claudetools/projects/wrightstown-solar/documentation/system-design.md

# Wrightstown Solar - System Design

**Created:** 2026-02-09
**Status:** Planning

---

## System Overview

Whole-house UPS using DIY 48V LiFePO4 battery packs with Victron MultiPlus II inverter. Seamless grid-to-battery switchover with generator backup for extended outages.

---

## Cell Selection: EVE C40

| Spec | Value |
|---|---|
| Chemistry | LiFePO4 (LFP) |
| Form Factor | 40135 Cylindrical |
| Model | EVE IFR40135 / C40 |
| Nominal Voltage | 3.2V |
| Capacity | 20Ah (20,000mAh) |
| Energy per Cell | 64Wh |
| Weight | 366g |
| Max Continuous Discharge | 50-60A |
| Pulse Discharge | 100A |
| Standard Charge | 10A |
| Fast Charge | Up to 8A |
| Internal Resistance (DC) | <=8 milliohm |
| Internal Resistance (AC) | <=3 milliohm |
| Operating Temp | 0-55C |
| Certifications | CB, UL1642, UN38.3 |

### Why Cylindrical Over Prismatic

- Individual cell failure is contained (easier thermal runaway management)
- No compression hardware needed (prismatic cells need clamping)
- Modular -- add capacity by adding parallel strings
- Smaller units easier to handle
- Trade-off: More connections to make, need cell holders

---

## Battery Pack Design

### Per-Pack Configuration: 16S5P

| Parameter | Value |
|---|---|
| Series cells | 16 (51.2V nominal) |
| Parallel strings | 5 |
| Total cells per pack | 80 |
| Pack voltage (nominal) | 51.2V |
| Pack voltage (full) | 58.4V (3.65V/cell) |
| Pack voltage (empty) | 48.0V (3.0V/cell cutoff) |
| Pack capacity | 100Ah / 5.12kWh |
| Max continuous discharge | 100A (limited by BMS) |
| Weight (cells only) | ~29.3kg (80 x 366g) |

### Full System: 4 Packs (16S5P x 4)

| Parameter | Value |
|---|---|
| Total cells | 320 |
| Total capacity | 400Ah / 20.48kWh |
| Packs in parallel | 4 (at DC bus) |
| Total weight (cells) | ~117kg |

### Expansion Plan

1. **Pack 1** -- Build, test, run for 1 month to validate design
2. **Pack 2** -- Parallel with Pack 1 (now 10kWh)
3. **Packs 3 & 4** -- Build together once confident in process

---

## Inverter: Victron MultiPlus II 48/5000

| Spec | Value |
|---|---|
| Model | MultiPlus II 48/5000/70-50 |
| Continuous output | 4,000W |
| Peak output | 9,000W |
| Transfer switch | Built-in, <20ms switchover |
| AC inputs | 2 (grid + generator) |
| Battery voltage | 48V nominal |
| Max charge current | 70A |
| Transfer relay | Physical disconnect (no backfeed) |

### Key Features for This Build

- **UPS Function:** <20ms transfer time, seamless for all electronics
- **Anti-Island:** Physical relay opens on grid loss, zero backfeed risk
- **Generator Support:** AC Input 2 accepts generator, charges battery + powers loads simultaneously
- **Micro-Grid:** Creates stable AC output that Fronius IG Plus can sync to during outages
- **PowerAssist:** Supplements grid during high-demand periods

### Parallel Option

Two MultiPlus II 48/5000 units in parallel provide:
- 8,000W continuous / 18,000W peak
- Full whole-house coverage including AC units
- Consider for future expansion if single unit isn't enough

---

## GX Device: Cerbo GX

| Spec | Value |
|---|---|
| Price | $320-350 |
| BMS-CAN Port | Yes (500kbit/s) |
| USB Ports | 2 (for dbus-serialbattery driver) |
| Display | Built-in touchscreen or HDMI |
| Connectivity | Ethernet, WiFi, Bluetooth |
| Remote Monitoring | VRM Portal (free) |

### Functions

- DVCC (Distributed Voltage and Current Control) -- lets BMS control charge/discharge
- Battery monitoring and SOC display
- Remote monitoring via Victron VRM portal
- Generator auto-start/stop control
- System configuration interface

---

## Existing Solar: Fronius IG Plus

- **Type:** Grid-tie string inverter (NO battery port)
- **Status:** Keep as-is
- **Normal Operation:** Feeds solar to grid, offsets electricity bill
- **During Outage:** Victron creates micro-grid, Fronius syncs to it and produces solar to charge battery and power loads
- **Future:** Replace with hybrid inverter when Fronius reaches end of life

---

## System Architecture

### Normal Operation (Grid Available)

```
Solar Panels
     |
[Fronius IG Plus] ---> Grid (net metering)
                          |
                    [Victron MultiPlus II] ---> House Panel
                          |                     (pass-through)
                    [48V Battery Bank]
                    (trickle charge / standby)
```

- Fronius produces solar, feeds grid
- Victron passes grid through to house
- Battery maintains float charge
- Victron can charge battery from grid if configured

### Grid Down (Battery Mode)

```
Solar Panels
     |
[Fronius IG Plus] ---> [Victron MultiPlus II] ---> House Panel
     (syncs to            |   (relay OPEN,         (powered from
      Victron              |    grid isolated)        battery)
      micro-grid)          |
                    [48V Battery Bank]
                    (discharging)
```

- Transfer relay opens (grid physically disconnected)
- Battery powers house via Victron
- Victron creates AC micro-grid
- Fronius detects stable AC, resumes solar production
- Solar charges battery + powers loads

### Extended Outage (Generator Mode)

```
[Generator] ---> [Victron MultiPlus II] ---> House Panel
                       |
                 [48V Battery Bank]
                 (charging from generator)
```

- Generator connects to AC Input
- Victron simultaneously powers loads AND charges battery
- When battery full, can signal generator to shut down
- Battery takes over, generator restarts when battery low (auto-start capable)

---

## Runtime Estimates (5kWh Single Pack)

| Load Scenario | Consumption | Runtime |
|---|---|---|
| Essentials (fridge, lights, internet, outlets) | ~500W | 8-10 hours |
| Moderate (+ TV, computers, fans) | ~1,000W | 4-5 hours |
| Heavy (+ AC or electric cooking) | ~2,000W | 2-2.5 hours |
| Full house (everything) | ~4,000W | 1-1.25 hours |

### At Full 20kWh (4 Packs)

| Load Scenario | Runtime |
|---|---|
| Essentials only | 32-40 hours |
| Moderate use | 16-20 hours |
| Heavy use | 8-10 hours |

---

## Installation Plan

### Electrical Work Required

1. **Critical Loads Sub-Panel** -- Move essential circuits to new sub-panel (or use whole panel)
2. **Victron Placement** -- Between grid and sub-panel
3. **Battery Location** -- Garage, basement, or utility room (ventilated, temperature controlled)
4. **Generator Connection** -- Dedicated inlet with proper gauge wiring
5. **Grounding** -- Victron system ground per local code

### Safety Requirements

- DC disconnect between battery and inverter
- Proper fusing on each battery pack (class T fuse recommended)
- Breaker on AC output
- Ventilation for battery area
- Fire extinguisher (Class D) nearby
- Temperature monitoring on cells

---

## Cell Voltage Reference (LiFePO4)

| State | Voltage/Cell | Pack Voltage (16S) |
|---|---|---|
| Full Charge | 3.65V | 58.4V |
| ~90% SOC | 3.35V | 53.6V |
| ~50% SOC | 3.30V | 52.8V |
| ~20% SOC | 3.20V | 51.2V |
| Low Cutoff | 3.00V | 48.0V |
| Absolute Min | 2.50V | 40.0V |

Note: LFP has a very flat voltage curve between 20-90% SOC (~3.2-3.35V). This makes SOC estimation from voltage alone unreliable -- coulomb counting via BMS is essential.