Series wiring a battery bank increases your system voltage while keeping the amp-hour (Ah) capacity identical to a single cell. For a 5,000W off-grid inverter, series wiring four 12V 100Ah LiFePO4 batteries yields a 48V 100Ah (4.8kWh) bank. This configuration cuts your DC current in half compared to a 24V system and quarters it compared to a 12V system, drastically reducing I²R heating in your busbars and allowing you to use smaller, more manageable wire gauges.
The Physics of Series Wiring: Voltage Stacks, Capacity Stays
When designing a DC power system, you have two primary methods to scale your battery bank: series and parallel. Understanding the exact consequence of each on voltage (V) and capacity (Ah) is the foundation of off-grid system design.
Think of electricity like water in a plumbing system. Voltage is the water pressure, and amp-hours represent the total volume of water in the tank. When you wire batteries in series (connecting the positive terminal of one battery to the negative terminal of the next), you are stacking pumps to increase the pressure (voltage), but the total volume of water (Ah) remains exactly the same as a single tank. When you wire in parallel (positive to positive, negative to negative), you keep the pressure the same but connect multiple tanks together to increase the total volume.
| Configuration | Voltage Consequence | Capacity (Ah) Consequence | Total Energy (Wh) | Best Application |
|---|---|---|---|---|
| Series | Multiplies (V × N) | Stays the same | V × Ah | High-power inverters (>2000W) |
| Parallel | Stays the same | Adds (Ah × N) | V × Ah | Low-voltage DC loads, RVs, marine |
| Series-Parallel | Multiplies by series string | Adds by parallel strings | Series V × Parallel Ah | Large residential solar storage |
For any inverter load exceeding 2,000W, series wiring is mandatory. A 5,000W load on a 12V bank requires over 450 amps of continuous DC current, which necessitates massive 4/0 AWG cables and generates dangerous heat at termination points. By series wiring four 12V batteries to create a 48V nominal bank, that same 5,000W load draws roughly 115 amps, which is safely handled by 2/0 AWG wire.
Sizing the Bank: Peukert, DoD, and C-Rate Math
Sizing a battery bank requires more than just multiplying voltage by amp-hours. You must account for inverter efficiency, Depth of Discharge (DoD), and the chemical limitations of the cells expressed as C-rates and Peukert's Law.
Peukert's Law dictates that as you increase the discharge current, the effective capacity of the battery decreases. This effect is severe in lead-acid batteries (Peukert exponent k ≈ 1.3) but negligible in Lithium Iron Phosphate (LiFePO4) cells (k ≈ 1.05). If you pull 100A from a 100Ah flooded lead-acid battery, Peukert's law reduces your usable capacity to roughly 60Ah before the voltage sags below the inverter's low-voltage cutoff. A 100Ah LiFePO4 battery under the same 100A load will deliver nearly 95Ah of usable energy due to its flat discharge curve and low internal resistance.
| Battery Chemistry | Configuration | Nominal Voltage | Total Ah / kWh | Usable DoD | Max Continuous Discharge (1C) | Min Wire Size to Inverter |
|---|---|---|---|---|---|---|
| LiFePO4 (12V 100Ah) | 4S 1P (Series) | 51.2V (48V nom) | 100Ah / 5.12kWh | 80% (4.09kWh) | 100A (BMS limited) | 2/0 AWG THHN |
| LiFePO4 (12V 200Ah) | 4S 1P (Series) | 51.2V (48V nom) | 200Ah / 10.24kWh | 80% (8.19kWh) | 200A (or 100A BMS) | 2/0 AWG THHN |
| Flooded Lead-Acid (6V 225Ah) | 8S 1P (Series) | 48V | 225Ah / 10.8kWh | 50% (5.4kWh) | 56A (C/4 rate) | 2/0 AWG THHN |
| AGM (12V 100Ah) | 4S 1P (Series) | 48V | 100Ah / 4.8kWh | 50% (2.4kWh) | 50A (C/2 rate) | 2/0 AWG THHN |
Depth of Discharge (DoD) is the percentage of the battery you can safely use without degrading its lifespan. LiFePO4 batteries routinely support an 80% to 90% DoD, whereas lead-acid chemistries should be limited to 50% to prevent sulfation. Therefore, a 5.12kWh LiFePO4 bank provides roughly the same daily usable energy as a 10.8kWh lead-acid bank.
C-Rate defines the charge and discharge speed relative to the battery's capacity. A 1C rate for a 100Ah battery is 100 amps. A 0.5C rate is 50 amps. Most off-grid LiFePO4 batteries feature a Battery Management System (BMS) rated for 100A continuous (1C). If your inverter requires 115A continuous, you must either parallel two series strings (4S 2P) or purchase 12V batteries with a 150A+ BMS.
Inverter/Charger Sizing and System Block Routing
Once the series bank is configured, you must route the DC power safely to the inverter and integrate it with your home's AC subpanel. Here is the exact system block description from source to load for a 5,000W 48V system:
- Source (Battery Bank): Four 12V 100Ah LiFePO4 batteries wired in series using 2/0 AWG Class K fine-stranded welding cable and adhesive-lined heat shrink terminals.
- DC Protection: A 150A Class T fuse installed on the positive main output, as close to the final battery terminal as possible. Class T fuses have a high Ampere Interrupting Capacity (AIC) of 20,000A at 125VDC, which is critical for safely clearing a dead-short on a massive lithium bank.
- DC Routing: 2/0 AWG THHN wire run inside flexible metallic conduit from the battery busbar to the inverter/charger DC terminals.
- Conversion (Inverter/Charger): A 5,000W 48V DC to 120/240V AC split-phase inverter/charger. Assuming 90% inverter efficiency, a 5,000W AC load requires 5,555W of DC input (5,555W / 48V = 115.7A).
- AC Protection: The inverter's AC output feeds into a 50A 2-pole AC breaker inside a dedicated AC subpanel.
- Load (Subpanel): The subpanel distributes power to critical branch circuits (well pump, refrigerator, lighting, outlets).
Charger Sizing: If you are using a standalone AC-to-DC battery charger (or sizing the internal charger in a hybrid inverter), the ideal charge rate for LiFePO4 is between 0.2C and 0.5C. For a 100Ah series bank, this means a 20A to 50A charger. A 50A charger at 51.2V requires roughly 2,560W of AC generator or grid input power. Sizing the charger larger than 0.5C (50A) on a single 100Ah string will cause excessive heat generation inside the cells and prematurely trip the BMS high-temperature cutoff.
Fire Safety, BMS Limits, and Mismatched Cell Risks
LiFePO4 cells are inherently safer than NMC lithium-ion, but a short circuit on a 48V series bank can deliver thousands of amps of fault current, instantly vaporizing copper and igniting surrounding insulation. Never work on series-connected battery terminals without removing the main Class T fuse and verifying the busbar is dead with a CAT III multimeter. Always install batteries in a steel or fire-rated enclosure, and never parallel mismatched cells or strings of different ages, capacities, or chemistries.
The Battery Management System (BMS) inside each 12V battery is the ultimate authority on charge and discharge limits. When wiring in series, the BMS of every single battery in the string must be capable of handling the total system current. If one battery has a 100A BMS and another has a 50A BMS, the 50A BMS will shut down under load, instantly dropping the entire 48V system to zero volts and potentially damaging the inverter's DC input capacitors due to inductive kickback.
Charge and Discharge Limits to Monitor:
- Low-Temperature Charge Cutoff: LiFePO4 cells cannot accept a charge below 0°C (32°F). Attempting to charge a frozen lithium battery causes lithium plating on the anode, which creates internal dendrites that pierce the separator and cause a dead short. Ensure your inverter/charger has a temperature probe wired to the battery BMS or an external low-temp cutoff relay.
- High-Voltage Cutoff (HVD): In a 4S (48V nominal) configuration, the series string reaches full charge at roughly 57.6V to 58.4V (14.4V to 14.6V per battery). If your charger's absorption voltage is set too high, the first battery to hit 14.6V will trigger its BMS over-voltage protection, breaking the circuit and causing the charger voltage to spike, potentially destroying the charger.
- Mismatched Parallel Strings: If you need more capacity and decide to parallel two 4S series strings, the strings must be identical in brand, capacity, and age. If you parallel a new 100Ah string with an older, degraded 80Ah string, the newer string will constantly push current into the older string to equalize voltage, leading to chronic overcharging and thermal runaway in the weaker cells.
For comprehensive wiring standards and DC overcurrent protection guidelines, refer to the Battery University guide on series and parallel configurations and the Victron Energy Wiring Unlimited documentation, which provides exhaustive diagrams for busbar routing and Class T fuse placement in multi-string series-parallel banks.
