Resound — Power & Battery Subsystem Design
Research/design doc. Current figures at the stated rail; assumptions called out. Sources at end.
0. Summary of the load
| Board | Role | Radio | Rail |
|---|---|---|---|
| SINK x1 | A2DP sink (iPhone -> I2S) | Classic BT RX + some TX | 5V or 3V3 |
| BROADCASTER x3/6/12 | A2DP source (I2S -> speaker) | Classic BT TX (bursty) | 5V or 3V3 |
| HUD x1 | ESP32-S3 + 1.28" LCD + touch | radio idle | 5V or 3V3 |
Dominant load is classic-BT A2DP on the ESP32-WROOM, replicated per board.
1. Current budget
ESP32-WROOM A2DP (3.3V rail): BT TX@0dBm ~130 mA peak; RX 95-100 mA; CPU 30-68 mA@240MHz. A2DP is bursty; continuous average ~110-130 mA, peaks ~130-160 mA (sub-ms 200mA+ spikes absorbed by decoupling).
| Board | Avg | Peak |
|---|---|---|
| SINK (RX-dominant) | 120 mA | 150 mA |
| BROADCASTER (bursty TX) | 130 mA | 160 mA |
| idle/connected no audio | ~80 mA | 130 mA |
| HUD (UI+backlight, radio idle) | 70 mA | 120 mA |
Assumption: TX at default 0 dBm (short range). 3.3V-rail figures; LDO boards draw ~same mA from 5V but burn (5-3.3)*I as heat (~0.2W/board).
Config totals @5V: Small (HUD+SINK+3) = 580 mA / 2.9 W; Beast-6 = 970 mA / 4.85 W; Beast-12 = 1.75 A / 8.75 W. BT bursts uncorrelated -> size regulator ~1.5x average, not Nxpeak.
2. Battery sizing
18650 Li-ion (not LiPo pouch) — vibration/puncture tolerant, energy-dense, mature packs. Cell: Samsung INR18650-35E (3500 mAh, 12.6 Wh, 8 A). 2S topology throughout (always-buck, scales). Derate: 0.90 (buck) x 0.85 (usable) x 0.80 (reserve) ~= 0.61 usable -> nameplate = load Wh / 0.61.
| Config | Target | Load Wh | Req. nameplate | Pack | Capacity |
|---|---|---|---|---|---|
| Small | >=12 h | 34.8 | 57 Wh | 2S3P (6 cells) | 75.6 Wh / 10.5 Ah |
| Beast-6 | >=24 h | 116 | 191 Wh | 2S8P (16) | 201.6 Wh / 28 Ah |
| Beast-12 | >=24 h | 210 | 344 Wh | 2S14P (28) | 352.8 Wh / 42 Ah |
Flags: Beast-12 ~1.3 kg, >100 Wh (air-travel restricted) -> consider modular hot-swap 2S packs or relax target. Small (75.6 Wh) stays <100 Wh.
3. Power path
USB-C -> protection -> CHARGER IC -> 2S pack <-> BMS/balance -> buck -> 5V bus (-> board LDO -> 3V3) [+ optional LED rail]
- Feed 5V (Small): simplest, uses boards' USB/5V pin; ~0.2W/board LDO loss. Feed 3V3 directly (Beast): saves ~13%; bypass each board's LDO, single high-current 3V3 buck.
- Charger: Small = TI BQ25887 (2S, I2C, balancing, ~2A). Beast = BQ25756 buck-boost + USB-PD sink (TPS25750/CYPD3177) for 3-5A (plain 5V/3A USB-C can't charge 28-42 Ah packs in reasonable time). Use NTC + JEITA profiling (hot bike).
- Buck: Small TPS563201 (3A); Beast-6 TPS54560 (5A); Beast-12 TPS546B24A (~10A). Bulk >=470 uF bus + 100 uF/0.1 uF per board for BT bursts.
- LED allowance: budget +0.5A (Small) .. +2A (Beast); ideally a dedicated LED buck so transients don't disturb audio rails.
4. Power BOM (indicative USD)
| Item | Example | Qty S/B6/B12 | ~$ |
|---|---|---|---|
| Cells | Samsung INR18650-35E 3500mAh | 6/16/28 | 7 ea |
| Pack/holder | 2S3P/2S8P/2S14P spot-welded | 1 | 8-40 |
| BMS | 2S 10-20A w/ balance | 1 | 2-8 |
| Charger | BQ25887 (S) / BQ25756+TPS25750 (B) | 1(+1) | 3 / 9 |
| Buck | TPS563201 / TPS54560 / TPS546B24A | 1 | 1-4 |
| (opt) LED buck | TPS563201 3A | 0-1 | 1.5 |
| USB-C | recept + CC/PD | 1 | 0.5 |
| Fuse/protect | PPTC + blade fuse + TVS + revpol P-FET | 1 set | 1-3 |
| Power switch | soft-latch P-FET load switch (TPS22965) | 1 | 0.5-2 |
| Decoupling | 470-1000 uF bulk + per-board | - | <3 |
Rough subsystem cost: Small ~$70-90; Beast-6 ~$160-200; Beast-12 ~$260-320 (cells dominate).
5. Safety / thermal (Li-ion on a motorcycle)
- Vibration: spot-welded nickel packs (not spring clips); pot/foam the pack; strain-relieve leads.
- Temperature: charge 0-45 C, discharge -20..+60 C. Mandatory NTC on pack -> charger TS (JEITA) so charging inhibits when hot/cold. Mount pack away from engine/exhaust; copper-pour + airflow for the buck/charger ICs; switch-mode (not linear) to avoid hot dies.
- Charging: BMS is last-resort protection, not the charger; 2S must balance. Input: reverse-polarity P-FET + TVS + fuse. Charging from the bike's 12V needs an automotive front-end (load dump 40V+), not bare USB-C.
- Pack: inline fuse on pack+ (5A Small / 10A Beast); vent a 28-cell beast pack away from the rider; soft-latch switch for clean HUD shutdown + SOC readout.
Sources
ESP32 datasheet RF current table; Last Minute Engineers ESP32 power; Hubble ESP32 BLE power; pschatzmann ESP32-A2DP; ESP-IDF S3 current measurement; Waveshare ESP32-S3-Touch-LCD-1.28; GC9A01 LCD spec (LCDwiki); Samsung INR18650-35E datasheet; TI BQ25895/BQ25887/BQ25756 datasheets; TP4056 ref; TPS6300x buck-boost; 2S BMS module ref.