# Need info on dual power system

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Registered Users Posts: 21
Hello: Am new, just found this forum. My question is about a small pontoon boat that I plan on using solar panels (around 6 amps) and the standard 12V deep cell battery to power a small electric motor. My question is if I have a 6 amp solar cell set up feeding a battery and the load is something between 5 and 30 amps, will the 5 amp solar cell system short circuit and become non-working or ruined if the motor uses 20 or so amps? My thinking is that the solar cells will just feed into the battery and the battery will allow the higher load to be handled until it can not handle the load and will then load down the solar panel cells. What do those of you that understand this subject think? Thanks.

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Re: Need info on dual power system

Welcome to the forum!

First, a solar PV panel is, for the most part, a current source. Once the voltage is at Vmp or below, the output current is proportional to the amount of sunlight hitting it.

As long as you don't connect the solar panel to the battery backwards--Any normal loads you place on the panel will be just fine (including a dead short).

For your system... Usually it is best to think of the battery as the "heart" of your system. All your loads will feed from the battery--and the battery will keep the voltage between ~11.5 to 14.5 volts (discharging under heavy load, to charging).

If you take the battery below ~20% state of charge--you run the risk of ruining it (i.e., taking a battery to 10.5 volts/dead). On the other hand, if you charge the battery for hours/days at a time over 14.6 volts or so--You will damage it too from overcharging (damage the plates, "boil" out the water from the cells).

A battery is the "voltage regulator" for your boat.

Charging sources (AC battery charger, solar PV panels, etc.) basically put current into the battery to bring it back to full charge. The regulator reduces the current into the battery as it approaches the charging voltage (~14.5 volts is "bulk/absorb" voltage for quick charging; ~13.7 volts is "Float" voltage that keeps the battery charged, but does not boil off water).

You can connect a solar panel directly to a battery bank, but if left connected without any battery charger, it is possible that the battery will be overcharged (most 12 volt panels can charge a battery to ~17.5 volts) if the panel wattage is large enough and there are light to no loads.

Generally, before you purchase a solar PV system for your boat--You should know your loads, battery bank size (voltage/AH), and your expected usage (winter/summer/2 days a week/7 days a week/panels will get full sun but mounted flat/etc.).

In general, most people underestimate their loads and overestimate how much power is available from a solar panel.

Lets say you want run 10 amps 8 hours a day on a 12 volt battery bank. That is:
• 10a*8h*12volts=960 WH per day
And you run the boat from April through August which means you average 6+ hours of sun per day for panels mount flat on the top of the boat. Assuming 0.52 system derating (yes, about 1/2 of the energy is lost (solar panel to charge controller to battery bank to AC inverter):
• 960 WH per day * 1/6 hours of sun * 1/0.52 derating = 308 watt minimum solar panel
For the battery bank, you will want the battery to be ~2x to 6x your daily load (2x is one day storage capacity and 50% maximum discharge, 6x is 3 days of storage and 50% maximum discharge).
• 960 WH * 2 battery capacity * 1/12 volt nominal bank voltage = 160 Amp*Hour @ 12 volts
• 960 WH * 6 battery capacity * 1/12 volt nominal bank voltage = 480 Amp*Hour @ 12 volts
So, a good battery range would typically be between 160 to 480 AH for the mythical load.

Anyway, that is the basic starting point--Please feel free to ask questions and correct my assumptions.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Thanks for the information. There is so much that I do not know that it is daunting. Had an idea on the current load, not a whit on the % of power generated at different seasons. I will have to rethink the boat thing as there may not be enough room on my little 8 ft rubber pontoon boat. I was not really thinking of charging the battery, more like just getting enough current out of the panel to run the motor on low when there was enough light and the battery when sky is overcast or I need to go actually faster. Just trying to extend the length of time fishing if I am in a place with no AC for a charger. So, on a 3 or 4 day trip may have to have the thing just setting on the shore line to generate power again. Which, still won't be all that bad. My camp site panel will use 6x6 8 amp cells and will have double the current capacity as the load. So, may just be a little better on that one. I am glad to hear that the panel will not be overloaded if the current runs to the max on it, allowing the battery to handle the extra current. Thanks again. Oh, another question: I was thinking of using an .060" or an 1/8" acrylic for two 24" x 24" panels. Should I be thinking of glass instead? These will not be in hail areas and if so, I will fold them up.
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Re: Need info on dual power system

In general, you will be very disappointed with the energy from a solar panel driving a tolling motor for several reasons.

First, solar panel just do not output very much energy... A good sized panel will output 7.7 amps between 0 and 17.5 volts (typical 135 watt "12 volt" panel).

And, a solar panel is a current source... Assume that its output current is 7.7 amps in full summer noontime sun:
• Power=17.5 volts * 7.7 amps = 135 watts
• Power=12 volts * 7.7 amps = 92 watts
• Power=6 volts * 7.7 amps = 46 watts
• Power=0 volts * 7.7 amps = 0 watts
When you connect a load to a solar panel (with no battery)--the operating voltage will depend on the load. If you have a large motor, it will pull Vpanel below Vmp... A very big motor will pull it to near zero volts.

From the chart above, you see that the lower the operating voltage, the less power is actually available for the electric motor.

In general... Solar panel + charge controller + battery bank will be the only useful way you could use your trolling motor. Charge for several days, and use the motor for an hour (made up numbers).

Running a trolling motor directly from the panel will (my guess) be close to useless.

-Bill

PS: I am not a big fan of DIY solar PV panels. It tends to be very difficult to build a well sealed, electrically/physically sound panel that is relatively fire resistance.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

In thinking on the small trolling motor solar panel, may just put that on hold. Maybe I just need to buy a 2nd deep cell battery and just make the camp site panel large enough to replenish the power from the earlier days use. Main reason for wanting to make a panel is due to the lack of AC for chargers in remote camp/fish areas and this will allow the use of 12V items (am not going to use inverter/120V stuff, can get all I need with 12V items). Think I will not have the normal issues with sealing the thing as it will not be outside more than 6 or 7 times a year and only a few days on each trip. So, if it rains, will just fold it up or cover it with a tarp. Cause after my little walks in SE Asia in the Army......well I just ain't gonna walk or fish in any rain.
I do have a question on the back of the cells, can something be used to heat sink temp away from the cell by say using heat sink thermal compound on the back touching an aluminum heat sink? Was wondering about compound migrating and damaging the solar cells. Was thinking about using a galvanized sheet of metal as a rear cover touching the heat sinks pulling heat out of the cells. Was thinking that the actual contact to the cell back could be copper fingers just gently touching the cell back and the other side to heat sinks bolted to the back metal sheet. The copper finger I was thinking of like the copper shielding fingers that are used to shield RF room entry doors, thin strips curved and have little force, but would make some thermal transfer.
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Re: Need info on dual power system

So far, no form of solar thermal heat sinking really seems to worth the cost and weight for simple solar pv panels.
And the cells need to be electrically isolated.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Thanks, if the thermal heat sink was just on the back on the cells, and not touching the tabs, wouldn't that be electrically isolated? I was just going to use aluminum channel extrusions on the back metal plate as heat sink to air. Only reason I am thinking of going through this extra is just to get as much power I can out of a rather small (90 watt) assembly and being out in a remote area, it is not a matter of cost, rather the need to charge battery.
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Re: Need info on dual power system

As long as you have some sort of sheet insulator behind the cells that is good for, at least, 70 VDC or so, should not be a problem.

What kind of charge controller are you planning on using? With PWM type charge controller, cooling the cells will not really give you any more energy (as long as the Vmp of the array at operating temperature is above Vbatt-charging+Vcontroller-drop).

Is there some sort of size limitation/weight for the panel?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Hi, well I am just buying a cheap ebay controller (Solar Panel Charge Regulator Controller 12V/24V 200W) and don't even know just what that is versus the PWM controller is that you mentioned. I was only trying to watch out for cell heat up and reducing power output if I could (never knew of the problem). There is no real limit to the size or weight of the unit except it is just for camping and replacing of LED lighting in my tent, any radio power and re-charging a battery that I use on a small pontoon boat. Now, am thinking of buying a 2nd battery and just exchanging each day to keep a working battery. Have not really measured the current on the electric boat motor, but understand it can be 10 amps in the low gears, which I will be using. So, just do not want to run out of battery power to get back to the camp site as I am a disabled Vet and can not think of rowing back, even in a small boat.
I was not thinking that the black part of the cells was conductive as I see small square spaces to solder the positive tabs to the next cell.
Thanks for the replies.
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Re: Need info on dual power system

OK, I see a small 15 amp two speed trolling motor... So lets assume low speed it 10 amps.

Assume a solar panel to charge controller to battery to motor is:
• 0.77 panel+charger eff * 0.80 flooded cell efficient = 0.62 derating.
Lets say you expect to use the motor two hours per day and you have 4+ hours of sun per day. Plus you want to run a few LED's and a radio at night in the camp. Assume another 2 amps for 5 hours a night.
• 2 hours * 10 amps motor + 5 hours * 2 amps camp = 30 AH per day
Assume 12 volt battery bank and 4+ hours of sun per day:
• 30 AH per day * 12 volts * 1/0.62 derating * 1/4 hours of sun per day = 145 watt panel minimum
That is a fairly good size solar panel--Now you are getting in to the realm of asking the question of bringing two good sized batteries plus a solar panel, or just bring two good sized batteries that will give you enough power for your 3-4 day outings.

In general, your camp power should be designed around conservation (LED lights, walkman/ipod type radio/music player, etc.) which will consume nearly no power... LED Head Lamp with 3xAA cells will last for quite a few nights on low.

I have an LED Flash light that will last for a month on low (just enough to wander around camp/home with night adjusted eyes), or several hours on high with two lithium CR123 batteries (high is really bright--lots of light out to 100+ feet). They also have versions that run off of AA cells too (not as much light or run-time on high).

For the boat--if you can measure your power usage (amps * hours) per day--that will tell you about how big and now many storage batteries you need to bring...

If your "off the grid" camping trips are few and relatively far between--I am not sure a solar panel is really worth the hassles or the expense for your needs.

A DIY panel tends to be very fragile (may not last long being moved around). And even commercial panels which will last 20-40 years--are still tempered single weight (~1/8") window glass... A gust of wind, dropped corner, pocket knife dropped on top of the glass just wrong--and you will have a very expensive piece of modern art.

There are several ways to go about measuring your daily power use... One of these DC Amp*Hour / Watt*Hour meters would be ideal (short of purchasing a "real" Battery Monitor--which is probably too much for your needs).

Until you know your real loads--you will run the risk of over buying (too large of battery and solar panel--hard to move/expensive)... Or too small of battery and panel (short battery life, may leave you stranded--and a panel too small to really do you much good on shore).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Bill:
Wow, am I in a area which I should not have tread in! The more I read, and get information on (thanks a lot to you for info) it seems that I have invested in a time limited unit with a half life of a year. So, now it would seem I try should try to sell the stuff coming in or just see the dust settle in the future. Now, it seems to not be as great idea as I thought it was. Guess the old adage: the best plans of mice and men are sometimes equal!
Thanks, again
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Re: Need info on dual power system

BigT,

I am not trying to discourage you--Really just asking questions to help you plan out what it is you need from your system.

Personally, I am not a big fan of DIY panels--although others are. If nothing else, it is a useful learning tool and keeps you off the streets at night.

In the end, I am a big believer in a little back of the envelope planning before jumping off in a, potentially, expensive or hazardous project (don't want to see you stranded in your boat).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Thanks, your information was great! I should have looked into this forum before I jumped into this arena!! Guess, I will embark on a most likely will just have a "standard" run of those who run before they can walk. Thanks again.
• Solar Expert Posts: 1,341 ✭✭✭
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Re: Need info on dual power system
a1bigtuna wrote: »
Hi, well I am just buying a cheap ebay controller (Solar Panel Charge Regulator Controller 12V/24V 200W) and don't even know just what that is versus the PWM controller is that you mentioned.

PWM stands for Pulse Width Modulation.

The cheapest controller just connects the PV to the battery until the battery voltage gets up to a certain point.

The PWM does the same thing, but it does it in pulses. As the voltage rises, the pulses get shorter and shorter. Finally, the controller is doing very short pulses, which are just enough to keep the battery floating at full charge.

Then there is the new style - MPPT, which stand for Maximum Power Point Tracking. That is basically a computer controlled transformer that will adjust the voltage load as needed to squeeze every possible amp out of the PV. They can increase the harvest from a PV quite a bit, on the order of 10%-20%. The PV puts out the same watts, but by lowering the voltage the amps go up.
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Re: Need info on dual power system

To add to DWH's explanation--Here are a couple of solar charge controller FAQ's from our host's, Northern Arizona Wind & Sun / NAWS, webstore:

I knew I forgot to answer something with that torrent of words.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
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Re: Need info on dual power system

Hi, and thanks for all the input. I now have another question: I just got my 6x6" cells and am really surprised at how thin they are, but all the posts state that. My question is if I use EVA for the back side to sort of hold the cells in place and help moisture issues, if I also put it on the front side of the cells, will I loose a lot of power? Or will just laminate it to the front tempered glass be just as good for moisture? Would Tedlar tape seal the panel/frame against moisture as good as anything else? And, would Tedlar tape expand and contract enough if the panel is taken from sea level to say 5,000 feet above sea level so as not to break the seal in expanding air and then when returning to sea level bringing in moisture? Or would the EVA keep any moisture from doing harm. You can see I am woefully dumb in this arena. Just want to thank all of you out there helping someone who got in to something before knowing anything.