Portable AP Power for the modern Wi-Fi Professional

There have been various methods used to power APs over the years when doing a wireless survey. One of the first was very much home-brew; a 12V car battery and if you were powering most indoor Cisco APs, an inverter to achieve the required 48 volt input was required. I found this to be a messy, heavy, unscaleable solution. You could find a more elegant alternative such as a sealed motorbike battery but this solution was still far from fantastic. Another solution was to use a UPS; this provides you with a more complete solution as the UPS includes the inverter however you still need to connect either a PoE injector or a power brick to power your AP. On top of this requirement, the weight and size of the UPS is a big concern. Sure, if all your work is in your local area or accessible by car, transport is less of an issue to the site, but moving this equipment around with the AP once you’re on-site and performing the survey - well that is another matter. Various other methods have been used by people over the years but I am yet to see any as elegant as the solution I wanted to detail in this post.

An all-in-one solution came along in the form of the Terrawave PoE Battery. This has been a favourite amongst WLAN professionals for many years, not least because it is bundled with the popular Caster Tray survey rig. You get a fairly compact piece of equipment, you get the inverter (built-in) and you get a convenient RJ45 output. I’ve been happy with this battery for many years but I feel it’s had its day. It has a special output for Cisco’s first 802.11n AP (the 1250) which shows you that it has had a good run but like the 1250, maybe it is time it is put to rest.

The main reason I started look for alternatives to the Terrawave was because it is just too big to travel with in many cases. As I’ve gathered more Wi-Fi tools, the Terrawave has been squeezed out of my survey case as it takes up far too much room and adds too much weight. In addition, there was always a question mark when flying with it. It contains a sealed lead-acid battery and whilst I put it in the same category as a wheelchair battery (which airlines typically have no issues with), I know at some point I will lose it to airport security – you know how they love random boxes of electronics these days, after all! The last problem is that, due to its size and weight, only in certain circumstances could I justify brining a second Terrawave. When travelling for Wi-Fi work I like as much redundancy as possible which a second battery affords. When I did travel with a second Terrawave I had AP battery redundancy that allowed for any issues with one Terrawave as well as extended days surveying (12+ hours) that a single battery couldn’t offer.

My requirements for a Terrawave replacement were as follows:

1. Small
2. Lightweight
3. Airline-friendly
4. Ability to power an AP for a similar period to the Terrawave (quoted as 6 – 8 hrs depending on the number of radios enabled)
5. Ability to power a Cisco AP with its non-standard voltage requirements

Fortunately I found a solution in the Energizer XP18000AB external laptop battery – thanks to @keithparsons for pointing me towards it. As I intended this post to be short I am going to cut to the main highlights of the battery. The main purpose of this post was demonstrate how this battery can power a Cisco AP (due to the non-standard voltage input requirements) so if you need more information on the battery generally, take a look at the numerous online reviews or get in touch. If you just want to see the uptime achieved on both a Cisco and Aruba AP, scroll to the bottom of the bullet points:

• It has a USB output, 9-12V DC output and 16-10V DC output;

• If offers additional flexibility over the Terrawave in its ability to power laptops and other devices. Whilst I survey with a laptop that can see through a full day of surveying (assuming a lunch break charge), an extended day will require more power. Being able to throw one of these into my backpack and tether my laptop to it in order to finish, for example, the last hour of a survey is mighty useful (see comments on this post);

• It comes with adapters (referred to as tips) for most common laptops and devices but additional tips can be purchased from Energizer if required;

• It can power an Apple laptop however unfortunately this requires a few more adapters, one of which can only be bought second-hand and isn’t particularly cheap;

• I won’t list the specific dimensions as you can compare yourself however as you can see in the photo below I can easily fit two of these in my survey kit whereas I would have to ‘vacate’ a large amount of tools in order to fit the Terrawave;

Snug as a... pair of batteries... in a survey case!

• When it comes to improving over the Terrawave when it comes time to fly, it is a mixed bag. Looks-wise, it is obviously much more likely to fly under the radar however as it uses lithium batteries that presents another set of challenges. Airlines and specifically, the International Air Transport Association (IATA) has started to toughen restrictions on lithium batteries. The specific issue with the Energizer is whether it is considered to be equipment or a spare. If it is considered equipment then is can be carried in checked-in luggage whereas if it is considered a spare then it must be packed in carry-on luggage. The major issue is that the spares that the IATA refer to are from an era when a spare lithium battery was commonly a spare internal battery for a laptop, complete with exposed connectors. A more recent example would be a spare (internal) mobile phone battery, once again with exposed connectors. It seems clear that an external lithium battery with no ability to power itself on automatically, nor any exposed connectors, should be classed as equipment and not their traditional interpretation of a spare battery but that doesn’t appear to be the case, at least with some of the airlines I’ve looked at. Note, some airlines I looked at actually had stricter regulations than those dictated by the IATA though most were inline with the IATA.

• All of the APs I’ve looked at besides the Cisco’s (*sigh*) can be powered via 12V. This includes Aerohive, AirTight, Aruba, Meraki, Motorola and Ruckus. With any luck, you’ll be able to power your non-Cisco AP out of the box with the included tips. At worst, you may need to order an additional tip from Energizer.

• In order to power the Cisco APs I bought a Tycon TP-DCDC-2448GD-HP PoE Injector. Yes this does mean I lose the ‘all-in-one’ solution offered by the Terrawave but the numerous other advantages make up for that shortcoming. This PoE injector is unique in that it steps up from 24V (more correctly 18 – 36V) to 56V used to provide 802.3at power. If you only require 802.3af or 100 Mbps support there are additional models available but I think it’s worth paying the extra because I know at some point the Gigabit, 802.3at support will come in handy. Another alternative model, the Tycon TP-DCDC-1248GD-HP takes a 12V input and could also be used with the Energizer. The PoE injector uses a screw terminal for input and therefore I bought a DC power cable with a 5.5mm (OD) / 2.1mm (ID) connector at one end and bare wires at the other. The DC connector plugs into one of the Energizers included DC power cables and I screwed the bare wires into the injectors screw terminal.

The Tycon TP-DCDC-2448GD-HP PoE Injector with DC cable attached to screw terminal

• And finally, the most important information, AP uptime! A few comments first – obviously the uptime you achieve will depend on the AP model, radio transmit power, number of radios enabled and so on. These numbers give a reasonable indication of what can be achieved across both a Cisco and non-Cisco (in this case, Aruba) APs that you may use for an AP-on-a-stick survey, measuring wall attenuation or whatever other purpose you may have for portable AP power. This was not meant to be provide strict scientific results but merely provide a ballpark figure that you may be able to achieve. Adjustments to the configuration I used would obviously yield higher or lower overall AP uptime. My test setup was as followes:

• 2.4 GHz TxPower - 4 dBm
• 5 GHz TxPower - 14 dBm
• Wired uplink - Disconnected
• Uptime determination method – Pinging from associated client to AP (Obviously this additional traffic would lower the uptime of the APs a little so you can expect higher uptime than I achieved during your average passive survey or wall attenuation measurements)

• Cisco 3602i uptime from a single Energizer XP18000AB: 7.75 hrs (with required PoE injector connected)

• Aruba IAP-225 uptime: from single Energizer XP18000AB: 10 hrs (without PoE injector connected as it is not required)

The complete solution

I will keep my existing Terrawave batteries as they certainly can still be useful, but as with a few other pieces of bulky survey equipment, they will remain in the cupboard and only brought with me when required for a specific job.

All in all, I am happy with the two Energizer XP18000AB + Tycon PoE injector solution that comes in at a quarter of the weight of the Terrawave, provides much more versatility and leaves me with much more room in my survey case!

Lastly, I should mention, the homebrew or UPS solutions mentioned at the start of this post do still have a place today. There are always going to be scenarios where a custom power solution may be required and there isn’t anything wrong with homebrew!