Resilience (n): an ability to recover from or adjust easily to misfortune or change
As we discussed in our last energy resilience blog post, two common uses of household power were notable for how little power they use. We labeled these “low-hanging fruit” since making them resilient is both easy and inexpensive. One is lighting, which will be the topic in the next installment of the series. The other is the modem that provides internet connectivity and often the household WiFi connection to it.
A cable or fiber-optic modem device draws minimal power. My fiber-optic modem, for example, pulls only about 12 Watts but is still punching out a strong WiFi signal. Many use even less power.
Backup Power for Modems
While there was a time when a power outage would cause the complete loss of work if you were working on a computer, laptops completely resolved the problem by having built-in batteries capable of operating the equipment for a few hours. In fact, the trend for many equipment manufacturers is to build their equipment with sufficient backup power to be able to survive short outages. As a result, many modems and other individual components, now come with built-in internal batteries.
Some of the “disadvantages” of internal batteries include the need to replace the battery, and also the need to occasionally “reboot” the equipment. In many cases, rebooting equipment with internal batteries entails the temporary removal and reinstallation of the batteries unless a switch has been provided by the manufacturer.
For a modem without internal batteries, there are two easy plug-and-play ways to provide backup power to a modem.
Battery: The first option is a small external battery. Most modems operate off a Direct Current (DC)[1] power supply, usually 12V but sometimes 5V or other low voltage. They are, however, normally very tolerant of voltage variations. Thus, a small backup battery is a compact and simple solution that can be charged and provide power at the same time.[2]
Also, they often come with a USB port that can act as a backup charger for portable devices like a phone or tablet.
The image to the right depicts a battery I bought to back up my home automation system. It cost me less than $40 and works fine for the automation but can run my 12W modem for only 6 hours.
To get longer backup capability one needs a larger capacity battery like the one shown on the left. Capacity is measured in Ampere-hours (Ah) or Watt-hours (Wh). A 142Wh battery unit, as shown on the left, costs less than $100 in the United States, and is otherwise similar to the smaller one above. That would keep my modem running for more than 10 hours.
In fact, these types of batteries can back up many different devices since most electronic devices use a DC supply.
Uninterruptable Power Supply (UPS): For those wanting to back up a modem along with other network devices such as Network Attached Storage (NAS) or computers, a battery is not the solution. That’s because the NAS or desktop computer typically requires mains electric service at 110V Alternating Current (US) or 230VAC (Europe).[3]
The simple option is a UPS. A UPS is just a battery with associated charging circuitry and an inverter on the battery. For those of us with NAS, because the NAS draws much more power than a modem, the backup time is much more limited. This, however, can be mitigated by having the NAS sense the power outage and initiate shutdown long before the UPS is discharged.
A basic yet effective UPS can be purchased for less than $200 in the United States, depending on storage capacity. And even a basic UPS often has a USB connection designed to be connected to a NAS to tell the NAS to shut down when the UPS battery reaches a threshold level of discharge.
UPS or Battery?: Advantages and Disadvantages
Efficiency: A UPS takes power in through a rectification circuit to a battery and then must provide power through an inverter from the battery. This is less efficient than a simple battery which gets power from a rectifier circuit but provides DC power without having to invert it back into an alternating waveform. Inverter inefficiency also means that less of the stored energy in the UPS battery will be available to the backed-up devices relative to the DC battery backup.
Supply Continuity: A simple battery is connected in parallel with the DC power supply, so if utility power to the house or office is lost, the battery is still there. That means there is no discontinuity in the supply of DC power. A modern UPS is not much worse. To maximize energy efficiency, many UPS’s are in standby mode and switch on the inverter circuitry when power is lost from the utility due to storms or other powerline damage. However, this happens so fast that few devices may be sensitive enough to be impacted.
Power Quality: While some UPS systems provide high-quality, alternating current (AC) power with close to a perfect sine wave shape (which 
optimizes efficiency and is less potentially damaging as a result), others only approximate the sine wave to cut costs. This may not have a negative effect on Network equipment and NAS (Network Attached Storage) systems which have sophisticated power supplies, hardened against less-than-perfect supply.
Later in this series we will be dealing with more sensitive systems, so let’s explore sine waves further.
Sine waves of varying frequencies (and other characteristics) can combine to form a wave shape of any form. This is a property of sine waves called orthogonality. But the converse is true. If you have a waveform that is not an exact single-frequency sine wave, it contains waves of a different frequency.
So how would we create a square wave? We would start with a sine wave of the same frequency as the square wave. We would add a smaller sine wave of 3x that frequency (the green line) and then add an even smaller sine wave of 5x the frequency (the orange line) and so on for all odd multiples of the primary frequency. You may continue plotting out the outcome but you will quickly see a square wave emerging. These higher-frequency sine waves are called harmonics, and we engineers go to some lengths to eradicate them.
Why am I banging on about square waves? It is really wasteful to drop voltage across a semiconductor. The voltage drop times the current flow is quickly going to be Watts of waste heat (“Watts and Watts of waste,” get it?). So some UPS manufacturers simply switch the power one way and then the other way–a square wave. This means the user is pumping a lot of harmonics into the backup power being served. Many computers and other power supplies, especially DC, can handle it but motors and other essential equipment hate it. They get mechanically stressed and thermally stressed from losses, which is wasteful and in most senses not ideal.
On a DC battery backup, the battery voltage can fluctuate quite widely. On a nominal 12V circuit, for example, the voltage may be anywhere from 9 – 12.6 Volts depending on the state of charge and the current being drawn. Most often, however, this will not have a negative effect on the supplied devices.
Battery Degradation: The simple battery (DC) solution loses storage capability over time if the batteries are left fully charged for too long. To prevent this, one should fairly regularly disconnect the charger and run the system off the battery alone for a while. This is a problem most often associated with the lithium-ion batteries typically used. Many UPS systems use sealed lead acid batteries, similar to the conventional car battery, which are more forgiving of this, but otherwise not as long-lasting as lithium batteries.
Other Protection: Should a power surge somehow make it through the DC power supply, a battery will do almost nothing to protect the device it is backing up. In contrast, a UPS system typically has surge protection connections. One can connect the external cable or other connections, such as coaxial cable connections or ethernet, to the house to insulate appliances from surges that are high voltage, such as those caused by nearby lightning strikes.
[1] Direct Current (DC) supply is common in electronics. A battery is a DC device. So is a PC power supply, albeit only after it has accepted the incoming mains supply and converted it. One terminal is always and invariably positive and the other is invariably negative.
[2] As pointed out in this article Lithium Ion Battery Charger Circuit: Load Sharing – MicroType Engineering, it is not ideal to connect the battery and charging supply to the load at the same time. But the same article makes the excellent point that battery backup is always about trade-offs. The overriding concerns in this application were simplicity and cost.
[3] Alternating current (AC) is the primary mode of moving power over long distances on a high-voltage electric grid. The reason is that, before the advent of semiconductors, the easiest way to switch voltages was through a transformer. And a transformer requires a fluctuating magnetic field. The half of a transformer that is an electromagnet powered by an electric current that flips from positive to negative fairly frequently is ideal. This is why you hear the hum when you are close to large powerlines or substations.
Part 1 | Part 2 | Part 3 | Part 4
