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Power Supply Deep Dive

The HALPI2 power supply is designed for the unstable electrical environments of boats and vehicles: it tolerates voltage spikes and glitches, limits inrush current, and provides enough stored energy to shut the system down safely when input power is lost.

For the electrical specifications, see the Hardware Reference. For the state machine that acts on the measurements described here, see the Carrier Board Controller reference.

Input Stage

The nominal input range is 10–32 VDC, covering both 12 V and 24 V systems. The input stage is protected against reverse polarity and against transient overvoltage events of up to 100 V, such as alternator load dumps.

Current Limiting

An input current limiter controls the maximum current drawn from the supply, selectable between 0.9 A and 2.5 A with a switch on the carrier board. The limit serves two purposes:

  • It caps the inrush current when the discharged super-capacitors begin charging at power-on.
  • It keeps the total draw within the power budget of the source — the 0.9 A setting (LEN 18) makes HALPI2 safe to power from an NMEA 2000 bus.

The default setting is 0.9 A. Select 2.5 A when the system powers high-current peripherals or when faster startup is desired. The switch location and the procedure for changing it are described in the Hardware Guide.

Super-Capacitor Backup

A bank of super-capacitors provides backup energy for graceful shutdowns. Unlike a battery-based UPS, super-capacitors do not wear out, work across the full temperature range, and charge in seconds — at the cost of a much smaller energy reserve.

Charging

The super-capacitors charge whenever input power is present. From empty, charging takes approximately:

  • 25 seconds at the 0.9 A current limit
  • 9 seconds at the 2.5 A current limit

The front-panel LEDs show charging progress as a red bar filling up. The Compute Module is powered on once the super-capacitor voltage reaches the power-on threshold (default 8.0 V).

Backup Duration

When input power is lost, the super-capacitors carry the full system load. They provide 30–60 seconds of runtime, depending on system load and connected peripherals — sufficient for a graceful operating-system shutdown with margin.

Not a UPS

The super-capacitor system is designed to bridge glitches and to power a safe shutdown. It is not designed for continued operation through extended outages.

Power Loss Detection

The controller measures the input voltage continuously and considers input power lost when the voltage drops below 9.0 V. A blackout timer (default 5 seconds) suppresses shutdown for brief interruptions: the super-capacitors bridge the gap, and operation continues undisturbed if power returns in time. Longer outages trigger the automatic shutdown sequences described in the Carrier Board Controller reference.

Monitoring

The controller measures input voltage, input current, and super-capacitor voltage, and exposes them through the HALPI daemon:

halpi status

The values are also available programmatically through the daemon's REST API — see the Software Guide.

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