If you crack open a Tesla EV skateboard chassis or a Powerwall 3 enclosure, a Tesla battery looks like a dense, highly engineered brick of cylindrical or prismatic cells bonded in structural foam, interlaced with glycol cooling ribbons, and managed by a distributed Battery Management System (BMS). In older Model S and X vehicles, it looks like a heavy metal box divided into 16 distinct, removable 5.2kWh modules. For DIY solar builders using salvaged EV modules or building 48V home storage, it looks like a tightly packed array of 18650 or 2170 cells wired with thick copper busbars and wrapped in Kapton tape.
Understanding the physical and electrical architecture of these packs is critical whether you are tearing down a salvaged Model S module for an off-grid cabin or spec'ing a commercial Powerwall install. Below, we break down the exact cell layouts, system block wiring, and the sizing math required to integrate Tesla-style lithium architectures into your 48V DC or 120/240V AC system.
The Physical Anatomy: EV Packs vs. Home Powerwalls
Tesla's battery design has evolved from modular metal boxes to fully integrated structural components. In the early Model S, the battery was a flat, rectangular enclosure containing 16 individual modules wired in series. Each module contained hundreds of 18650 cylindrical cells arranged in a 6s74p configuration (6 series groups of 74 parallel cells). If you pulled a module out, it looked like a dense, brick-like tray with a prominent cooling tube snaking through the bottom and a slave BMS board mounted on the end.
Today, the 4680 structural pack (found in the Austin-built Model Y) looks entirely different. The cells themselves are larger and feature a tabless design. Instead of being housed in removable modules, the 4680 cells are glued directly into the vehicle's chassis using a rigid polyurethane foam. The pack acts as a structural floor member, with cooling plates bonded directly to the cell bodies. According to teardown analyses by Argonne National Laboratory, this cell-to-chassis design eliminates module housings, reducing dead weight and increasing volumetric energy density.
For home energy storage, the Tesla Powerwall 3 uses a dense matrix of 2170 NMC (Nickel Manganese Cobalt) cells. Unlike the EV packs that operate at 350V-400V, the Powerwall 3 and most salvaged DIY modules are stepped down or configured for 48V-52V nominal architectures to interface with standard low-voltage hybrid inverters.
| Cell / Pack Type | Form Factor | Nominal Voltage | Typical Configuration | Energy Density | Max Continuous C-Rate |
|---|---|---|---|---|---|
| Model S/X (Legacy) | 18650 NCA | 3.6V (Cell) | 96s74p (85-100 kWh) | ~250 Wh/kg | 1.0C |
| Model 3/Y Long Range | 2170 NCA/NMC | 3.6V (Cell) | 96s46p (75-82 kWh) | ~260 Wh/kg | 1.5C |
| Model Y (Austin) | 4680 NMC | 3.6V (Cell) | Structural (Cell-to-Chassis) | ~272 Wh/kg | 2.0C+ |
| Model 3 RWD | Prismatic LFP | 3.2V (Cell) | ~106s1p (60 kWh) | ~160 Wh/kg | 0.5C |
| Powerwall 3 (Home) | 2170 NMC | 52V (Pack) | ~15s (13.5 kWh usable) | N/A (Stationary) | 0.8C (11.5 kW out) |
System Block Architecture and Sizing Math
When adapting Tesla-style lithium modules for a home solar or backup system, you must map the DC source to the AC load through a strictly ordered system block. The standard signal and power flow is:
Solar Array / Grid → MPPT Charge Controller / Grid Tie → Hybrid Inverter/Charger → BMS → Battery Pack → AC Load Panel
The BMS must sit between the inverter and the battery pack, monitoring individual cell group voltages and temperature. If a cell group hits the low-voltage cutoff (typically 2.8V for NMC, 2.5V for LFP), the BMS opens the main contactor, physically disconnecting the pack to prevent irreversible copper dissolution inside the cell.
Series vs. Parallel Consequences
Understanding how to wire salvaged modules requires a strict grasp of series and parallel rules:
- Series (S): Voltages add, Amp-hours (Ah) remain constant. Wiring fifteen 3.2V LFP cells in series (15s) yields a 48V nominal pack (15 × 3.2V = 48V). If each cell is 100Ah, the total pack is 48V at 100Ah.
- Parallel (P): Amp-hours add, voltage remains constant. Wiring four 48V/100Ah strings in parallel (4p) yields a 48V pack at 400Ah.
Sizing Math: Peukert, Efficiency, and Inverter Matching
Let's size a 48V battery bank and inverter for a stated continuous load of 4,000W (with a 6,000W surge for well pumps or compressors) running for 4 hours.
1. Inverter Sizing:
Continuous load is 4,000W. Applying a 1.25x safety factor for continuous duty (NEC-style guidance), we need an inverter rated for at least 5,000W continuous. A standard 6,000W / 48V hybrid inverter covers the 6,000W surge requirement.
2. Battery Ah Sizing:
Base current draw: 4,000W / 48V = 83.3A.
Base capacity for 4 hours: 83.3A × 4h = 333.2Ah.
3. Peukert's Law and Efficiency Derating:
In lead-acid batteries, Peukert's exponent (typically 1.2 to 1.3) drastically reduces usable capacity at high discharge rates. A 333Ah lead-acid bank pulled at 83A might only deliver 220Ah in reality. However, lithium NMC/NCA cells have a Peukert exponent near 1.05. According to MPowerUK's lithium-ion engineering data, this means a 333Ah lithium pack delivers nearly its full rated capacity even at high C-rates. We can ignore heavy Peukert derating, but we must account for inverter efficiency (typically 93-95%) and Depth of Discharge (DoD) limits.
Assuming 95% inverter efficiency and a maximum 90% DoD to preserve cycle life:
Adjusted Ah = 333.2Ah / (0.95 × 0.90) = 390Ah.
You would build this using four parallel strings of a 15s 100Ah LFP module, or wire salvaged Tesla 5.2kWh (6s74p, ~145Ah at 22V) modules in a 2s3p configuration to hit roughly 44V nominal and 435Ah.
Charge Limits, C-Rates, and Thermal Runaway Safety
Tesla cells are high-performance, but they demand strict adherence to charge and discharge limits. Pushing a cell beyond its rated C-rate generates excessive internal heat, accelerating calendar degradation and risking thermal runaway.
- NMC/NCA (2170/4680): Nominal 3.6V, charge to 4.2V. Max continuous discharge is typically 1C to 1.5C. For a 100Ah pack, do not pull more than 100A-150A continuously. DoD should be limited to 80-90% for daily cycling.
- LFP (Prismatic): Nominal 3.2V, charge to 3.65V. Max continuous discharge is usually 0.5C to 1C. LFP tolerates 100% DoD far better than NMC, making it the superior choice for stationary solar storage where weight is irrelevant.
Lithium-ion cells contain highly flammable organic electrolytes. If a cell is punctured, overcharged, or short-circuited, it can enter thermal runaway—a self-sustaining exothermic reaction that burns at over 1,000°C and generates its own oxygen. Standard water or ABC fire extinguishers will not stop a lithium cell fire; they only cool adjacent cells to prevent propagation. Always install stationary lithium packs in compliance with NFPA 855 spacing requirements, utilize BMS units with secondary over-voltage contactors, and never sleep in the same enclosed, unventilated room as a large, unenclosed DIY lithium bank.
The Mismatched Cell Hazard
When building parallel strings from salvaged Tesla modules, never parallel mismatched cells or modules. If you wire a degraded 80Ah module in parallel with a fresh 100Ah module, the lower-impedance fresh module will dump massive current into the weaker module during charging and discharging. This unbalanced current flow bypasses the BMS's ability to protect individual cells, leading to localized overheating, venting, and catastrophic failure. Always capacity-test and internal-resistance-match every module before bolting them together on a common DC bus.
Whether you are marveling at the polyurethane-bonded 4680 structural pack in a modern Model Y or bolting together salvaged 18650 modules for a 48V off-grid array, the underlying physics remain identical. Respect the C-rates, size your busbars for the surge current, and let the BMS do the heavy lifting of keeping the chemistry stable.
