# Wrightstown Solar - Session Log 2026-02-09

**Session:** Initial Research and Planning
**Machine:** ACG-M-L5090

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## Work Completed

### 1. Cell Selection Research
- Researched EVE C40 (IFR40135) cylindrical LiFePO4 cells
- Specs: 3.2V, 20Ah, 64Wh per cell, 366g, 50-60A continuous discharge
- Selected for home solar battery pack build

### 2. Pack Design
- Determined 16S5P configuration for 5kWh packs (80 cells each)
- 48V nominal system, 51.2V actual
- Modular design: build in 5kWh increments up to 20kWh (4 packs)
- Chose 5kWh over 10kWh packs for: cost spreading, manageable builds, fault isolation, single-person liftable weight

### 3. Inverter Selection
- Existing solar: Fronius IG Plus (grid-tie only, no battery port)
- Selected Victron MultiPlus II 48/5000 for:
  - <20ms transfer (UPS-grade seamless switchover)
  - Physical relay disconnect (zero backfeed)
  - Generator input support
  - Creates micro-grid for Fronius to sync during outages
- Architecture: whole-house UPS with generator failover

### 4. Salvaged EV Battery Analysis
- Evaluated Tesla LFP, Chevy Bolt, Nissan Leaf salvage options
- Conclusion: New C40 cells are cheaper ($190-282/kWh vs $365-780/kWh for salvage)
- Salvage has hidden costs: voltage conversion, custom BMS, cooling, unknown SOH
- Most salvage is NMC chemistry (less safe than LFP for home use)
- Decision: Proceed with new EVE C40 cells

### 5. BMS Research (Victron CAN Bus Compatible)
- Comprehensive comparison of 8 BMS models
- Recommended: JK BMS B2A8S20P 150A ($80-150)
  - 2A active balancing (critical for cylindrical cells)
  - Victron integration via dbus-serialbattery driver + USB-UART cable
  - Known gotcha: CAN pinout reversed (use USB-UART path instead)
- Alternatives: SEPLOS V4 (native CAN, $150-250), REC Q16 (plug & play, $559)
- Cerbo GX ($320-350) selected as GX device

### 6. Budget Estimates
- Phase 1 (5kWh + full Victron system): $2,175-2,945
- Full 20kWh system: $3,635-5,750
- Battery-only $/kWh: $86-153

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## Decisions Made

| Decision | Chosen | Over | Why |
|---|---|---|---|
| Cell type | EVE C40 (new) | Salvaged EV packs | Cost, safety, lifespan, simplicity |
| Pack size | 5kWh modules | 10kWh modules | Modular, manageable, fault isolation |
| Inverter | Victron MultiPlus II 48/5000 | Other options | UPS switchover, relay disconnect, generator support |
| BMS | JK BMS B2A8S20P 150A | REC, SEPLOS, Daly | Active balancing, price, community support |
| GX device | Cerbo GX | Raspberry Pi | Easier setup, native BMS-CAN port |

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## Files Created

- `projects/wrightstown-solar/PROJECT_INDEX.md`
- `projects/wrightstown-solar/documentation/system-design.md`
- `projects/wrightstown-solar/documentation/bms-comparison.md`
- `projects/wrightstown-solar/documentation/ev-salvage-analysis.md`
- `projects/wrightstown-solar/documentation/parts-list.md`
- `projects/wrightstown-solar/session-logs/2026-02-09-session.md` (this file)

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## Next Steps

- [ ] Finalize BMS selection
- [ ] Source EVE C40 cells (Grade A, matched, with test data)
- [ ] Order Victron MultiPlus II + Cerbo GX
- [ ] Design physical cell holder / bus bar layout for 16S5P cylindrical pack
- [ ] Plan sub-panel / transfer switch wiring
- [ ] Build Pack 1, test for 1 month before expanding