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aronhk_md 05-10-2013 08:02 PM

another solar question, beating the dead horse
Ok, so in searching I have found lots of talk about solar being great for topping off batteries, but usually insufficient to power anything with substance, like refrigerators or A/C. The usual statements are that for the average user, its not possible to have enough juice produced to make it happen with the number of panels the average boat can reasonably carry.

So I say, ok...makes sense. Takes lots of room and lots of panels.

I have lots of room on my deck canopies. So would you folks mind carrying me through an exercise in "what if?"

*An average small home fridge uses 10 amps on 120v.
*An average home 8000 BTU A/C unit uses 7.5 amps on 120v (I think marine reverse cycle A/C is more efficient but haven't researched it yet completely, just going off the fact that instead of using 90 degree ambient air for your heat sink you are using cooler water)

Lets take a solar array hooked to a bank of 12v batteries. I don't understand the conversion from 10 amps at 120v to what the draw would be through an inverter at 12v DC on those batteries. Can anyone explain that for me? With today's panels, how many panels would be needed, and how much surface area?

What I'm wondering is.........what WOULD it take to run that fridge? Or the A/C? Not necessarily together. I am assuming you would want to match the current draw of what you are using to what you are producing, though I imagine if you came close you'd slowly drain the batteries and that would not be good.

Honestly just looking at the stuff on ebay and conversations about it here leads me to confusion as far as how many panels can be hooked together, run by a single controller, what type of panels, etc. Not sure where to go to research it more carefully even since there is SO much info out there.

deckofficer 05-10-2013 10:26 PM

Looking at your deck canopies from your avatar I would say you have plenty to power a fully electric galley along with A/C. The biggest draw would be A/C, so lets crunch some numbers there. 12,500 btu will draw around 1500 watts and depending on outside temp could cycle 50% for 10 hours per day. 7500 w-hr on the A/C, and no more then 2500 w-hr for everything else including electric cooking and fridge and freezer. I set every load on the high side to show max draw. 2000 watts of solar that would fit on your boat's canopies coupled with a large battery bank would make you self sufficient. If you stay with a 12 volt bank for inverter loads I would go with LiFePO4 cells instead of lead acid, which sags under load.

At 90% conversion efficiency a 10 amp load at 120 VAC would draw from a 12 volt battery 110 amps.

aronhk_md 05-10-2013 10:34 PM

So you are saying that a 2000 w system on 12 volt would run a 12,500 BTU A/C unit, a fridge, and most of everything else in the galley with occasional use? Something like a 3000w inverter would be needed I guess? Does producing and drawing that much energy through the batteries shorten the life of the batteries?

Also, what kind of controller would that require? So far rthe HD controllers I see for example by Tristar only control 60 amps...which equates to 720w at 12v.

What size battery bank would be needed?

deckofficer 05-10-2013 10:50 PM

I think the largest 12 VDC input inverter is 4000 watts cont, surge 8000 watts. Most inverters this size are 48 VDC input, so the amp draw on the battery bank is 1/4 of a 12 volt battery bank. Lets say your A/C is running at the same time you use your microwave and coffee maker. That comes to 3900 watts, so at 90% efficiency of the inverter would draw close to 400 amps, and any lead acid battery would experience serious voltage sag along with de-rating of a-hr capacity due to the Peukert effect. This is why for such a large system at 12 volts, lithium cells are the way to go, almost nil Peukert, and minimum voltage sag. If you feel more comfortable with lead acid instead of LiFePO4, then I would go with a 48 volt bank to feed a 48 volt inverter. A good choice would be (24) 700 a-hr 2 volt AGM cells in series. This would give you 33.6 Kw-hr bank which is large, but you could go with a 17 Kw-hr LiFePO4 bank and have the same usable a-hr with 4 times more cycle life.

Starting load of an A/C can be twice the running load.

aronhk_md 05-10-2013 10:56 PM

Which Li cells would be needed and how many? Don't you need special lithium chargers and controllers too?

deckofficer 05-10-2013 11:09 PM

I have used Winston LiFePO4 cells. I don't use any BMS (battery management system) but I did perform a bottom to top balance of my cells in parallel before putting them in service. The only thing I use is a cell logger with digital readout and high and low cell alarm. You could assemble a LiFePO4 700 a-hr 12 volt battery that would have more usable capacity than a 1400 a-hr lead acid bank for $2240 here Balqon - Advanced Transportation Solution

Charge cells to 3.55 volts (14.2 Volts for 4 in series) and don't allow them to discharge below 3.0 volts (12 volts).

deckofficer 05-10-2013 11:24 PM

The reason you can use a much smaller a-hr rated LiFePO4 battery vs lead acid is how these two different batteries get their advertised rating. With lead acid for decades it has been calculated on a 20 hour rate to completely drained. So on a 100 a-hr lead acid, that is 5 amp draw for 20 hours. LiFePO4 use a brutal 1 hour rate and stop when the battery is 80% discharged instead of the 100% discharge for a LA rating. LA should not be discharged deeper than 50% for it to give 500 cycles, while a LiFePO4 can be discharged to 80% for 2000 cycles.

If you discharge a LA at a 1C load (100 amps for a 100 a-hr battery), then the Peukert effect would de-rate that 100 a-hr battery to about 30 a-hr at best.

Big difference between LA and LiFePO4. It has only been the last year that due to major price drops of LiFePO4 cells, they have become the least expensive battery option when you factor cycle life. I've used all types of batteries in the electric vehicles, and now that I've used LiFePO4 I will never go back to lead. Lead is dead.

aronhk_md 05-11-2013 12:20 AM

Without some sort of battery management system I cant imagine knowing when the cells have reached their charge threshold. It would require frequent checking, and THEN turning on the charging system when it gets there, no? Or do you allow the system to charge WHILE using the current to power stuff on board? Doesn't Li also require a special charger?

deckofficer 05-11-2013 12:39 AM

With your solar you can enter bulk and float voltages into the MPPT controller. Due to the high charge acceptance of LiFePO4 you will need to reduce the output of an engine driven alternator. On a LA bank, the surface charge helps to cut down on charge rate, but a 80% DOD LiFePO4 bank will pull whatever the alternator will put out without taper so after a period of time the alternator that has thermal protection will back itself off. I mentioned above that a cell logger with high and low alarms will notify you with an alarm that one cell has slipped below 3.0 volts. For charging, if you set the solar controller and alternator for 14.2 volts, there won't be a chance of any single cell going over 4.0 volts. For everything you might want to ever know about the care and feeding of these cells, here is a 2800 post thread just on LiFePO4 cells for house banks. LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks - Cruisers & Sailing Forums

deckofficer 05-11-2013 12:49 AM

BTW member rjtrane just upped the ante on his boat, going with (16) 1000 a-hr Winston cells for a 48 volt 1000 a-hr bank. He not only has a fully electric galley, A/C, electric heated hot water, he also has diesel-electric propulsion, thus the big bank. He is also running 6 Kw of solar, whereas I mentioned for your needs of A/C and electric galley, 2 Kw of solar would fit on your canopies and do the job for you of no gen set run time while on the hook.

Insequent 05-11-2013 01:03 AM

I have just installed 7 x 260 W panels on my boat, and removed a 7.5 kW and a 2 kW generator. So far I have seen the solar putting 60 A into the battery bank in recent weeks in Puget Sound. When I get to somewhere with more sunshine, or sun angle higher in the sky, I expect to easily double that.

In due course I'll collect some data for analysis, but initial thoughts are that I have plenty for LED lighting, DC refrigeration and assorted electrical stuff. But not enough to run an electric range/oven or airconditioner - i dont have, or want, those things, so no problem.

deckofficer 05-11-2013 01:12 AM

1820 watts of solar is a nice array. Get out of the PNW and with your efficient lighting and fridge you should be able to run the A/C and electric kitchen appliances.

FF 05-11-2013 06:35 AM

*An average small home fridge uses 10 amps on 120v.

50% to 90% of the time!

*An average home 8000 BTU A/C unit uses 7.5 amps on 120v (I think marine reverse cycle A/C is more efficient but haven't researched it yet completely, just going off the fact that instead of using 90 degree ambient air for your heat sink you are using cooler water)

The first solution is always to trash the home cheapo equipment for far more efficient alt energy gear.

A better fridge like a Sun Frost may require 1/2 to 1/5 the energy of a house reefer.

Thicker insulation , no need for heat to defrost internal thin insulation and a more efficient compressor all help.

There are 12 Truck air cond, built for battery operation.

The use of water cooling does make any refrigeration/air cond slightly (5%-10%?) more efficient , BUT the main use is to get the heat out of the boat.

You will still need a Honda for those days when there is not 12 hours of sun shine..

A simple cost analysis may show you are spending $50 to save $4.00 worth of fuel per hour..

aronhk_md 05-11-2013 08:59 AM

Thanks guys....this is just an exercise in "what if" as I mentioned. Sure, it might be cheaper to run a generator in the short run, but what if you are living aboard? With the price of solar panels coming down to less than $1 per kw, its not so far fetched to put together a 2000 kw system for under $4000 (not including batteries). Even a new Honda 2000 generator is $1000, and to run it all the time costs money.

If you are on the grid while plugged in, that costs money.

Its an interesting exercise in "what if" to see what it would take to be self sufficient part of the time, even if it turns out "not worth it". Most of the time in these threads its left at that, rather than actually tallying up what it would take in equipment to make it work. In other words FF....actually DOING that cost analysis. As an exercise if nothing else.

Now I do realize its not sunny every day. And there are months in our seasons here in the northeast where the days are short. But I do wonder what my local marina would say if I asked them about feeding back into the grid on sunny days. (lol, yes I know its probably not possible even though they have metered slips)

motion30 05-11-2013 09:33 AM

that seems like a high current draw for small 120 volt fridge

aronhk_md 05-11-2013 09:49 AM

You may be right. I just did a search to see if I could find average draw for a home type fridge. FF....I do realize there are much more efficient appliances out there too. Good stuff. Just tried doing the setup without going to "best of the best" scenario.

So its sounding to me like a 2000 kw system with Lipo batteries could be assembled for around $6000? Doing it all oneself that is. It would be able to run at least some A/C, a fridge and some other on demand galley equipment as needed. I'm sure for the average user it would have no problem with LED lights on board, and probably a computer. Maybe an LED tv?'d be able to power those items on sunny days in your region. Less so on winter days obviously due to the angle of the sun. My guess is too, if you used "best of the best" equipment, you might be able to get away with less power.....say 1600 kw?

Phil Fill 05-11-2013 10:00 AM

One big factor is you are in Delaware. Take a walk around and see how many solar panel there are. In the PNW, not many and if there are to charge the batteries.

deckofficer 05-11-2013 10:08 AM

Just a few corrections. The average home fridge consumes 180 watts when running which is 1.5 amps at 120 volts, not 10 amps. I quoted a 2 Kw solar array (2000 watts) not 2000 Kw (2,000,000 watts).

Underway 05-11-2013 10:34 AM

I spent much of this week speaking with the technical staff of several large insurance companies about the confusing and potentially dangerous situation that is developing for Lithium batteries amongst do-it-yourself boaters. Every one of them is extremely concerned with the "wild west" (their words) information scenario that is developing in the marine community... primarily via boating web sites. ABYC is in the loop and I suggested that they step up their efforts to clarify design, component certification, installation, monitoring, maintenance, and inspection criteria for a minimum acceptable system. I have also alerted the Coast Guard Engineering and Boating Safety offices to the potential safety scenario that is clearly developing. I specifically requested that both ABYC and the Coast Guard conduct a serious engineering assessment of the Greenline boats that are being imported. Awards in the categories of design innovation and eco-friendly are nice, but the important categories are performance, safety, and quality. Greenline might set the example for the industry...good or bad. Information in these threads is heavy on performance potential and light on safety. The word cavalier comes to mind.

deckofficer 05-11-2013 10:48 AM


And as a kid you were the hall monitor, right? I don't know of anyone using any other lithium battery for a house bank other than the perfectly safe LiFePO4 chemistry. We give up some energy density with these cells over lithium ion, but have no thermal issues. Yet you felt it was your duty to cast doubt over all lithium battery installations? Politicians use scare tactics for their agenda, what is your agenda?

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