Wiring a battery in series circuit multiplies the system voltage while the amp-hour (Ah) capacity remains identical to a single cell. If you connect four 12V 100Ah LiFePO4 batteries in series, you get a 48V nominal (51.2V actual) bank with 100Ah of capacity, yielding 5.12kWh of total energy. This voltage scaling is the foundational architecture for modern off-grid solar, marine, and RV power systems, as it drastically reduces DC current and allows for smaller, safer wire gauges.

Below is the exact sizing math, C-rate limitations, and safety protocols required to design a reliable series battery bank, moving past generic advice into datasheet-level engineering.

System Block Description: Source to Load in Series

Before calculating wire sizes, you must define the physical current path. A properly engineered series bank follows a strict source-to-load topology to ensure overcurrent protection and clean disconnects.

  • Source (The Bank): Individual 12V batteries connected positive-to-negative via identical-length, identical-gauge interconnect cables (e.g., 2 AWG stranded copper). The final negative and positive terminals form the main bank output.
  • Primary Protection: A Class T or ANL fuse rated for the inverter's maximum continuous DC draw plus 25%, placed within 18 inches of the main positive terminal.
  • DC Disconnect: A heavy-duty rotary switch (e.g., Blue Sea Systems 400A) to isolate the bank from the inverter and charge controllers during maintenance.
  • Inverter/Charger: The DC-AC conversion point. High-frequency pure sine wave inverters (like the Victron MultiPlus-II 48/3000) draw heavily from the bank during surge loads.
  • AC Distribution: The inverter's AC-out feeds a subpanel with standard thermal-magnetic breakers protecting branch circuits.

Series vs. Parallel Consequence: Wiring in series adds voltage (V) while keeping capacity (Ah) static. Wiring in parallel keeps voltage static while adding capacity (Ah). A 48V system requires series wiring to achieve the voltage threshold, while parallel strings are added only when the single-string Ah capacity is insufficient for the required runtime.

Sizing Math: Peukert, Efficiency, and Inverter Matching

The most common failure point in DIY power systems is undersizing the wire and battery bank for the inverter's DC draw. To size a battery in series circuit correctly, we must account for inverter efficiency (typically 90-93%) and the Peukert effect.

Peukert's Law dictates that a battery's usable capacity drops as the discharge current increases. The formula is t = C / (Ik), where k is the Peukert exponent. Lead-acid (AGM/Gel) batteries have a k value around 1.3, meaning heavy loads severely cripple their capacity. LiFePO4 chemistry has a k value near 1.05, making it vastly superior for high-draw inverter applications.

Worked Example: Pulling 100A from a 100Ah AGM battery (k=1.3) yields only 15 minutes of runtime before voltage collapse. Pulling 100A from a 100Ah LiFePO4 battery (k=1.05) yields roughly 48 minutes. This is why series-wired lithium banks dominate modern high-load systems.
Bank Configuration Nominal / Actual V Total Ah & kWh DC Draw @ 3000W (90% Eff) Min THHN Wire (75°C Col)
12V (1P1S) 12V / 12.8V 100Ah / 1.28kWh 260.4A 4/0 AWG
24V (1P2S) 24V / 25.6V 100Ah / 2.56kWh 130.2A 1/0 AWG
48V (1P4S) 48V / 51.2V 100Ah / 5.12kWh 65.1A 6 AWG
48V (2P4S) 48V / 51.2V 200Ah / 10.24kWh 65.1A 6 AWG

Assumptions: Copper conductors, 75°C ampacity column per NEC Table 310.16, 30°C ambient, continuous load. Inverter efficiency calculated at 90%. Actual voltage reflects fully charged LiFePO4 resting state.

Charge, Discharge Limits, and C-Rate Constraints

Voltage and wire sizing are only half the equation. The battery management system (BMS) inside each 12V series block enforces strict C-rate limits. The C-rate defines the maximum safe charge and discharge current relative to the battery's capacity.

  • Discharge C-Rate: Most commercial 12V 100Ah LiFePO4 batteries feature a BMS limited to 1.0C (100A continuous discharge). Looking at the table above, a 24V series bank pulling 3000W requires 130.2A. A single string of 100Ah batteries will trip the BMS overcurrent protection and shut down your inverter mid-load. You must either parallel a second string (2P2S) or use 12V batteries rated for 200Ah with a 200A BMS.
  • Charge C-Rate: Lithium iron phosphate accepts charge efficiently up to 0.5C. For a 100Ah series bank, your MPPT charge controller and inverter/charger combined should not push more than 50A into the bank. Pushing 1.0C (100A) charge current regularly degrades the cell anodes and generates excess heat, shortening cycle life.
  • Depth of Discharge (DoD): While LiFePO4 can physically be drained to 100%, the BMS low-voltage disconnect (LVD) usually triggers at 10V per battery (40V for a 48V series bank). To maximize cycle life (achieving 4,000+ cycles), configure your inverter's low-voltage cutoff to 11.5V per battery (46V total), limiting real-world DoD to roughly 85-90%.

BMS Architecture and Lithium Fire-Safety Protocols

When wiring a battery in series circuit, the BMS topology becomes critical. Standard 12V LiFePO4 batteries contain an internal BMS designed to protect that specific 4-cell block. When you wire four of these in series to make 48V, you are relying on four independent BMS units communicating indirectly through voltage thresholds.

LITHIUM FIRE-SAFETY WARNING: Never parallel mismatched cells or batteries of different ages, chemistries, or internal resistances. If a 100Ah new battery is paralleled with an aged 80Ah battery, the new battery will forcefully push current into the old one during charging, bypassing the BMS charge limits and risking thermal runaway. Furthermore, always install a dedicated lithium fire suppression system (like aerosol or clean agent extinguishers) in enclosed battery compartments, as LiFePO4 off-gases flammable electrolyte vapors prior to venting. Consult NFPA 855 for stationary energy storage spacing and ventilation requirements.

Series Charging Imbalance: In a 4-battery series string, the batteries do not age or drift in internal resistance identically. During the constant voltage (CV) absorption phase, one battery might hit 14.4V (triggering its internal BMS to cut off charging) while the other three are only at 13.8V. The entire series circuit is then broken, and the bank fails to reach 100% State of Charge (SoC).

The Fix: For series strings larger than 24V, you must install an active battery balancer (e.g., Victron Battery Balancer or equivalent capacitive balancers). These devices bleed current from the highest-voltage battery and route it to the lowest-voltage battery, ensuring the entire series string charges evenly without tripping individual BMS high-voltage disconnects.

For ultimate reliability in 48V systems, many professional installers bypass 12V series strings entirely, opting instead for single 48V server-rack batteries (like the EG4 48V 100Ah or SOK 48V). These utilize a single, unified BMS monitoring 16 cells in series, eliminating the multi-BMS desync issues inherent in 12V series blocks while maintaining the exact same voltage and capacity math outlined above.