Building A Small Off Grid PhotoVoltaic Power Supply
A small solar power system is a “must-have” and does not have to cost more than $1,000 USD. Here are a few tips that might help. I’ve lived off-grid for years and learned by doing.
If in a sunny part of the U.S., then 200 watts would be my minimum. In the north, I would want 400 watts minimum, and two small inexpensive generators, and two 20-amp automotive battery chargers for redundancy sake, and 50 gallons of fuel for two years. One, or both sets can be used at the same time to charge two separate battery banks, or a larger battery bank of 400 amp-hour capacity. On cloudy days, panel production is about 10 percent of the panel’s rating. I do not need an expensive Honda, when a good used and inexpensive 1,000 watt generator is more cost-effective as a 200 battery bank should only receive a 20 amp-hour charge rate to avoid damage. An automotive battery charger that provides up to 20 amps per hour, requires almost 400 watts from the generator. If only powering a few radios, I should not need to run the generator, except occasionally during the darkest and most cloudy months– typically December through February. A Baofeng or analog scanner only draws .075 amps per hour when ‘listening’.
Two 6 volt golf cart batteries are deep cycle batteries that as a wired-in-series pair that can have about 200 to 220Ah capacity at a nominal 12 VDC when wired in series. I will choose old tech lead acid over modern choices as these are more cost-effective and foolproof. Other forms are attractive, yet they also have special requirements and quirks. Good old wet cell lead-acid batteries are hard to beat, and the maintenance required is well worth it, given their low price and the proven reliability. Also, we do not want a marine battery or a car battery, but a true deep cycle battery, if possible. Starting batteries will not last long if used in the same way as deep cycle batteries are used. Starting batteries will have a Cold Cranking Ampere (CCA) rating. Marine batteries now come with a CCA rating denoting that it is a ‘starting’ battery. These will have a short service life of only months if used as if they were deep cycle batteries.
Deep cycle batteries can provide 3 to 5 years of service if used to their full capacity daily, yet not abused. We must avoid letting the voltage drop lower than 12.4 to 12.3 volts, or to a depth of discharge of no more that 50 percent as indicated by voltage, or better yet, use a hydrometer. Additional photovoltaic (PV) panels will reduce the depth of discharge, and ensure that it is possible to recharge the batteries to 100 percent each day before drawing them down again. Recharge to 100 percent each day to get the full potential service life. If you live in a sunny part of the U.S., then we want, at a minimum, 1 watt of solar power for every 1 ampere hour of battery storage. In Montana, I want 2 to 4 watts of PV power, for every 1 ampere hour of the battery’s rated capacity. We need enough wattage to fully charge up the batteries by midday after period of discharging. PV power is reliable and requires no fuel. It is now much less expensive per watt, and now harder to justify a generator because we can afford additional panels, yet during the darkest part of a long snowy winter in the American Redoubt, a generator will likely be needed.
If a 220aH deep cycle battery bank is used to it’s full capacity, it can be drawn down to a 50% depth of discharge, or about 12.4 to 12.3 VDC (volts direct current), and can provide 100 to 110aH daily. The voltage does not have a direct correlation to the battery’s actual state of charge. It is only a rough approximation. These batteries can provide about 50 percent of their rated capacity if the depth of discharge is no more than 50 percent for up to for 3 to 5 years. To extend the service life of the batteries greatly, reduce the depth of discharge to no more than 20 percent, and the service life could be up to 10 years, if properly maintained. A ‘depth of discharge’ that would be 20 percent can be indirectly indicated by a voltage of about 12.5 VDC. If the amount of power needed each day is known requirement, and we desire up to 10 years of service, then we must increase the size of the battery bank until the depth of discharge will not be more than 20 percent on average.
I have batteries that are almost eight years old that have a voltage of 12.6 VDC. Given the old age of this battery set, 12.6 VDC does indicate a fully charged state, yet 12.3 VDC no longer indicates a 50 percent ‘depth of discharge’. Even if the voltage is 12.6 VDC, the actual capacity, that is the power available is no longer half of the rated capacity once the batteries are a year or more old. Voltage readings are only accurate for practical purposes, when batteries are relatively “young”, or new. As batteries sulfate, or age, it may have enough voltage to do the job, but the amount of power (amp hours at or above the nominal voltage, 12 VDC) that it can provide is reduced.
My 8-year-old batteries still produce 12.6 volts, but only have about 1/4th their original capacity, or approximately 25 amp hours, instead of the 100 amp hours available before it reaches a 50 percent discharge. We would need a ‘load’ tester and hydrometer to determine the actual capacity, as voltage is not a precise indicator, as the correlation between voltage and capacity becomes weaker over time. Even my hydrometer does not tell me how many amp hours are available. A hydrometer will tell me when the battery is fully charged or the percentage charged, but it will tell me how many amp hours I can use before it is completely or is partly depleted. To find out how much capacity a battery actually has, we can use a known load measured by a multimeter that measure amperage that is timed by a clock to see how long it takes to drop to 12.0 VDC, or better yet, when a given specific gravity as indicated by a hydrometer is achieved. Multiply the amperage that the load used by the number of hours it took to drop to 12.0 VDC to derive the actual and usable amount of power the older battery bank can actually provide. If we do not have a multimeter, we could also use a lamp rated in watts. For example, a standard 1134 automotive bulb uses 35 watts. 35 watts divided by 12 VDC = the number of amps that it uses per hour.
Basic Battery Maintenance
Distilled water needs to be added periodically so that the water level does not drop and expose the plates. Water should only be added after the battery is fully charged. Baking soda dissolved in water will neutralize the acid, and remove all corrosion from exterior metal surfaces and connections. To remove all the corrosion deposited from a battery cable end, simply immerse the connector end into a container of water that has a water and baking soda solution, and let it ‘soak’ until all the corrosion is gone. Refresh with new baking soda solution every 15 to 30 minutes if needed. It works wonders. Be patient and persistent, and eventually all the corrosion will melt away exposing a clean metal surface. A battery terminal tool, a knife, or sandpaper can finish the job by making the lead metal ends shiny and able to conduct electricity once again. The baking soda solution should also be used to remove corrosion on the battery terminals after physically removing a much as possible with a wire brush, or cloth and gloved hand. The sulfuric acid and dirt that builds up on the top surface of the battery should also be removed in a similar way. This dust and dirt in combination with the sulfuric acid, even when dry in appearance, conducts electricity between the battery terminals increasing the rate of self-discharge.
Cable, Charge Controllers, and Panel Prices
To reduce the cost of a small ‘solar system’, use automotive fuses, connectors, and standard outdoor extension cord as inexpensive wiring to make ‘home runs’ between each panel to the charge controller. Standard extension cord wire is usually 14 AWG. This means that 100 watt panels should be located no more than 40 feet from the charge controller otherwise there will be a greater than 5 percent drop in the voltage, and the system becomes inefficient. Use heavier gauge wire to extend this distance. Use an online voltage drop calculator to find when there is a 5% drop in voltage.
I can afford replacement PWM charge controllers, but not a spare MTTP charge controller. MTTP is justified in systems over 400 watts, if the cost of the wattage is near $1 per watt, and the difference in price of the MTTP is not more than another panel, or the cost of the heavier wire. It could be that we will need an extremely long run of wire to attain full sun. That fact alone could easily justify the cost of a lower-cost MTTP charge controller. The cost of a particular MTTP charge controller can be offset by the 15 to 30 percent increase in production, and by the reduction in expense if large gauge wiring must be used to limit the voltage drop.
Lastly, PV panels are not all the same. Some are fragile, or their cells or other construction can be of low quality. Renogy is known to be a durable and generally good overall quality panel, and is less than $120, delivered. This class of panel is ideal for small systems. Panels over 100 watts are usually the lowest per dollar/watt, however, must be purchased in quantity, or by the pallet, to realize an actual savings as these must be delivered by freight and have additional shipping costs. We would need to order perhaps 1 to 2kW, or ten to twenty 100 watt to 300 watt panels to beat the price of a Renogy panel because shipping by truck (freight) can be expensive. It has been years since I’ve order panel shipped by freight. I expect freight charges are now much higher. Discover the shipping cost and determine the break-even point against the cost of 100 watt panels that are shipped by UPS.
How Much Power Do We Need?
Without power for comms, we would be ‘hurt’n for certain’. We cannot coordinate our best defense without it. So how would we know how large or small our ‘solar’ system should be? Optimally we need to scan to detect threats, and run two or more Baofengs 24/7, and charge headlamp batteries. To estimate the least amount power needed to accomplish our goal, we can do some simple math if we know a few key numbers. A Baofeng radio on ‘standby’ or ‘listening’, when no audio is heard, consumes 75mA, or 0.075 amps. .075 x 24 hours = 1.8 amps per day. A simple analog handheld scanner will also draw 75mA per hour when scanning. .075 x 24 hours = 1.8 amps per day. A high power mobile or base station radio would use approximately 250mA (milli amp hours) when listening. It is not nearly as efficient as a receiver as the former two examples, so I’ll not use this transceiver when power is scarce to ‘listen’. And instead of charging headlamp battery that are AA or AAA batteries for lighting, I’ll use candles, or kerosene lamps instead. If we do not have enough power to run the radios, then our security operation could fail. I also do not want to run a noisy generator and use precious fuel if I can avoid it. We might be in a long-term and austere environment for many years. Conserving resources must be a daily consideration.
It is also best to estimate potential photovoltaic power production during the least productive conditions imaginable. A 100 watt panel is typically producing power under less than ideal conditions. In real-world conditions, it usually produces at minimum in full sun around 70 watts, or 4.025 amps per hour, for up to 6 hours peak summer, and 2 to 3 hours in the winter, when the sun is shining, and there is no cloud cover. In Montana, I would assume cloud cover for the 3 peak hours during winter months. A 100 watt panel might produce as much as 10 percent of its rating, or less, in full cloud cover, or up to 10 watts, or .575 amps. During periods of the heaviest possible cloud cover expect no power to be produced. This is why a generator is necessary.
Having a gas engine generator assures that critical power needs will be met. It is an alternate means. There are also several contingency means to generate power. One example is a one-wire GM alternator turned by a bicycle. Either or both methods can be more cost-effective than a large solar system, and a large battery bank that might be lost or impaired in some way. During peak hours, with cloud cover, during the winter, we can estimate production. 3 hours x .575 amps = 1.725 amps per day. Power is lost when converted into chemical form in the storage battery, and visa versa, so deduct 30%. I would rather error on the side of caution. 1.725 amps per day – 0.51 amp hours (30% loss) = 1.21 amp hours net power stored per day.
Base Power Requirement
At a minimum, we need 2 Baofengs listening 24 hours per day: One at the OP (Observation Post), and one at the base station. Two Baofengs x 1.8 amps per day = 3.6 amps total amp-hours per day. To produce 3.6 amps per day we need to know how many 100 watt solar panels are needed. 3.6 amps(2 Baofengs) divided by 1.21 amps from one 100 watt panel = 2.97 (three) 100 watt solar panels. To run a scanner in addition to two Baofengs or other handhelds, we will need more than three 100 watts panels during the darkest part of the winter in Montana without running a generator. Running a small 1000 watt generator for 15 minutes per day could produce 5 amps, or 20 amps per hour using one 20 amp automotive battery charger.
A safe rate of charge should be limited to 20% of amp hour rating, or 20 amps for a 200aH battery bank. A 1000 watt generator could easily make the additional power needed every few days, and use only 1/2 pint to one quart of fuel when used on an as-needed basis. Use a generator in an efficient way by charging the batteries only when the battery voltage is 12.5 VDC, or lower, and stop charging immediately when 12.6 VDC is first indicated. This is most fuel-efficient method. Use the generator for bulk charging only, and let the panels top off the batteries.
Applying P.A.C.E to A Power Supply Plan
Military planners often use the P.A.C.E acronym. Our primary (P) source of power generation could be PV Panels that charge a small storage battery bank. The alternative (A) means could be a gas-powered generator, and a contingency means could be a one-wire GM alternator turned by a bicycle, or we can do without a generating source, if our battery bank is large enough to supply all the basic power needs, if all else fails. Should our only gasoline generator fails, we might like to know how long our small 200 amp hour battery bank could provide two Baofengs with power without being recharged. A storage battery bank that is 200aH divided by 3.6 amp hours consumed daily = 55 days! If we had storage battery bank twice that size, or 4 golf cart batteries, or 4 Trojan T=105s, then we would have more than 400aH, or more than 110 days of power. This is enough for 3 months and would see us through the darkest part of winter in the event that we could not afford a second generator, or fuel, or a bicycle-powered generator. I therefore submit an argument for a larger battery bank as a contingency (C) source of power.
It is unlikely that a small amount power will not be generated by our panels during a period of 55 days, and certainly some power will be generated by the panels during a 110 day period. And by spring, there will be enough power to keep two handhelds in operation. We might not need to run a generator at all except to keep the batteries from aging prematurely. An uncharged battery will ‘sulfate’, that is begin to deposit material on the lead plates at an increasing rate when it is not fully charged. This is the process that decreases the battery’s capacity as it ‘ages’. So as a contingency plan, we can discharge the battery to very low levels or completely. It would reduce the life of the battery by perhaps a year or two, but it would still be useful for several years once charged back up.
The “E” in PACE
A larger battery bank of 400ah, or larger, would suffer less from less sulfation, and provide enough power for a long dark winter in the event no power was generated. Finally, emergency (E) communications could be rope, or strong cordage run through a pipe of some kind if the distance to the OP were practical. And if the alternative for radio communications could be a homemade, or surplus TA-1 field telephone that uses no battery power, or a TA-312 that requires just a little battery power from D cells, then a PV system with batteries might not be needed at all. However, considering that I would need to bury about a mile of wire, that might not be feasible. To use either, or both of these options, I would have to move the OP to within practical distances. Given your situation, the choices could be different.
It would be best to maintain a layered defense if at all possible. Radios make that easier. In a total collapse society, we should not attempt to defend ourselves at our mailbox. That would be a very bad plan.