Note: This is still in draft form. Some sections may need completion and/or editing.
An often overlooked but critical system for amateur radio sites is the power supply.
AC or DC, utility or renewable, which battery technology?
Let’s examine the fundamentals.
In the perfect world that only exists in physics textbooks where every system is friction-less and all experiments perfectly match the corresponding equation, all power systems would be equal and our only concerns would be convenience and cost. In the real world, things are messier and we have to deal with the electrical equivalent of friction — heat.
Whenever we change from one voltage to another or switch between alternating current and direct current, we create heat as a byproduct. This heat has to come from somewhere — either it makes your utility meter spin faster or it eats away at your power generating capacity and power storage. It also has to go somewhere. If a site is in an arctic winter, it may help to warm your equipment to keep it within its environmental operating limits. If a site is in a desert summer, you may have to double your power budget to extract that heat from your equipment with air conditioning or a heat exchanger.
Whether they have an AC power cord or DC terminals, all of our modern radio equipment runs off of DC internally. AC was chosen for our power grids in the early days of electrification for its ability to travel long distances, turn motors and change voltages with a simple transformer. Now, almost everything we plug in has some digital components and therefore also has an AC to DC power converter.
Not all power conversions are equal. Inverters, which convert from DC to AC are notoriously lossy — in other words, they create a lot of waste heat or are inefficient. This is why the inexpensive uninterruptible power supply, or UPS, under your desk has such a short runtime even under a small load. Also, it’s generally more efficient to go from a higher DC voltage to a lower DC voltage than the other way around.
Let’s examine how a UPS works to understand its inefficiencies. UPSs come in three flavors: standby where, as the name implies, the UPS does nothing but monitor the input power and charge its batteries until the power fails, line interactive, which is a more modern take on the standby UPS and can help boost power during brownouts and online, which is the gold standard for providing the cleanest AC power, but also the least efficient. All three technologies are converting AC power to DC to charge their batteries whenever it is available. In a standby or line interactive UPS, the inverter which converts DC battery power to AC output power remains idle unless line power fails. The downside to this arrangement is an imperfect switchover. The power supplies in your equipment have to use their internal capacitance to keep everything from rebooting when power switches from utility to battery and back.
Online UPSs always feed clean inverter power to their loads, while only using input power to charge their batteries. This keeps your equipment and your electrical utility’s finance department happy and your wallet empty. Remember that when you’re powering your equipment this way, you’re going from AC to DC to AC to DC all the time. UPS manufacturers do their best to minimize these inefficiencies. For example, while that little standby UPS under your desk probably runs off of a 12 volt battery that has to be stepped up to 120 or 220 volts, a lot of online UPSs use high DC voltages close to their AC line voltages by putting their batteries in series.
In the early days of telephony, phone company engineers realized that they could keep their DC-powered equipment online and safe from instabilities of mechanical generators and the emerging power grid by running it off of large battery banks while relegating generator and grid power to charge the batteries. This concept is similar to some diesel submarines where a battery-powered electric motor turns the propeller and the diesel generator is used when at or near the surface to charge the batteries. Fundamentally, this is the same technology in any modern DC power system.
Power sources can be rectifiers (AC to DC converters), DC generators which convert a combustible fuel to mechanical energy then to DC current, solar panels, wind or water turbines and thermoelectric generators which convert heat (usually from combustion) directly to electric current. In DC, these are easy to parallel. In AC, they have to be synchronized.
Batteries can be “floated” in a system where a constant power source is expected — for example, in a site with utility power and a backup generator. In an off-grid site, a charge controller intelligently provides batteries the current they need to recover from their discharged state after a night without solar, a day without wind, etc. The current state of the art is the Maximum Power Point Tracking charger.
A variety of chemistries are readily available for DC power systems. The most common and economical is Sealed Lead Acid (SLA). These are a deep-cycle maintenance-free version of the battery commonly used for starting your car. (Note the differences — do not use a cranking-optimized car battery for your DC plant.) SLA batteries should always be installed with adequate ventilation.
Lithium Iron Phosphate (LiFePO4) batteries have a much higher up-front cost, but have a number of advantages including lighter weight and longer life. They require intelligent charging — failure to use an intelligent charger can result in a fire. Many have charge circuits built in, so be sure to take this into account when you design your system.
Batteries should be kept from freezing and over-heating. On a cold mountain top, a thermostatically controlled battery heating pad is a simple and inexpensive option. In a hot climate, thermal mass can help regulate temperature if HVAC isn’t available. Battery boxes can be buried or drums of water can be strategically placed around the enclosure. Shade and enclosure paint color can also be used to help with thermal stability.
Connections from batteries to your power bus should be protected with a fuse or circuit breaker. Fuses between batteries wired in series are also a good idea so that a failed battery doesn’t damage the rest of the string.
If you install quick disconnects, such as Anderson Powerpole connectors between your batteries and your power bus, battery replacement will be both easier and safer.
Plan on checking your battery plant at every visit to the site and put a minimum interval on your calendar for rarely-visited sites. Be sure to put expected replacement dates both on your calendar and your budgets. No battery technology lasts forever.
Low Voltage Disconnect
When charge sources are lost, voltage drops as battery power is depleted. Draining a battery below it’s minimum permitted voltage will damage it. This also has the potential to damage attached electronics by running them below their minimum required voltage. It can even blow breakers and fuses, since amperage of a given load will increase as voltage falls to maintain the same wattage. To prevent this, we use a low voltage disconnect, which will power everything down if battery voltage reaches this critical level.
Just as we install breaker panels on our AC power system, we must do the same with DC. Fuses and breakers prevent excessive current from melting the wires from the power supply to the load, shut off power in the event of a short, protect personnel if they inadvertently become part of a circuit and isolate faulty equipment from taking down larger parts of your system. Since a lot of DC connections are made with screw terminals instead of plugs, they also give us a way to power off individual connections to service them.
More advanced power distribution devices also include remotely controlled relays which allow you to remotely power cycle devices that are misbehaving, preventing a site visit and requisite travel time just to press a reset button.
Monitoring devices provide voltage, temperature, amperage and other readings in multiple points within your power system to alert you to problems, hopefully before they cause an outage.
Integrated systems are readily available. While the added expense may seem excessive, the manpower savings when they are deployed and serviced often makes up for it. This is especially true when you have trouble finding qualified personnel in your market and you have to optimize the use of your existing talent pool. Repair times are also reduced when field replaceable modules can be quickly swapped.
Ultimately, there is no perfect power system for every application. In fact, hybrid systems are common, where AC is used for HVAC and larger loads and DC is used for the most critical radio components.