Cabin Solar Power System
This is a modest 12VDC cabin power system with a 1500W inverter to produce 120VAC power for running conventional loads. Some AC loads we will be operating on this system are: laptop computer, compact fluorescent lights (CFL), radio, and a sleep apnea machine. We can use the 12VDC wall outlet for powering cell phone chargers and other small appliances that would plug into a standard car cigarette lighter adapter. In the future we would like to install a 12VDC ceiling fan to increase airflow and better circulate heat from the loft bedroom.
The system is designed for mostly summer time weekend use where we run the batteries down to 50% during our stay and allow the solar modules to charge the batteries while we are away. If the batteries should get run down during our stay, we have the backup generator to power our cabin while the solar modules are given a chance to charge the batteries back to full.
In the future we plan to add a 55A external battery charger (1500W load) that will run on the generator (2000W capacity) to charge the batteries. The generator has an 8A built-in battery charger, but this is too small for charging our battery bank and is designed for trickle charging car batteries. You typically want your battery charger to be rated at 10% of your battery bank capacity as a minimum (44A for us) and up to double that as a maximum.
Our batteries are housed in a custom, sealed battery box to prevent hydrogen off-gassing to our home and the box is vented to the outdoors to allow the gas to escape. This is an essential safety feature for any battery-based system that used flooded lead-acid batteries.
Maintenance on our power system is minimal.
The solar modules need to be clear of debris and will be washed off with water whenever necessary. If needed, we can use simple household dish soap to remove more difficult obstructions. You should never use harsh cleansers on the solar modules. It is very important the solar modules are clear and free of shading because each one operates with two strings of solar cells. If one cell is covered, then we lose almost 50% of the charging capacity from the module. If a hard shadow from a branch falls across the end of the module, that can take out the entire module even though only a small portion of it is covered. Our solar modules are mounted in a shade-free area, so this will not be a problem for us. However, in the case where shading cannot be avoided (like a sailboat), you should use Amorphous type modules instead of Crystalline modules. Amorphous modules are larger per Watt and cost more per Watt than crystalline modules, but they handle hard shadows much better. With an amorphous module, if 5% of the module is covered by debris or shade, then you will lose about 5% of your charging capacity; much better than 50-100% of lost power with a crystalline module depending where the shading falls.
The batteries will need to have the electrolyte/water level checked from time to time. About once per month if you are using the cabin on a continuous basis or at least twice a season (once in the Spring and once in the Fall) for occasional weekend use. Use a hydrometer to take a reading of the specific gravity of the electrolyte/acid for each cell (3 in each battery) and record these readings. All of the readings should be close to one another. The batteries will need to be topped up with distilled water only (or de-mineralized water). You should never use spring, well, or tap water because the minerals in the water will contaminate the battery electrolyte and reduce the battery lifetime. Once the batteries have been topped up, we will use the generator to bring the batteries up to full charge and take hydrometer readings again. At full charge the batteries should all read at about 1.265 SG. This will be lower for batteries that are not at full charge or for batteries that are getting older. That is all the maintenance we can do on our batteries with our system.
If we had a battery charger with an Equalization charge cycle (ours does not have this feature) and the specific gravity at full charge is below 1.265, then we can perform an equalization charge to try to bring this up higher. An equalization charge is an intentional overcharge that will boil the acid in the batteries and shake some sulfate off the plates inside the battery. This can revitalize your batteries and increase their lifetime. The 6V golf-cart batteries in our system will have a 5-6 year lifetime with proper maintenance and used on a continuous basis. Since we will mostly just be using them on weekends in the summer, we could get 8-10 years out of these batteries.