The Solar Power Expert Blog

My Solar Off Grid Living Plans

by Chris - December 18th, 2011.
Filed under: Boating, Projects. Tagged as: , , .

This article walks through my off grid solar system that I’ll be installing over the next few months.

I’m really excited because I finally saved up enough to by four marine gel batteries for my off grid house boat. Most people think of houses or off grid cabins when they think of off grid solar power systems. In my case, my boat is also my house (or will be when I’m done), but it will also have electric drive motors, which makes it’s design similar to an electric car.

The Batteries

I bought four 8D (form factor) gel deep cycle batteries from West Marine. Each battery is rated for 238 amp hours and I’ll be stringing them in series to create a 48 volt pack in order to power a Torqueedo Cruise 4.0R electric outboard. That’s almost 650 lbs of batteries! They should be able to provide at least 2 hours of continous run time for one electric outboard motor. I expect to get about 1 hour of run time on two electric outboards, but I may double my battery power when I can afford two outboards.

This amount of battery power is adequate for powering a large off grid house. It works out to be roughly 10kWh (kilowatt hours) of battery power. They will be powering the 12 volt off grid appliances on my boat in addition to providing power to the electric outboards, which is why I went with batteries of such high capacity.

I chose to use gel batteries because of their superb deep-cycle life. Depending on the level of discharge I actually subject them to, they can last three times longer (or more) than AGMs (which are known for their great deep cycle life) according to this chart from East Penn:

marine gel batteries cycle life

Deep cycle gel batteries lifespan vs depth of discharge - table from East Penn

The Battery Charger

Note: I no longer recommend this charger for gel batteries. I’ll update this page soon with details as to why this charger is a bad choice for gel batteries.

I also got a great price on this NOCO Gen4 battery charger. Its designed for exactly my type of battery pack – four 12v deep cycle batteries in series to create a 48 volt pack. It’s designed to handle all type of lead acid chemistries (including gel), and will equalize each of the batteries as it charges them at 40 amps.

Equalization is important because it prevents too much charge from building up on one battery. Charge build up shouldn’t happen (theotically), but occurs frequently in practice. Rechargable batteries of all types (including NiCd, NiMH, and LiFePO) will frequently build up charge on one cell in a series of batteries. This can lead to early failure of that cell as it will become overcharged while the other batteries in the series will be undercharged.

Battery chargers that equalize the batteries are able to shift current around to the cells that need it and ensure that each cell in the series is maintained at the same voltage. This significantly improves the working life of the battery.

The Off Grid Power System

The schematic below is a quick sketch I made of the complete off grid power system. In the upper left corner, you can see transfer switch I use to switch the boat’s electrical system between shore power and generator power.

From the 120v power line, my 48 volt and 12 volt off grid power systems branch off. At the heart of each system is the battery pack. The 48 volt pack is made up of the batteries I already mentioned, and the 12 volt system is made up of two Group 31 deep cycle batteries from Costco. Each system has their own battery charger fed by the 120 volt input.

However, both system also have a transfer switch that can connect them to their own off-grid inverters for powering the other system. For example, if my 12 volt batteries are running low, I can power the 48 volt off grid inverter to charge them up. Similarly, if my solar panels have fully charged my 12 volt batteries, I can activate it’s transfer switch to pass energy to my 48v pack through a 12v off grid inverter. I don’t make use of a grid tie inverter since I’m not worried about passing any extra energy to the grid.

off grid power systems

Schematic of my off grid power sytem

The Off Grid Solar System

With the ability to pass power back and forth between the 48v and 12v systems, I can iteratively add more solar and wind power to my boats 12 volt system and pass it along to the 48v system.

Once the battery system is in place, I plan to install an ‘awning’ of sorts above the upper deck which will be covered in my own DIY solar panels. Each panel will run independently at 12 volts, which means if one has a bad day, it won’t effect the others. It also means I can add one panel at a time, which is much easier on the pocket book.

I also plan on installing at least two marine wind turbines on my boat, which will also charge the 12 volt system. Here in the Puget Sound, the sun doesn’t always shine, but the wind always blows. From talking with other boat owners who have used these wind turbines, I can expect a significant portion of my energy to come from the wind.

Of course, I’ll always have my generator on board too. But that will be a last resort.

I’ll be sure to post updates to this blog as well as my twitter account if you’d like to follow along with the install!

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