# 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

The IP5389 uses cells in **series** (2S to 5S), not parallel. A synchronous buck-boost converter handles all voltage conversion in one IC with a single inductor.

```
USB-C wall charger → IP5389 board → 18650 cells (3S2P configuration)
                       ↓
              USB-C PD output → OPi3B (default 5V)
              USB-A output    → Display controller (5V via micro USB)
              5V rail         → USB hub, audio amp, fans
```

One board, one cell pack, one inductor. No separate charger, BMS, or boost modules needed.

### Why Series, Not Parallel

| Config | Voltage range | Regulator mode | Efficiency |
|--------|--------------|----------------|------------|
| Parallel (1SxP) | 3.0–4.2V | Must **boost** to 5V | ~85-90% |
| 3S (series) | 9.0–12.6V | **Bucks** down to 5V | **Up to 97%** |

With a 3S pack, battery voltage never drops below 9V even at cutoff. The regulator is always stepping down — the easy, efficient direction. The IP5389's synchronous buck-boost handles 3.0V–21V input range and can output 5V/9V/12V/15V/20V via PD negotiation.

## Power Budget

Updated estimates with all components known:

| Component | Voltage | Current (est.) | Power (est.) | Source |
|-----------|---------|----------------|-------------|--------|
| Orange Pi 3B | 5V | 2-3A | 10-15W | USB-C from IP5389 |
| Display + controller | 5V | 2A | 10W | Micro USB from IP5389 or 5V rail |
| USB hub (powered) | 5V | 2-3A | 10-15W | 5V rail via barrel adapter |
| Audio amp (PAM8403) | 5V | 0.3A | 1.5W | 5V rail direct |
| Cooling fans (x2) | 5V | 0.2A | 1W | 5V rail or GPIO |
| **Typical total** | **5V** | **~3-4A** | **~14-20W** | |
| **Peak total** | **5V** | **~7-8A** | **~35-40W** | |

Note: Display controller and OPi 3B can share the IP5389's USB ports (USB-C + USB-A). The hub, amp, and fans tap the 5V distribution rail with a simple barrel-jack breakout.

## Cell Configuration

**Decision: 3S2P** — 6 cells total, two parallel strings of three series cells.

| Config | Cells | Weight (50g ea) | Energy (2500mAh cells) | Energy (3500mAh cells) | Est. runtime (~14W) |
|--------|-------|-----------------|----------------------|----------------------|---------------------|
| 3S1P | 3 | ~150g | 27.75 Wh | 38.85 Wh | ~2-2.5 hr |
| **3S2P** | **6** | **~300g** | **55.5 Wh** | **77.7 Wh** | **~4-5.5 hr** |
| 3S4P | 12 | ~600g | 111 Wh | 155.4 Wh | ~8-11 hr |

**3S2P** (~300g, 55.5 Wh with standard cells) is the pick. Matches a typical laptop battery capacity. With quality 3500mAh cells (Samsung 35E, LG MJ1, or equivalent), the pack delivers **77.7 Wh** — enough for a full café session without recharging.

**Important:** Cells in the same parallel group must be matched (same model, age, capacity). Label and pair them during assembly. Never mix old and new cells in the same pack.

## 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 OPi3B
- Premium price for unused GPIO header
- Capped at 5V output, no 12V rail without extra boost converter

## Safety Notes

- Cells in a parallel group must be matched (same model, age, capacity). Mixing old and new is a fire risk.
- 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)
- IP5389 modules vary in quality — buy from reputable sellers, check reviews for "clone" ICs

## TODO

- [x] Cell configuration: 3S2P (6 cells, ~300g)
- [ ] Select specific IP5389 board model with verified reviews
- [ ] Source matched 18650 cells (Samsung 35E, LG MJ1, or equivalent)
- [ ] Verify passthrough charging on selected board
- [ ] Select PAM8403 audio amp module + small 4Ω speakers
- [ ] Select powered USB hub with 5V DC barrel input (not AC wall wart)
- [ ] Order board + cells + amp + hub
- [ ] Test charging and output before case integration
