had been running 24/7 and the battery bank was at 80%.
I would guess that annothe week or two and I would have had to run the engine to charge back to 100%.
So the system would have eventually gotten run down to a state of either having to run the engine or turn everthing off for a day or two.
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Old 22-01-2011, 10:25   #8
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Oh, PS...
With the ability to turn the panels from time to time to point directly at the sun (by way of the SolarStik) and the use of the MPPT controller I am getting 7-8 amps continuous for 10-12 hours a day.
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Old 22-01-2011, 10:30   #9
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As Liam mentioned the illumination of screens can take a lot of power. especially the plotter! Turn it down and see what happens.

Or turn it off when you can.

Or pretend its night
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Old 22-01-2011, 10:47   #10
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Paul-
" I know that if I run my chartplotter (2A), below-deck autopilot (3A), refrigerator (2A), and LED tricolor (0.1A) it adds up to about 170Ah / day. "
Bear in mind that the duty cycle on all of those can be highly variable. The tricolor, OK, that's going to be xx hours per day at a straight power load, sure. And to some extent so will the chartplotter, assuming you're using it.
But the rated power on the reefer will be the max when it is running, and depending on how well the box is insulated, you could easily double or halve the effective power consumption conmpared to an identical reefer with a different box.
Ditto on the autopilot, if a boat is well balanced, trimmed properly, and experiencing light weather, it may pull less than half the power needed by "the same" autopilot fighting rough water with a poorly trimmed boat. Or one that doesn't track well.

The numbers just get you into the ballpark.
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Old 22-01-2011, 10:57   #11
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Quote:
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Oh, PS...
With the ability to turn the panels from time to time to point directly at the sun (by way of the SolarStik) and the use of the MPPT controller I am getting 7-8 amps continuous for 10-12 hours a day.
That is pretty impressive for a solar panel. That means 70 - 96 amp hours or about 1000 watts per day.
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Old 22-01-2011, 11:06   #12
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"With the ability to turn the panels from time to time to point directly at the sun (by way of the SolarStik) "
Keeping the panels aimed does make a huge difference. IIRC, you lose/gain about 5-10% power for every 15 degrees (one hour) that the sun has been "moved" from optimum. A good payback for a little tweaking during the day.
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Old 24-01-2011, 19:46   #13
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Thanks for the replies. I'm going with 400 amp hour AGM's for now and working out my amp management as I go along.

Lennie
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Old 24-01-2011, 21:02   #14
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Old 24-01-2011, 22:23   #15
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Originally Posted by hellosailor View Post
Paul-
" I know that if I run my chartplotter (2A), below-deck autopilot (3A), refrigerator (2A), and LED tricolor (0.1A) it adds up to about 170Ah / day. "
Bear in mind that the duty cycle on all of those can be highly variable. The tricolor, OK, that's going to be xx hours per day at a straight power load, sure. And to some extent so will the chartplotter, assuming you're using it.
But the rated power on the reefer will be the max when it is running, and depending on how well the box is insulated, you could easily double or halve the effective power consumption conmpared to an identical reefer with a different box.
Ditto on the autopilot, if a boat is well balanced, trimmed properly, and experiencing light weather, it may pull less than half the power needed by "the same" autopilot fighting rough water with a poorly trimmed boat. Or one that doesn't track well.

The numbers just get you into the ballpark.
My numbers *are* actual in-use values, not just the published specs. For example:

Refrigerator: 5A when running. In the tropics it runs with perhaps a 40% duty-cycle. When the temperature and water are cooler it cycles less often, but 2A is the number I use for my budgeting.

The autopilot averages about 3A when we are running with medium seas on the quarter. (To be honest, this is not a carefully measured number, as the current duty-cycle is pretty variable.) I can change the autopilot settings to reduce the power consumption, but then the seas kick us around more than I like. When conditions are easier the autopilot can draw much less. Actually, I prefer to use the Monitor windvane when I can.

The LED tricolor draws about 200mA, so I cut that in half for my daily estimate. The tricolor obviously isn't a significant factor. However, the old incandescent tricolor used to be one of my biggest power consumers.

My power budget philosophy tells me to size my batteries so that they can support my likely worst-case at-sea consumption for 24 hours (starting at 80% charge and not dropping below 50%). This is without factoring in my solar panels, as there are often cloudy days. Under more typical conditions I can go several days before I have to run the engine for an hour or two for battery charging.
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