Universal Mini Router UPS Pro v1.0 — Feasibility and Architecture Notes
Current Architecture Decision
User selected a 3S Li-ion architecture for smooth router and ONU backup with automatic power-failure switchover. The design priority is that the router/ONU must not restart when adapter power fails.
Updated Battery Architecture
Battery pack: 3S Li-ion / 18650 pack.
Nominal pack voltage: 11.1V.
Full charge voltage: 12.6V.
Cutoff voltage target: 9.0V pack-level, assuming 3.0V/cell minimum.
Pack connection: use welded pack pads or a rated 3S pack connector. Avoid spring holders for production/current reliability.
User Requirement Summary
12V DC adapter input via 5.5mm x 2.1mm barrel jack.
3S Li-ion battery pack.
Safe battery charging with temperature monitoring and protection.
Seamless automatic switchover to battery.
Regulated outputs: 12V at 3A continuous / 4A peak, 9V at 2A, 5V USB at 3A.
A 3S pack makes the power current practical compared with 1S, but it introduces a charger constraint: a simple buck charger cannot fully charge a 3S pack from a 12V adapter because the pack requires 12.6V at full charge plus charger headroom. Therefore the design must use one of:
A buck-boost 3S charger / power-path controller that can charge a 12.6V pack from a 12V input.
A higher-voltage adapter input such as 15V to 20V, allowing a simpler buck charger.
A separate boost-precharger stage feeding a 3S charger, which is less preferred for compact production.
Because the user wants 12V adapter input, the preferred production architecture is a buck-boost charger / power-path controller.
No-Reboot Switchover Strategy
To avoid router/ONU reboot during adapter failure:
Use an always-regulated 12V system output rail from a 4-switch buck-boost regulator. It regulates 12V whether the source is 12V adapter or 3S battery.
Use ideal-diode / power-path control so the adapter powers the system when present and the battery takes over when the adapter fails.
Add low-ESR bulk hold-up capacitance on the 12V output rail to cover controller handoff, load transient, and cable inductance.
Keep the 12V rail continuously enabled while the main power switch is on.
Detect adapter present and battery mode with comparator/MCU GPIO, but do not rely on firmware timing for the actual power-path switchover.
Proceed with 3S architecture using an always-regulated 12V buck-boost output and a buck-boost 3S charger/power-path design so router/ONU loads do not see a power gap during adapter failure.