How to build a cheap UPS for PC Engines ALIX


ALIX is the name of a nice motherboard family designed by PC Engines to build small fanless, quiet, cheap and still powerful servers. Those motherboards drain so little power that they can be powered by a simple 9V battery ! Typical power usage sits around 3W for a 500 MHz processor.

I recently experienced a power outage which made me realize that not having a battery backup for such servers is a shame, given how easy it is to power them. I experienced a bit with complex circuits involving MOSFETs and transistors in order to achieve the lowest power drop but finally went back to a very simple 3-component design.


Circuit diagram

The principle is very simple. The ALIX power supply delivers 18V to the motherboard. In parallel, a 9V rechargeable battery (B1) is installed in series with a current-limiting resistor (R1). The charging current is defined as the the difference between the power supply's voltage and the battery's voltage, divided by the resistor. I chose 1k5 for R1, which sets the charging current to (18-9)/1500 = 6mA.

A Zener diode installed in parallel with the battery prevents it from over-charging, by draining all the charging current when the battery is fully charged. The diode's voltage must equal the battery's full charge voltage. 9V NiCd battery packs are not 9V in reality, but 8V4 assembled from 7 1V2 batteries. Since they reach 9V6 when fully charged, I used the same value for the diode. The diode may heat a little bit depending on the charging current and the voltage. The power dissipated by the diode equals the difference between the power supply's voltage and the diode's multiplied by the current, which is (18-9.6)²/1500 = 47mW. A small 100mW diode is enough.
A low voltage drop Schottky diode is installed in the discharge path between the battery and the power line so that nearly all of the battery power goes to the motherboard during power outages. The diode has to support at least 1A of continuous current depending on how the motherboard will operate. Also, since the motherboard is equipped with a switching voltage regulator, it will drain a higher current when the battery is low. The diode I used only shows a drop of 0V2, which is quite acceptable.

Test results

My first tests show that the cheapest 120mAh battery maintains the motherboard online for about 10 minutes until the voltage drops to 7V. It requires about 24 hours for a complete charge with the 1k5 resistor. The charging rate may be increased to 10mA with a 820 ohms resistor, but above, a bigger Zener diode will be required.
I also noticed that my SpeedTouch ADSL modem uses a 9V input... I tried the same module on it. Bingo! it worked too. This means that with a few of those modules, I can maintain my network access up during short power outages.

Possible improvements

The 8V4 nominal voltage of this battery pack is very close to the low voltage limit of the motherboard (7V). This leaves a very small work margin. Other battery packs from 9V6 to 12V may be more interesting to experiment with. However, they will require a bigger Zener diode and will probably cost much more.
An interesting enhancement consists in daisy-chaining as many of such modules as there are motherboards to power. This will ensure that all power supplies are able to backup any one which would fail, and it will also optimize the offline duration of the batteries in the even that some batteries are less sollicited. In this case, it would also make sense to use a bigger battery such as a 12V/7Ah as commonly found in medium-sized UPS. Such a battery could power an ALIX motherboard for a full day!

Approximative cost

I got this 9V battery for 1 Euro, but they are generally sold around 5 Euros. There are between 1 and 2 Euros of components, including the male and female jacks. A small plastic box and a PCB to hold the battery and the connectors would be a nice improvement. What would be very nice would be if PC Engines could sell such a module as an option, just as they do with the PoE injector.

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