[Server-devel] 12 Volt power system for School Servers.

David Leeming david at leeming-consulting.com
Thu May 30 03:24:14 EDT 2013

We have a primary school in PNG with school server driving 9 wireless access
points with PoE to cover a 200 laptop deployment with 7 classrooms 


Powered by 12V solar using the excellent Sundaya kits. The wiring looks
untidy, I know, but they have worked very reliably over 2-3 years. Also the
integrated batteries and proprietary DC connectors make it difficult to
misuse the power (a common cause of early demise of batteries)


The Mikrotik GrooveA range of outdoor wireless access points are really
good. They will work on 9-27V so even with 12V, you can get very long cable
runs, and you can screw in a vertical omni or your antenna of choice. We
didn't use them in the deployment above but I would do in the future.

David Leeming
Solomon Islands

-----Original Message-----
From: server-devel-bounces at lists.laptop.org
[mailto:server-devel-bounces at lists.laptop.org] On Behalf Of James Cameron
Sent: Thursday, 30 May 2013 4:17 p.m.
To: Tony Anderson
Cc: server-devel
Subject: Re: [Server-devel] 12 Volt power system for School Servers.

Tony makes an interesting point that may be new to some:

On Thu, May 30, 2013 at 06:47:26AM +0200, Tony Anderson wrote:
> This discussion is relevant because 12vdc does not travel well over
> long distances. It may be necessary in a school with multiple
> classrooms to use an inverter to provide 110 or 220vac to the
> routers via PoE.

As the distance between the battery bank and the access point
increases, so too does the required _diameter_ of copper cable.

Copper cable is quite valuable, and easily traded.  It can be a good
security practice to avoid installing too much of it.

If the cable is too thin, the power demand by an access point causes
too much of a voltage drop.  For instance, one might put 12V DC in one
end, and get 10V DC out the other end.  The access point may still
work fine.  But some of the power will be lost as heat in the cable,
lowering efficiency.

At some point, depending on cost, reliability, and availability of
equipment, it is worth operating at a higher voltage.

The options would seem to be Power over Ethernet (PoE), which will
yield 12.95W (IEEE 802.3af-2003) or 25.5W (IEEE 802.3at-2009), or an
inverter with a separate power circuit.

My best guess is that PoE will be cheaper, because a power circuit
installed to meet electrical regulations would tend to be designed for
the minimum domestic power standard of the country.  For example, in
Australia the minimum is 10A at 240V, or 2400W.  The mass of copper
is far more than what PoE would require.

Then there's the cost of a 12V input PoE power sourcing equipment

James Cameron
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