Design Log DEC 004 supercap charge current 05a under poe - thepinkmile/Enigma-NG GitHub Wiki

DEC-004 - Supercap Charge Current 0.5A Under PoE

  • Status: Superseded by DEC-029 (cell specification updated; 0.5A charge current constraint retained)
  • Date: 2025
  • Category: Electrical
  • Area: LTC3350 supercap charger, PoE power budget

Decision

When running on PoE (802.3bt Type 4, 72W budget), the supercap charge current is reduced to 0.5A (vs. up to 2A on USB-C/Battery). This limits peak PoE utilisation to 73.9% (53.2W / 72W) - within the 75% design rule โœ” (see Certification_Evidence ยง3.5).

Rationale

  • Full 2A supercap charging on PoE would push utilisation to ~98%, leaving <2W margin for transient loads.
  • 0.5A charge current charges the 8x 25F supercap bank (2S4P, 50F total - Abracon ADCR-T02R7SA256MB) in approximately 9 minutes from depleted.
  • Normal system usage is expected to exceed 30-45 minutes per session (startup + configuration + use), making a 2-minute charge time acceptable.
  • This limitation should be documented in the User Manual with guidance that maximum system load is not recommended during the initial PoE power-up window.

Constraints

  • Steady-state PoE load (after caps charged): 50.3W / 72W = 69.9% utilisation โœ”