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Old 27-09-2011, 14:02   #1
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Stacked Inverters

Hi all,

My name is Bob, and have been into EVs and off-grid for close to 30 years. Latest project is to retro-fit and older 12 meter Catalac catamaran for world cruising. I do not want to use anything but wind and stored electrons to provide transportation, galley needs, refridgeration and freezer, hot water, and believe it or not, propulsion for both the cat and digny. I have picked (4) PM, brushless DC motors, rated at 12KW cont, 30 KW peak for 1 minute. One motor and controller for starboard and port drives. Two motors via a single controller for the tunnel hulled (cat) digny. All will be the same so interchangeable and all will run on 96 VDC. The Catalac will have its diesel engines, fuel tanks pulled for this electric drive. The battery string will be (48) 700 a/hr AGM 2 volt batteries.

This is my first question to the pros here on this forum. I know that a battery bank that is a string of single batteries all in series need to be charged and discharged at the same rate. Problem is, even though I have found 96 VDC input pure sine wave inverters, they are all made in China. Can I run (2) 5KW (surge to 10KW) inverters with an input voltage of 48 VDC, that in a stacked configuration, will pull equally from the battery banks? I need 96 volts for the motors but could tap the 96 volt bank into (2) 48 volt banks for the twin inverters as long as the current draw will be equal on each bank.

Thank you
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Old 28-09-2011, 20:34   #2
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Re: Stacked Inverters

I’m by no means a professional marine electrician but if the inverters are your main problem I would most probably use a series parallel configuration. I assume that you will sail most of the time and therefore can keep the battery bank in the parallel position and only put it in series when you’re going in and out of port. I would most probably used to 48 goal alternators that can be isolated above ground as you’re charging system. This should allow you to motor as long as you need to on the rare occasion that you have no wind available for several days. Just my two cents, Mike.
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Old 28-09-2011, 21:40   #3
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Re: Stacked Inverters

I have a pair of Trace 4024's that have been running in "stacked" configuration for 14 years. The issue is that in my case "stacked" means something different than you are suggesting. They are both connected to the 24 Vdc house bank, and the AC side is stacked to supply a 120/240 volt panel. It has worked well with little trouble over the entire time.

I believe that you simply need to review the installation manuals for the inverters that you are considering to see if they can be stacked on the dc side.

Gord May could be chiming in soon with links to such data.
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Old 28-09-2011, 21:46   #4
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Re: Stacked Inverters

I have to wonder how you're going to power all this? 12kW motors, at least 1kW cooking, 1kW kettle, microwave, etc, probably around 1kW hot water......

A kilowatt of solar panels is a lot to fit on a 12 metre cat, and they won't run just one of those motors for long.
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Old 28-09-2011, 23:22   #5
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Re: Stacked Inverters

Prices on solar panels and LiPo batteries have dropped a lot in the last 5 years. I plan on using (16) 185 watt panels @ $5600. I hope the LiPo batteries keep dropping in price, but at current price breaks I'm willing to set up the dinghy this coming spring. Able to pick up (30) 3.2 volt, 160 a/hr ThunderSky LiPo for $6000. That would be 96 volts, 15.4 Kwh. On the new cat/tunnel hulled dinghy, I could get it to plane with just a 5 hp electric outboard, draw was so light that it only took 300 Whr per mile at 17 kts. So if driven easy, a range of 50 miles, not bad. Of course spirited speeding around would cut that down to 10~15 miles. The house bank would consist of (150) of the same cells for 96 volts and 800 A/hr, 76.8 Kwh. My goal is no propane, gas, diesel, or kerosene and even though the price of these batteries are still rather high, if only cycled to 50% DOD will give 8000 cycles. If cycled once per 24 hours, that gives a service life of 20 years. 20 years of never buying or lugging around petrol based consumables or water is IMHO, worth the cost.
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Old 28-09-2011, 23:33   #6
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Re: Stacked Inverters

I agree with cruising cat I do not think what you are proposing is possible.
I think you should look again at your energy budget.
I would be a shame to see the project fail after so much work.
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Old 29-09-2011, 00:07   #7
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Re: Stacked Inverters

I have crunched the numbers on prop pitch, gear ratios of the lower drive of a 50 hp outboard and tested hull efficiency of the cat/tunnel dinghy. It is doable on that scale and should scale up. I can live on the hook for 12 Kwh per day without AC, about 21 Kwh with AC. Panels at 5 hours of average production will bring in 14.8 Kwh. This was the minimum number I would consider for passage making because if required I want the ability to motor at 2 kts 24/7. Of course if I have to, cold cuts for meals, no hot showers, no water making, and I'll even use the SSB without the amp, and THAT would be a hardship because I've always run at least 1000 watts as WL7GS Maritime Mobile.
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Old 29-09-2011, 01:00   #8
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Re: Stacked Inverters

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Prices on solar panels and LiPo batteries have dropped a lot in the last 5 years. I plan on using (16) 185 watt panels @ $5600.
No doubt solar is getting cheaper. But have you looked at where you'd physically fit 16 x 185 Watt solar panels on a 40 foot cat? Do you still plan on having a rig/sails?

Also, unless you plan on following the sun and staying in the tropics over summer, you shouldn't count on getting 5 hours at 100% (or it's equivalent) per day. Well I very rarely do. In fact, only once in the past 2 years. (Yes I have an MPPT regulator)
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Old 29-09-2011, 04:20   #9
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Re: Stacked Inverters

Go ask this question on boatdesign.net, but not if you want confirmation of the validity your idea. There are dozens of threads and the overwhelming response is that it is not practical when applied in the real world
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Old 29-09-2011, 12:54   #10
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Re: Stacked Inverters

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Panels at 5 hours of average production will bring in 14.8 Kwh.
Umm, I usually hate to join choruses of skeptics, but I can't hold back here.

16 panels of 185 watt nominal produce theoretical maximum power of 2,960 watts. There is a big difference between theoretical maximum power and "average production". In fact, you will be lucky to get an average of 1,000 watts into your battery banks even in a sunny climate.

Then there is the question, raised by someone, of where are you going to put them? 16 185w panels will occupy about 21m2 of space. That's an area of something more than 10 feet by 22 feet. I doubt if you have that much deck space on a cat that size. How will you mount them?

I always love to see skeptics proven wrong, so I will cheer if you can make that work. But to be honest it doesn't sound very realistic to me.
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Old 29-09-2011, 15:39   #11
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Re: Stacked Inverters

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Umm, I usually hate to join choruses of skeptics, but I can't hold back here.
Skepticism is good. Cynicism is bad, don't confuse the two

Some quick cals. There may be mistakes so feel free to correct. I am not trying to be pessimistic. I am always interested in these ideas but they just never seem to add up.

48 AGM batteries x 700ah x 2V = 62.7kw hours. But the actual usable capacity is much less. Batteries never hold their rated capacity. So they might be 650ah at best. Then you should not discharge them past 70% discharged if you want them to last. So that makes about 455 real world ah per battery, or a total of 43.2 usable kwh.

How long will this power the boat and how far will you get? Lets say you fit 1 x 12kw motor to each hull. Lets be optimistic and say that you only need to drive each motor at 7kw to cruise at 5k. So that is 14kw total. You have 43.2 kwh available. To get 14kw output you will be using more than 14kw due to inefficiencies in batteries/systems and the motors themselves. Lets say 18kw. So with 43.2kwh available you will run 18kw for: 43.2 / 18 = 2.5hours.

So your boat will be able to motor for 2.5h with a range of 2.5h x 5k. 12 nautical miles on a full charge of batteries.

How long will it take to charge them up with the panels? You might get an average of 1.5kw for 6h a day when you include clouds and the fact it will be nearly impossible to keep them in the sun since this is a world cruiser and you will be sailing and have a mast and rig creating shadows. So that is 1.5 x 6 = 9kwh per day. But since there will be inefficiencies in getting that power into your batteries you might charge the batteries by 7kwh per day. So it will take you 43.2 / 7 = 6.2 days (ignoring all other power consumption) to get enough charge to move the boat 12nm.

So this boat will have less than a 2nm a day range and 12nm maximum range under power. And to achieve this you need to add 5000lbs of batteries to your boat plus somehow affix 16 x185w panels so they wont create windage problems and wont rip off the boat in bad weather.

That low range might work with the philosophy of hardly ever motoring unless its impossible to sail (which will be quite often in a catalac 12m with an extra 5000lbs of batteries and deck covered in frames with solar panels), but why go to such a huge expense in the motoring department when your philosophy is no almost never motor?

That sounds very compromised to me?
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Old 29-09-2011, 16:05   #12
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Re: Stacked Inverters

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20 years of never buying or lugging around petrol based consumables or water is IMHO, worth the cost.
But you'll be "lugging around" 2 tons of batteries and 1/4 acre of solar panels instead.

I know it would be great to be totally independant, off the grid, self reliant etc. You can do it on land, for sure, if you have the space for the solar, the crops, livestock... But on water it's not so easy.

We catch a lot of fish, crabs and so on, but we still have to go and buy red meat and veggies. Why not buy fuel at the same time? We have LPG cooking and hot water. A 9kg bottle lasts around 3 months. We carry 4. To me it's really no hardship to refill a couple every few months.

Our dinghy uses more fuel that the "mothership". Because we use it to catch the fish crabs etc, and because the big boat SAILS virtually everywhere. It sails because it's light. If we added a couple of tons of batteries and solar panels it certainly would sail as well.

I think we'd all love to get away from using hydrocarbons, but the fact is, the amount of energy you get per litre, kilogramme and dollar from them is VERY hard to beat.
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Old 29-09-2011, 16:14   #13
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Re: Stacked Inverters

This is more just my opinion rather than a calculation, but I would also wager that the environmental impact of leaving the diesel motors and tanks in the boat would be less than going electric. The diesel motors have already been made and the production of all the gear needed to create the elec system would create huge industrial waste that would not be created if you just left the diesels in. So rather than trying to be 100% diesel free, the money spent in other areas of sail efficiency would lead to a more green boat overall. Maximizing the performance under sail instead of hampering it with panels and batteries would mean the diesel engines will hardly every need to come on. Only in emergencies and docking.
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Old 29-09-2011, 16:29   #14
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Re: Stacked Inverters

Got any drawings of what this boat is going to look like both on deck and below Deck officer?<grin> Also what are your cruising intentions? I think this last question should be explained before any input would be meaningful.
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Old 29-09-2011, 16:45   #15
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Could work if...

I'd agree with the above that there just isn't enough energy for a cruising boat.

However if you could be less "pure" by, say, reducing the area of solar panels to what can be practicably accommodated, reducing the total battery weight, using gas for cooking and installing a small diesel generator (I'm guessing 6Kw?) it may be possible to "squeeze" the numbers enough to make it work.

For safety my opinion is that you'd need to size the generator and motors (again, less than 2*3Kw continuous?) such that the generator could power the motors continuously. It would be slow, though...

Cruising yachts sit for long periods at anchor/mooring/dock and many boats only use their engine for leaving/arriving so in practice the generator may not get that much use.

This would be an extensive modification to an older boat that was not designed for this so purpose building from scratch may be easier in the long run.

Letting my imagination run wild one could dispense with conventional sails altogether and build huge adjustable retractable solar panels that could also double as sails...
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