Power Systems for Off-Grid Vacation Cabins: Page 3 of 5

Intermediate

Inside this Article

In an off-grid, part-time cabin, battery bank sizing and care are often more important than PV array size.
This off-grid cabin has solar water heating collectors and solar-electric modules.
This tiny house, occupied part-time, requires only a couple of PV modules to meet its electricity needs. A solar water collector provides domestic water heating.
This compact, modified sine wave, off-grid-only inverter is offered in 600 W and 1,500 W versions. It has a built-in 120 VAC charger for charging the battery bank from any AC source, such as a generator.
This multimode inverter can receive grid power and send out PV power, and can also operate in off-grid mode.
Nickel-iron batteries are an “old” technology that offers superior longevity, but also has efficiency and financial costs.
High-performance lithium-ion batteries are light and small, but require sophisticated charge management and are expensive.
Saltwater batteries are quite new to the scene but the industry is hopeful about their efficacy.
AGS units usually work fine—it’s other, less reliable parts of the system (like the generator and fuel supply) that cause many pros to discourage using them.
Though often less than desirable because of the noise and pollution it generates, a backup generator for battery charging during times of little sun can be a necessary addition.
The author’s family cabin, at 8,200 feet of elevation in the northern Colorado mountains, uses a small PV array to keep full-time loads running and batteries well-charged, even when it’s vacant.
Most inverter manufacturers and third-party companies offer remote system monitoring, but you’ll need always-on internet access at your cabin.

Operations & Maintenance When Vacant

PV systems require very little regular operations and maintenance care, with one exception: the battery bank. Lead-acid batteries—no matter what type—will be permanently damaged by sulfation (deposits of sulfur on the lead plates, which partially block electrolyte contact with the plates) if left at a low SOC for an extended period of time. Simply shutting down the entire system including PV modules, inverter, and loads doesn’t help this situation, as all lead-acid batteries “self-discharge” when sitting unused, noticeably lowering their SOC as time goes by. Therefore, it’s essential to keep the PV to the battery-charging side of the system operational during absences.

Remember also that PV charge controllers themselves are phantom loads that use a small amount of power all the time, typically 1 to 4 W. That can be problematic in cold, remote locations where the PV array can be covered with snow for weeks or months at a time with nobody around to clear it. One solution sometimes used by wily remote system designers is simply a single PV module mounted vertically on a south-facing exterior wall. It doesn’t have to live there all summer long, and can be quickly deployed just before vacating for the winter. That single module can provide enough charge to make up for battery bank self-discharge, plus charge controller and (possibly) inverter phantom loads, over an entire winter. (Note: This strategy assumes the module, charge controller, and battery bank voltages are carefully selected to allow the single module to charge the battery bank.)

All lead-acid batteries emit gas when charging, and some require the electrolyte be topped off regularly with distilled water. If this maintenance isn’t performed and the electrolyte level drops below the tops of the internal plates, the batteries will be permanently damaged. Different strategies to solve this problem depend on exactly what type of battery is selected for the installation, and how often the batteries need to be topped off.

Temperature also plays a role in battery selection for an unattended system. Consistently high temperatures (greater than 80°F) cause all battery types to age prematurely, which can be a problem in desert and tropical locations. Cold temperatures won’t damage most fully charged batteries. Common lead-acid batteries are good down to -50°F (and lower)—but only if fully charged. If deeply discharged, their electrolyte is mostly water with the sulfuric acid essentially absorbed into the lead plates; they can freeze and even burst at temperatures near 0°F. Never try to charge a frozen battery; it must be removed from the battery bank, allowed to thaw, and then charged very carefully to see if it is salvageable. Most likely, it will need to be sent for recycling and replaced. I highly recommend a sealed, vented, and insulated battery bank enclosure for all battery types. This tempers both heating and cooling of the batteries.

Battery Types & Charging Strategies for Unattended Locations

Using the proper battery type is crucial when the system won’t be receiving regular maintenance for an extended period of time. Ambient temperature is an important consideration, as is the specific charging regime programmed into the charge controller.

Flooded lead-acid batteries are a common and cost-effective choice for many remote installations. They handle low ambient temperatures gracefully and without damage, as long as they are kept at full (or nearly full) by the PV array. Their biggest disadvantage is that they require regular watering—at least four times per year, and sometimes more frequently. Automated watering systems with a central reservoir and valves in each cap are available, but will not function if the battery enclosure can reach temperatures below freezing. Catalytic re-combiner caps are also available, but do not entirely eliminate the need for regular watering—they simply reduce the frequency.

Flooded lead-calcium batteries are another option, though they are more expensive and difficult to source. They use calcium instead of antimony in the plates, resulting in less water use and a slightly lower voltage versus SOC curve.

Clever PV installers have found that it is possible to change the charging regime to reduce gassing in flooded lead-acid and lead-calcium batteries by changing the charge controller settings. Setting the absorb voltage high (for example, 15.0 V for a 12 V system), absorb time to only an hour or two, and the float voltage quite low (around 13.0 V for a 12 V system), minimizes water loss. This runs the risk of not charging the batteries to 100% SOC, but during an extended absence of a month or more, they will likely fill with no trouble if there are no loads during this time.

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