# Wrightstown Solar - Project Index

**Project:** DIY Home Solar Battery Storage
**Location:** Wrightstown (home)
**Status:** Planning / Research Phase
**Created:** 2026-02-09
**Last Updated:** 2026-02-09

---

## Project Overview

DIY 48V LiFePO4 battery storage system for whole-house UPS / backup power with generator failover. Designed for modular expansion from 5kWh to 20kWh.

## Target System Specs

| Spec | Value |
|---|---|
| Chemistry | LiFePO4 (LFP) |
| Cell | EVE C40 (40135 cylindrical, 3.2V 20Ah) |
| Configuration | 16S5P per 5kWh pack |
| Nominal Voltage | 51.2V (48V system) |
| Pack Capacity | 5.12kWh per pack |
| Target Total | 20kWh (4 packs) |
| Inverter | Victron MultiPlus II 48/5000 |
| BMS | JK BMS B2A8S20P 150A (recommended) |
| GX Device | Cerbo GX |
| Existing Solar | Fronius IG Plus (grid-tie string inverter) |

## Design Goals

1. Seamless switchover when grid drops (<20ms, UPS-grade)
2. Zero grid backfeed (physical relay disconnect)
3. Generator input for extended outages
4. Modular expansion (build one 5kWh pack at a time)
5. Fronius solar continues working during outages via Victron micro-grid

## Architecture

```
                    NORMAL OPERATION
Grid -----> [Victron MultiPlus II] -----> House Panel
                    |
              [48V Battery]
              (EVE C40 packs)

Generator ---> [AC Input 2]

                    GRID DOWN
Grid --X    [Victron MultiPlus II] -----> House Panel
             (relay OPENS, no backfeed)
                    |
              [48V Battery] (discharging)

Generator ---> [AC Input] (if battery low)
```

---

## Files

### Documentation
| File | Description |
|---|---|
| `documentation/system-design.md` | Full system architecture and component specs |
| `documentation/bms-comparison.md` | BMS models compatible with Victron CAN bus |
| `documentation/ev-salvage-analysis.md` | Analysis of salvaged EV packs vs new cells |
| `documentation/parts-list.md` | Bill of materials and estimated costs |

### Session Logs
| File | Description |
|---|---|
| `session-logs/2026-02-09-session.md` | Initial research and planning session |

---

## Budget Estimate

### Phase 1 (5kWh + Victron System)
| Component | Est. Cost |
|---|---|
| Victron MultiPlus II 48/5000 | $1,400-1,800 |
| Cerbo GX | $320-350 |
| JK BMS B2A8S20P 150A | $80-150 |
| 80x EVE C40 cells | $240-400 |
| USB-UART cable | $15 |
| Cell holders, bus bars, wiring | $60-120 |
| Fuses, breaker, enclosure | $60-110 |
| **Phase 1 Total** | **$2,175-2,945** |

### Phases 2-4 (Additional 15kWh)
| Component | Est. Cost |
|---|---|
| 3x additional 5kWh packs (cells + BMS + hardware) | $1,140-2,010 |
| **Full 20kWh System Total** | **$3,315-4,955** |

---

## Decision Log

| Date | Decision | Rationale |
|---|---|---|
| 2026-02-09 | EVE C40 cells over salvaged EV packs | Lower $/kWh ($190-282 vs $365-780), longer life, LFP safety, simpler build |
| 2026-02-09 | 5kWh modular packs (not 10kWh) | Spread cost, manageable builds, fault isolation, one-person liftable |
| 2026-02-09 | Victron MultiPlus II for inverter | <20ms UPS switchover, physical relay anti-backfeed, generator support, micro-grid for Fronius |
| 2026-02-09 | JK BMS B2A8S20P (recommended) | 2A active balancing critical for cylindrical cells, best value, huge community |
| 2026-02-09 | Keep Fronius IG Plus | Still functional; AC-couple via Victron micro-grid during outages |

---

## Next Steps

- [ ] Finalize BMS selection (JK BMS vs REC BMS vs SEPLOS)
- [ ] Source EVE C40 cells (Grade A, matched, with test data)
- [ ] Order Victron MultiPlus II 48/5000 + Cerbo GX
- [ ] Design cell holder / bus bar layout for 16S5P cylindrical pack
- [ ] Plan sub-panel / transfer switch wiring
- [ ] Build Pack 1, test for 1 month before expanding
