Solar Charging Question

[GreggS]

I have an Oday 25 that needs new wiring and charging system. A few long weekend trips would be my longest-duration on the boat before heading back to the mooring. The electrical house loads that I have planned include:

1. VHF
2. Bilge Pump
3. Composting Head
4. LED cabin lights
5. Nav Lights (LED)

That's about it. Maybe a few additional plug-in devices at most, so minimal load. I would like to put in a 100 or 200 aH Lithium Iron Phosphate battery, knowing that it will carry me through a 4-day trip without needing to charge. Then, while it sits for several days on the mooring, I would like to charge it with a small panel (about 20W), basically trickle-charging the system.

Is a panel this small going to be able to juice the battery, even over several days or a week?

Thanks
Gregg

 

[Leeward Rail]

Battery size seems overkill but you won't know much unless you determine the draw for those specific devices from their documentation.
Similar devices vary.
Do the calculations.
Lots of spreadsheets online for easy calculation.

calculating solar panel and battery size spreadsheet at DuckDuckGo

DuckDuckGo. Privacy, Simplified.

duckduckgo.com

 

[Leeward Rail]

If you haven't chosen the specific battery yet, or need details on using a drop in lithium battery setup, check out these links.

Our resident professional @Maine Sail has some great info on his website.

Drop-in Lithium batteries

How are you feeling these days about drop-in Lithium batteries on boats? I'm thinking of changing a AGM house bank over to Battleborn drop in 100 amp G31's (US company assembling them in the US). These have a 10 year warranty (8 years full replacement). The charger settings are pretty simple -...

forums.sailboatowners.com

DIY LiFePO4 Batteries On Boats - Marine How To

DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals

marinehowto.com

Easy LiFePo4 Conversion - Marine How To

Easy LiFePo4 Conversion Before reading this article please read the 101 article on Drop-In's: Drop-In Batteries Be An Educated Consumer Hardly a day goes by where we don't see or hear

marinehowto.com

 

[Project_Mayhem]

On my O'Day 25 I had a 100w panel and an old deep cycle battery. It powered a small class D bluetooth amp, USB charging, voltmeter, LED nav and interior lights, bilge pump, VHF and engine starter (outboard starters draw even less than a Saildrive's). The batteries were always topped off within a few hours of solar charging. I'd go with a 50w panel if I were to do it again

 

[dlochner]

No one ever said they had too much electrical power on their boat. Many complain they don't have enough.

The 20 watt panel is too small for any serious charging. LiFePO4's do not take well to trickle charging. Trickle charging is a LA battery maintenance charge, i.e., keeping a fully charged battery fully charged. LiFePO4 want to be left alone in a particle state of charge for long term storage and charged very rapidly with a high current.

The first step in the design of an electrical system is to do a load analysis. As @Leeward Rail suggests set up a spreadsheet with a list of all the electrical devices, including tablets and cell phones, rechargeable flashlights and any other electrical devices including the ones you listed. Identify the current draw for each one and how long each day they will be used. This will give you a daily electrical budget. Multiply that number by the number of days you will be off-grid. This will give you the total consumption, multiply that by 1.2 to get the battery capacity you will need. Thus, if you use 40 amphours a day for 4 days you would need a battery with a 200 ah capacity.

On average, a decent quality solar panel will produce about 3 times it nominal capacity each day, A 100 watt panel will produce about 300 watt hours each day, or about 25 ah @12 v. Bright sunny days in the summer, probably a bit more. Cloudy rainy days, a lot less.

Assuming the panels are up and working even on days you are cruising, a 100 watt panel would leave you with a daily 15 ah deficit on average. Over 4 days that would be a 60ah deficit which would take 3 days to restore the battery to 100% SOC.

Your numbers will of course vary, I just pulled the starting point, 40 ah, out of the air to use as an example.

 

[Project_Mayhem]

Another little note... It may be easier to mount two smaller panels than one larger one. You can also have one mounted and one fold up one if that's more convenient for you

 

[jssailem]

To answer this query you need to estimate the Amp Hrs needed. Then you can choose the correct battery for your needs.

I would like to put in a 100 or 200 aH Lithium Iron Phosphate battery, knowing that it will carry me through a 4-day trip without needing to charge.

Armed with an idea of your needs and consumption, you can now identify the type of charge system you will need to prepare the batteries for your next sail.

Or you can just buy some batteries and a solar charge system and see what happens.

 

[Stu Jackson]

John's table is a good approach but he's wrong in including days. The way to do it correctly is by DAILY amp hour draw, not over the number of days you'll be out.

Here's how:

The All-Important Energy Budget:
Energy Budget

 

[jssailem]

You are correct in your approach, Stu.

Reading the OP's question, he stated his desire for a system that would last four days of cruising with charging at the end of the four days. Thus, a four-day consumption calculation is included to size his battery bank.

It is not as elegant as your spreadsheet; it is more of a directed response with a quick and dirty start to a specific solution.

A detailed solution is unattainable without the details of the systems consuming his battery bank charge.

 

[Caboteur]

With @jssailem ’s spreadsheet you may use about 100amp/hrs per 4 day trip. This is mostly summer sailing thus little use of lighting, led anchor light, thé big consumer would be the composting head’s fan.
A 200amp battery wouldn’t be overkill. A 75 to 100w properly regulated panel would keep it all happy. Wren you add refrigeration you just double everything.

 

[Leeward Rail]

Here's a worksheet I found last year:

https://www.custommarineproducts.com/uploads/1/4/3/7/1437708/solar_panel_sizing_worksheet_v1.8.xlsx

Here's a video that explains it:

https://www.custommarineproducts.com/uploads/1/4/3/7/1437708/using_worksheet_2020_v1.0_narrated.mp4

Here's more info:

Manuals, Wiring Diagrams, and Solar System Design Worksheets for Marine Solar Panel Kits, Marine Solar Kit Installation And Assembly InstructionsManuals

Manuals and information on Marine Solar Panel Kits for sailboats and boats who are designing, sizing, installing and selecting their marine solar system.

www.custommarineproducts.com

Seems like a good, old fashioned small business. Likely worth supporting, for those who live in the US

 

[Captain Larry-DH]

With @jssailem ’s spreadsheet you may use about 100amp/hrs per 4 day trip. This is mostly summer sailing thus little use of lighting, led anchor light, thé big consumer would be the composting head’s fan.
A 200amp battery wouldn’t be overkill. A 75 to 100w properly regulated panel would keep it all happy. Wren you add refrigeration you just double everything.

Refrigeration more than doubles everything, but he’s not going to have that. His other uses are not heavy loads and the VHF also uses much less than its rated draw when in listening mode.

 

[Timm R Oday25]

As a fellow owner of an Oday 25 , perhaps my observations might help .
There isn't a great deal of room for solar panels on a boat the size of ours . I mounted a 25 watt panel on an aluminum frame that is clamped out of the back of the stern rail . I ran the wires inside the rear pulpit to keep them out of harms way .
We have another panel that is tethered to the sliding hatch . Each panel charges a seperate battery .
Our outboard charges the first battery until it is 100% .The outboard at max rpm only puts out maybe 6 amps. Still better than the 2 amps from the 25 watt .A fifty watt panel is huge to be hanging off the back . The biggest user of electricity is the autopilot ,which we seldom use .
The next is the chartplotter .

 

[jssailem]

The biggest user of electricity

Beyond Refrigeration, the electronic gadgets - phones, watches, tablets, laptops - we bring are hungry and will devour your battery bank.

 

[GreggS]

There isn't a great deal of room for solar panels on a boat the size of ours . I mounted

This is why I asked about charging after a trip. Throwing solar panels on deck when I’m not on it is probably my best option. I don’t even have a stern pulpit, so my real estate for solar is minimal.

I think I may go with the oversized battery and 75-100 watt foldable panel to set out when the boat is not occupied.

I actually have done the calculations and the load is well under 200 aH for a weekend cruise.

 

[walt]

My .02 with a trailerable sailboat sometimes used similar..

I think what you said in your first post with the 100 to 200 ah li battery and the 20 watt panel and your overall small energy usage is not that far off for going out four days. Given what you said about power needs, I would guess you will maybe use 20 to 25 ah per day max if you also bring along something to entertain yourself like a laptop. Ive tried 10W, 20W, 30W, 40W with similar loads and even longer trips, 10 and 20 watts means you have to be a little energy frugal and monitoring charge usage becomes more important, 40 watt is somewhat "luxurious" power wise. With what you mentioned, you dont need 100 watt. But as someone suggested, anyone who has an overkill of solar never has an issue with it and the excess charge can be easier on batteries since the discharge is lower. If you just dont want to monitor things, go with larger. If you enjoy learning what different loads do and monitoring things, you can get by with the smaller panels.

My suggestions..
Focus on a panel location that gets very little partial shading. Example, mounting something under the boom might mean you need double or more the panel size compared to mounting the panel off the rear pulpit where it gets very good sun. If you get good sun, 40 or 50 watts is likely a little into the overkill range. Put the panel is a poor location and you may need significantly larger panels.

Get some sort of instrument to monitor your amps, amp hours, voltage etc in and out of the battery. These are usually called Charge monitors. This will really give you a good idea of what you are using in real time and over the length of the trip. When you buy a solar charge controller, get one that will link to your phone and allow you to monitor at least what the solar is putting out. The charge monitor tells what goes in and out of the battery, the solar monitor tells what the panels generated. All good info.

Focus on lower power. Example, if you have a car/marine stereo, get something with a class D amplifier. Use a direct 12V to laptop power brick if using a laptop instead of wasting a bunch of power with the intermediate 110V inverter step.

You mentioned all LED lights - good.

You were probably doing this no matter what but get a solar charge controller that allows for future solar expansion. For example, if you ever did decide you wanted an efficient fridge, you would need an additional 100 to 140 more watt panel. Easy to find a good quality charge controller like Victron that does this.

 

[jssailem]

I found "GrapeSolar" panels in 50 and 100 watt units at Home Depot.

https://grapesolar.com/products/

They were fairly priced and quickly available, and they did a good job of meeting the claimed specifications in tests at my house. It may be worth checking them out. There are a lot of options out there.

I decided to utilize Victron products to provide an MPPT controller.

 

[Caboteur]

Why I wrote 200 amp wasn't overkill it is because a 200A battery really has 100A of "usable" power. That would allow the solar panel to be stored for most of the cruise.

 

[jssailem]

a 200A battery really has 100A of "usable" power.

This is true when dealing with FLA (flood acid) batteries. The 50% discharge format has been discovered to maximize the performance of FLA batteries. Deep Cycle batteries used in boat house banks can last 3-5 years, sometimes longer. The long life depends on the charging system and proper maintenance of the battery (maintaining the water level in the cells and occasionally equalizing the battery minimizes sulfate crystals from forming on the lead plates). Deep Cycle batteries can have 500-700 charging cycles during their lifespan.

The LiFePO4 batteries are different. They can be discharged 100% and then recharged without damage. Recommendations are to begin recharging the batteries when they reach an 80% discharge state. These batteries can live through 5000 to 7000 plus charge cycles. These batteries need their own charge system to achieve top performance.

 

[Davidasailor26]

Why I wrote 200 amp wasn't overkill it is because a 200A battery really has 100A of "usable" power. That would allow the solar panel to be stored for most of the cruise.

With a LiFePO4 battery almost all of the 200 AH would be usable. Lead acid is usable to about 50% as a rule of thumb, but lithium is considered 80% or more.