To connect two batteries together, you must choose between a parallel configuration (which doubles capacity in Amp-hours while maintaining the same voltage) or a series configuration (which doubles voltage while maintaining the same capacity). For two standard 12V 100Ah LiFePO4 batteries, wiring them in parallel yields a 12V 200Ah bank, while wiring them in series yields a 24V 100Ah bank. The physical wiring path, terminal torque, and ground return routing differ drastically between the two.

This guide walks through the exact wiring diagrams, terminal mappings, and multimeter verification steps for both configurations, using modern 12V 100Ah LiFePO4 cells as our baseline.

Decoding Diagram Symbols and Physical Terminals

Before tracing the wires, you need to translate standard schematic symbols to the physical hardware on your bench. In any DC wiring diagram:

  • The Cell Symbol: Represented by parallel lines of alternating lengths. The longer line is always the Positive (+) terminal, and the shorter, thicker line is the Negative (-) terminal.
  • The Wire Node (Dot): A solid black dot where two lines intersect indicates a physical electrical junction (like a busbar or a split lug). Lines crossing without a dot do not connect.
  • The Load Symbol: Usually a circle with a cross, or a resistor zigzag, representing your inverter, charge controller, or DC distribution bus.

On physical 12V LiFePO4 batteries (such as those from Renogy or Battle Born), the terminals are typically M8 threaded stainless steel studs. The positive terminal is marked with a red plastic collar or a '+' stamp, while the negative is marked with black or a '-' stamp.

Safety Callout: Never clamp a bare wire directly under the factory terminal nut. You must use a properly crimped copper ring lug (tinned marine grade or AWG-rated tube lug). Furthermore, do not use the factory nut to clamp your lug; use a flat washer, your lug, a split lock washer, and a second stainless steel nut to achieve proper clamping force without stripping the internal BMS bus threads.

Master Terminal and Wiring Spec Table

Whether you are wiring in series or parallel, the physical hardware specifications for the interconnects and main feeders remain the same for a 100Ah bank pulling up to 100A continuous. Review this spec sheet before cutting your wire.

Connection Node Wire Gauge & Type Lug Size / Terminal Torque Spec Purpose in Circuit
Battery-to-Battery Jumper 2/0 AWG Fine-Strand Copper 2/0 AWG to M8 (5/16') Ring 10 - 12 Nm (88-106 in-lbs) Equalizes current flow between cells
Main Positive Output 2/0 AWG or 1/0 AWG THHN 2/0 AWG to M8 Ring 10 - 12 Nm (88-106 in-lbs) Carries total load to main DC fuse
Main Negative Return 2/0 AWG Fine-Strand Copper 2/0 AWG to M8 Ring 10 - 12 Nm (88-106 in-lbs) Routes ground path through BMS shunt
Shunt to Busbar 2/0 AWG Short Jumper 2/0 AWG to 3/8' Busbar hole 15 Nm (132 in-lbs) Completes the ground return to loads

Parallel Connection Walkthrough (12V System)

When wiring two batteries in parallel, you connect Positive to Positive, and Negative to Negative. To prevent one battery from doing all the work due to wire resistance, we use the diagonal wiring method. This means the main positive load is drawn from Battery B, while the main negative ground return is drawn from Battery A.

Node-by-Node Source to Load Trace

  1. Node 1 (Source Interconnect +): Start at Battery A Positive. Attach a 2/0 AWG red jumper wire and route it directly to Battery B Positive. Torque both M8 nuts to 12 Nm.
  2. Node 2 (Source Interconnect -): Move to Battery A Negative. Attach a 2/0 AWG black jumper wire and route it to Battery B Negative. Torque to 12 Nm.
  3. Node 3 (Main Positive to Load): At Battery B Positive (the diagonal opposite of our ground takeoff), attach the main red feeder wire. Route this to a Class-T fuse (e.g., 150A), then to the positive DC busbar.
  4. Node 4 (Ground Path Return): At Battery A Negative, attach the main black feeder wire. Route this to the Load side of your battery monitor shunt (like a Victron SmartShunt).
  5. Node 5 (Shunt to Busbar): Connect a short 2/0 AWG jumper from the Source side of the shunt to the negative DC busbar. This completes the ground path back to the inverter and loads.
Lithium Fire Safety Note: Before connecting LiFePO4 batteries in parallel, ensure both batteries are within 0.1V of each other. If Battery A is at 13.4V and Battery B is at 12.8V, closing the parallel jumper will cause Battery A to dump massive equalization current into Battery B, potentially tripping the BMS or melting undersized jumper wires. Top-balance them individually with a charger first.

Series Connection Walkthrough (24V System)

Wiring two batteries in series doubles the voltage. You connect the Positive of one battery to the Negative of the other. The remaining free terminals become your new 24V main positive and main negative. According to Battery University configuration guidelines, series strings require strict cell matching, as the BMS of each individual battery must handle the floating ground potential.

Node-by-Node Source to Load Trace

  1. Node 1 (Series Bridge): Start at Battery A Negative. Attach a 2/0 AWG jumper wire and route it to Battery B Positive. Torque both ends to 12 Nm. This node is now electrically 'dead' to the outside world; it is strictly an internal bridge.
  2. Node 2 (Main Positive Output): The only remaining positive terminal is Battery B Negative. Wait, standard convention dictates we use the unlinked terminals. Let's trace standard series: Link A(-) to B(+). The free terminals are Battery A Positive and Battery B Negative.
  3. Correction - Node 2 (Main Positive): Attach your main red feeder to Battery A Positive. Route to a 24V-rated main DC fuse, then to the inverter positive terminal.
  4. Node 3 (Ground Path Return): Attach your main black feeder to Battery B Negative. Route this through your high-voltage shunt to the inverter negative terminal.

In this 24V configuration, the ground path is entirely isolated from the 12V potential of the individual cells. If your inverter or charge controller requires a 12V accessory tap, you cannot tap into the middle bridge (Node 1). Doing so will unbalance the series string and destroy the BMS. You must use a 24V-to-12V DC-DC buck converter instead.

Verifying Connections with a Multimeter

Never apply a load until you have verified the wiring with a digital multimeter (DMM). Set your DMM to DC Volts (V⎓) for voltage checks, and Millivolts (mV⎓) for connection resistance checks.

Parallel Bank Verification

  • The Interconnect Test: Place the red probe on Battery A (+) and the black probe on Battery B (+). Your meter must read 0.00V. If it reads 12V, you have accidentally wired them in series. Repeat for the negative terminals.
  • The Total Voltage Test: Place the red probe on the main positive busbar and the black probe on the negative busbar. You should read between 13.2V and 13.6V (for fully charged LiFePO4).
  • The Voltage Drop Test (Under Load): Turn on a 500W load. Switch your meter to mV. Probe across the length of the Battery A to Battery B jumper wire. A healthy 2/0 AWG copper connection should show a voltage drop of less than 15 mV. If it reads higher, your lug crimps are loose or your terminal nuts are under-torqued.

Series Bank Verification

  • The Bridge Test: Place the red probe on Battery A (-) and the black probe on Battery B (+). It must read 0.00V.
  • The Total Voltage Test: Place the red probe on Battery A (+) and the black probe on Battery B (-). You should read between 26.4V and 27.2V. If you read 13.4V, your series bridge is broken or you are probing the wrong terminals.

For deeper architectural guidance on scaling these banks beyond two batteries, refer to the Battle Born Batteries series and parallel configuration guide, which outlines the limits of internal BMS balancing across larger strings. Always ensure your wire ampacity and overcurrent protection align with the lowest common denominator of your battery's continuous discharge rating and your inverter's peak surge requirements.