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
- Pack 1 -- Build, test, run for 1 month to validate design
- Pack 2 -- Parallel with Pack 1 (now 10kWh)
- 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)
- 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)
- 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 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
- Critical Loads Sub-Panel -- Move essential circuits to new sub-panel (or use whole panel)
- Victron Placement -- Between grid and sub-panel
- Battery Location -- Garage, basement, or utility room (ventilated, temperature controlled)
- Generator Connection -- Dedicated inlet with proper gauge wiring
- 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.
Mike Swanson
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