BarnacleBoy db353ad2d1 add power system: IP5389 board + 18650 array decision

- IP5389 power bank board selected (preliminary)
- All-in-one: charging, BMS, USB-C PD output
- Rejected K501U battery (integrated BMS not reusable)
- Documented alternatives considered (TP5100+2S BMS+buck, UPS HAT)
- Updated parts list with power system BOM
- Cost estimate now 06-230 (was 70-190)
- Added power-system.md, updated design-decisions.md

2026-05-12 06:02:51 +00:00

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Power System

Decision: IP5389 Power Bank Board (Preliminary)

All-in-one power management PCB. Replaces separate charger module, BMS, and boost/buck converter with a single board.

Why This Approach

Requirement	IP5389 Solution
Charge lithium cells safely	Built-in CC/CV with cell balancing
Battery protection	Overcharge, overdischarge, short circuit, thermal
Output usable voltage	USB-C PD (5V, 9V, 12V, 15V, 20V)
Wall power input	USB-C input from any charger
Charge while using	Passthrough charging
Repairable	Bare PCB, cells are replaceable 18650s

How It Works

USB-C wall charger → IP5389 board → 18650 cells (3S or 3S2P)
                       ↓ (passthrough)
              USB-C PD output → OPi5 (5V) + peripherals

One board, one cell pack, one output cable to the SBC. No separate modules to wire together.

Key Specs (varies by specific board)

Cell config: 3S–6S (most common: 3S or 4S)
Output: 45W–100W USB-C PD
Input: USB-C (bidirectional port)
Protection: overcharge, overdischarge, short circuit, thermal
Price: $8–15 bare PCB

Cell Configuration

TBD — need to select specific board first. Likely 3S (3 cells in series) or 3S2P (6 cells, pairs in series). 3S2P with quality 3000mAh cells gives ~6000mAh at 11.1V nominal.

Salvaged Battery: Not Usable

K501U battery (B31N1429, 4240mAh, 11.4V, 3S) assessed and rejected:

Integrated BMS is paired to the laptop mainboard
Most laptop BMS boards lock when removed from their system
Not worth the effort to hack around
Low capacity (4240mAh) doesn't justify the risk

Alternative Approaches Considered

Discrete Module Stack (TP5100 + 2S BMS + Buck Converter)

Three separate modules, more wiring
TP5100 charger (2S, 2A) + 2S 10A BMS + 5V/4A buck converter
~$10 in modules, works well but more complex assembly
Backup plan if IP5389 board has issues

Commercial Power Bank (decased)

Buy a power bank, remove plastic shell, integrate bare PCB + cells
Zero design work but constrained to whatever cell layout the bank uses
Passthrough charging not guaranteed on all models

UPS HAT (Waveshare / DFRobot)

Designed for Raspberry Pi, works via USB output with OPi5
Premium price for unused GPIO header
Capped at 5V output, no 12V rail without extra boost converter

Safety Notes

Always use 18650 cells with matching capacity and age in parallel pairs
Fuse between board output and SBC (5A automotive blade fuse recommended)
No puffy, warm-at-rest, or sub-2.5V cells — ever
Match cell count to board specs (3S board = exactly 3S, don't mix)

TODO

Select specific IP5389 board model with verified reviews
Confirm cell count (3S vs 3S2P)
Verify passthrough charging on selected board
Order board + cells
Test charging and output before case integration

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