Off-the-Grid Power

There are many situations where you want to install a wireless node in an area where the grid providing mains power is unstable or just not existing. This could be a remote wireless relay, or a developing country where the grid fails often.

An autonomous power system consists basically of a battery which stores electric energy that is produced by a wind, solar and/or gasoline generator. Furthermore, electronic circuitry that controls the charging/discharging process is necessary.

It is important to choose a device that draws a minimum of energy when designing an system for operation on solar energy or wind power. Every watt that is wasted on the consumer side causes high costs at the side of the power source. More power consumption means that larger solar panels and bulkier batteries will be necessary to provide sufficient energy. Saving power by choosing the right gear saves a lot of money and trouble. For example , a long distance link doesn't necessarily need a strong amplifier that draws a lot of power. A Wi-Fi card with good receiver sensitivity and a fresnel zone that is at least 60% clear will work better than an amplifier, and save power consumption as well. A well known saying of radio amateurs applies here, too: The best amplifier is a good antenna. Further measures to reduce power consumption include throttling the CPU speed, reducing transmit power to the minimum value that is necessary to provide a stable link, increasing the length of beacon intervals, and switching the system off during times it is not needed.

Most autonomous solar systems work at 12 or 24 volts. Preferably, a wireless device that runs on DC voltage should be used, operating at the 12 Volts that most lead acid batteries provide. Transforming the voltage provided by the battery to AC or using a voltage at the input of the access point different from the voltage of the battery will cause unnecessary energy loss. A router or access point that accepts 8-20 Volts DC is perfect.

Most cheap access points have a switched mode voltage regulator inside and will work through such a voltage range without modification or becoming hot (even if the device was shipped with a 5 or 12 Volt power supply).

WARNING: Operating your access point with a power supply other than the one provided by your manufacturer will certainly void any warranty, and may cause damage to your equipment. While the following technique will typically work as described, remember that should you attempt it, you do so at your own risk.

Open your access point and look near the DC input for two relatively big capacitors and an inductor (toroid with copper wire wrapped around it). If they are present, the device has a switched mode input, and the maximum input voltage should be somewhat below the voltage printed on the capacitors. Usually the rating of these capacitors is 16 or 25 volts. Be aware that an unregulated power supply has a ripple and may feed a much higher voltage into your access point than the typical voltage printed on it may suggest. So, connecting an unregulated power supply with 24 Volts to a device with 25 Volt-capacitors is not a good idea. Of course, opening your device will void any existing warranty. Do not try to operate an access point at higher voltage if it doesn't have a switched mode regulator. It will get hot, malfunction, or burn.

The popular Linksys WRT54G runs at any voltage between 5 and 20 volts DC and draws about 6 Watts, but it has an Ethernet switch onboard. Having a switch is of course nice and handy -but it draws extra power. Linksys also offers a Wi-Fi access point called WAP54G that draws only 3 Watts and can run OpenWRT and Freifunk firmware. The 4G Systems Accesscube draws about 6 Watts when equipped with a single WiFi interface. If 802.11b is sufficient, mini-PCI cards with the Orinoco chipset perform very well while drawing a minimum amount of power.

Another important strategy for saving power is keeping DC power cables short and using a good quality, thick cable. This will keep voltage loss at a minimum.