[Server-devel] smooth shutdown for 12V-DC powered XS

James Cameron quozl at laptop.org
Tue May 8 18:00:15 EDT 2012

I understand the requirement.  There are two approaches.

One measures voltage, and when it goes below a threshold begins
shutdown, and when it goes below a final threshold it disconnects the

The second approach is a charge controller, which estimates the
capacity of the battery bank by measuring the power input to the bank
against the power output.  When the estimated capacity drops below a
threshold, the shutdown is begun.

It is possible to use both approaches.  Each has advantages and

I don't know of any ready-made examples.

If I were using an Arduino, then I would need:

- measured proof that the USB ground is the same as the power supply
  input ground,

- a USB Arduino, such as the Uno,

- one spare USB port, to connect the Arduino to,

- a 10k trimpot, as a voltage divider, dividing the 0V to 15V input
  signal down to the 0 to 5V range,

- a sense cable from the trimpot to the battery bank (because if the
  computer 12V input is used, the resistance of the wiring between the
  battery bank and the computer would affect the result),

- software on the Arduino, which reads the voltage and sends it to the
  computer, (roughly half an hour of work for an Arduino expert,
  roughly two hours for a non-expert),

- software on the computer that reads voltage, averages several
  samples, and shuts down.

A wasteful alternative would be a UPS with no load, with the power
down signal cable of the UPS connected to the 12V DC computer.

James Cameron

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