Need info on dual power system

Re: Need info on dual power system

Welcome to the forum!

Regarding your questions...

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

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.

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

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

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

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