To be really useful in a school setting where laptops, and tablets often run on batteries, a school server needs to be running on battery itself. That also applies to any 3g modems, or wifi access points, that the students are using, as well.
This page shows a simple wiring harness which makes the connection between these three devices, and the 12v battery; which can itself be charged by a battery charger, if there is sufficient grid power, or by solar panels, and a charge controller, if the power is not reliable, or too expensive.
The coil in the upper left is #16 zip cord from the local hardware store ($.30/ft). Two 5/16-3/8 crimp on terminal rings, marked with red and black shrink tubing, and “+” and “-“, attach to one end of the zip cord. The other end is soldered to the three plugs. In this case, the 90 degree plug is for the school server (size 5.1×2.5mm), and the other two plugs are 5.5×2.1mm, for the 3g modem, and the access point.
In this photograph, a pair of Power Over Internet (POE) passive power inserters is shown. These are shown connected to the yellow cat5 ethernet bundle in the upper right portion of the picture. If the school server needs to be far from the classroom, in order to be close to the solar panels, and battery, the yellow ethernet wire can be replaced by a longer length of ethernet wire (up to 300ft), and the AP will still function correctly (that is, connections via wifi can be more reliable if the wifi signal path is short).
Besides just collecting the parts, the only tricky part in fabricating this power cable is soldering up the junction between the zip cord and the battery cable. I cut one of the conductors shorter than the other, and did the same to each of the plugs cables (chose the wire marked with the white stripe that connects to the inner contact on the plugs). This trick permits the native insulation of the wire to prevent shorts between the conductors. When it’s all done, you should verify with an ohm meter, or a battery and a light bulb, that the negative battery terminal connects to the outside of each of the plugs. If you apply the power backwards to a device, you run the risk of destroying it! Check your work after you are all done.
- 5.5×2.5mm plug for intel NUC (the more standard 5.5×2.1mm will not insert into this computer) http://www.parts-express.com/25mm-x-55mm-x-115mm-dc-plug-with-6-ft-cord–090-492 $2.00
- 2X5.5×2.1mm plug for AP and 3G modem http://www.amazon.com/gp/product/B000S7AQF4 $2.00
- passive POE inserter http://www.amazon.com/HCP05-Passive-Injector-Splitter-Connector/dp/B00DZLSRJC/ $7.09
- zip cord 10ft $3.00
- 5/16-3/8 terminal ring crimp on connector $2.18