Learning how to wire batteries in series parallel is the standard method for scaling both voltage and capacity in off-grid solar and RV power systems. To build a 24V system using four 12V batteries, you connect two batteries in series (positive to negative) to create two 24V strings, then connect those strings in parallel (positive to positive, negative to negative). For a standard 12V 100Ah LiFePO4 setup, this yields a 24V 200Ah bank (5,120Wh). You must use 4 AWG wire for the interconnects, 2/0 AWG for the main busbar feeds, and a 150A Class T fuse on the main positive line.
This guide walks through a 2S2P (2 Series, 2 Parallel) configuration using four identical 12V 100Ah LiFePO4 batteries (such as those from SOK, Ampere Time, or Renogy). We will cover the exact terminal landings, torque specifications, and the physics behind current sharing.
Tools, Materials, and Device Ratings
Before stripping a single wire, gather the correct materials. Undersized wire or missing fuses are the leading causes of melted terminals and lithium cell fires.
| Item | Specification & Rating | Purpose |
|---|---|---|
| Batteries | 4x 12V 100Ah LiFePO4 (Identical make/model/age) | Energy storage (2S2P configuration) |
| Interconnect Wire | 4 AWG Fine-Strand Silicone or THHN (Red & Black) | Series and parallel battery links |
| Main Feed Wire | 2/0 AWG Fine-Strand Silicone (Red & Black) | Bank to busbar/inverter runs |
| Terminal Lugs | 5/16" (M8) ring terminals, heat shrink sealed | Connecting wire to battery studs |
| Main Fuse | 150A Class T Fuse with block | Overcurrent protection on main positive |
| Busbars | 250A rated copper busbars (Positive and Negative) | Central distribution and parallel tie-in |
| Torque Wrench | 1/4" drive, 4-8 Nm range | Securing M8 battery terminals safely |
AC Mains and DC Safety Protocols
Batteries are live DC sources that cannot be "turned off." Always wear safety glasses and remove all metal jewelry. A dropped wrench across 24V terminals will cause an arc flash and weld the metal.
AC Mains Callout: If you are connecting this battery bank to a hybrid inverter/charger (like a Victron MultiPlus or Growatt SPF 5000ES) that ties into your home's AC electrical panel, you MUST de-energize the AC mains. Turn off the main utility breaker, lock/tag it out, and verify dead with a tested CAT III multimeter at the inverter's AC input terminals before making any AC connections. Never work on the AC side while the utility is live. Local code may require a licensed electrician for the AC tie-in; NEC-style guidance is provided here for educational purposes, but your local AHJ has final authority.
Step-by-Step 2S2P Wiring Procedure
For this procedure, we are building two series strings (String A and String B), then tying them together at a central busbar to achieve the parallel connection. This method is preferred over daisy-chaining parallel links directly on the battery terminals, as it prevents terminal overcrowding and ensures equal resistance.
- Prep and Top-Balance: Before wiring, ensure all four batteries are charged to the exact same voltage (e.g., 14.2V) and allowed to rest for 2 hours. Connect the Red probe of your multimeter to the Positive (+) terminal and the Black probe to the Negative (-) terminal of each battery to verify they are within 0.05V of each other.
- Wire String A (Series): Place Battery 1 and Battery 2 side-by-side. Crimp a 5/16" ring terminal onto both ends of a 4 AWG Red wire. Land one end of the Red wire on the Positive (+) M8 terminal of Battery 1. Land the other end of this Red wire on the Negative (-) M8 terminal of Battery 2. Torque both M8 nuts to 5 Nm (4.4 ft-lbs). You now have a 24V string. The free terminals are Battery 1 Negative (-) and Battery 2 Positive (+).
- Wire String B (Series): Place Battery 3 and Battery 4 side-by-side. Using another 4 AWG Red wire, land one end on the Positive (+) M8 terminal of Battery 3 and the other end on the Negative (-) M8 terminal of Battery 4. Torque to 5 Nm. The free terminals are Battery 3 Negative (-) and Battery 4 Positive (+).
- Connect String A to Busbars (Parallel Tie-in): Cut a length of 2/0 AWG Black wire. Land one end on the Negative (-) M8 terminal of Battery 1, and land the other end on the Negative Copper Busbar. Torque the battery nut to 5 Nm and the busbar nut to 10 Nm. Next, cut a length of 2/0 AWG Red wire. Land one end on the Positive (+) M8 terminal of Battery 2, and route the other end to the Positive Copper Busbar. Do not tighten the busbar end yet.
- Install the Main Class T Fuse: On the 2/0 AWG Red wire running from String A to the Positive Busbar, install the 150A Class T fuse block as close to the battery terminal as possible (within 7 inches per NEC Article 690/480 best practices).
- Connect String B to Busbars (Parallel Tie-in): Cut a length of 2/0 AWG Black wire. Land one end on the Negative (-) M8 terminal of Battery 3, and land the other end on the Negative Copper Busbar. Cut a length of 2/0 AWG Red wire. Land one end on the Positive (+) M8 terminal of Battery 4, and land the other end on the Positive Copper Busbar. Torque all battery nuts to 5 Nm and all busbar nuts to 10 Nm. Ensure the Red and Black main feed cables from the busbars to your inverter are exactly the same length to maintain balanced resistance.
Verify and Test the Bank
Never connect a load or inverter without verifying the wiring. A crossed series/parallel connection will result in a dead short, melting wires instantly and potentially triggering the BMS into a hard lockout.
- Open Circuit Voltage Test: Set your multimeter to DC Volts. Place the Red probe on the Positive Busbar and the Black probe on the Negative Busbar. You should read between 26.4V and 28.8V. If you read ~13.2V, you wired the strings in parallel but forgot the series links. If you read ~52V, you wired all four in series. Disconnect immediately and re-check.
- String Voltage Verification: Measure across String A (Battery 1 Negative to Battery 2 Positive). Expect 26.4V - 28.8V. Measure across String B (Battery 3 Negative to Battery 4 Positive). Expect the exact same voltage, within 0.1V.
- Current Sharing Test (Under Load): Turn on your inverter and apply a steady AC load (e.g., a 1000W space heater). Clamp your DC clamp meter around the 2/0 AWG Red wire from String A, then around the 2/0 AWG Red wire from String B. Both strings should be carrying roughly 50% of the total DC current (e.g., if total draw is 40A, each string should read ~20A). If one string reads 35A and the other reads 5A, you have a resistance imbalance.
The Most Common Botch: Cable Length Imbalance
The most frequent mistake DIYers make when wiring batteries in series parallel is using different lengths of wire for the parallel connections to the busbars.
The Symptom: Under load, String A gets noticeably warm while String B stays cool. The BMS on String A eventually triggers a low-voltage cutoff, shutting down the whole system, even though the total bank capacity is only half depleted.
The Physics: Electricity takes the path of least resistance. Wire resistance is calculated as R = ρ(L/A). If the Red and Black cables connecting String A to the busbars are 2 feet long, but the cables for String B are 6 feet long, String A has one-third the resistance of String B. String A will do the heavy lifting, discharging faster and experiencing higher voltage sag.
The Fix: Always measure and cut your main parallel feed wires to the exact same physical length. If String B is physically further from the busbar, route the cables for String A in a loop or zig-zag so the total copper length from the battery terminal to the busbar lug is identical for both strings. For a deeper dive into balanced busbar wiring, consult the Victron Energy Wiring Unlimited guide, which provides excellent diagrams on symmetrical busbar tie-ins.
Series-Parallel Battery Wiring FAQ
Can I wire different capacity batteries in series parallel?
No. Never mix different capacities, chemistries, or ages of batteries in a series-parallel bank. If you put a 100Ah battery in series with a 200Ah battery, the 100Ah battery will hit 0% State of Charge (SoC) while the 200Ah battery is still at 50%. The BMS on the 100Ah battery will disconnect to prevent cell damage, instantly dropping the voltage of that entire string to zero and causing a massive current surge from the parallel string into the dead string. Always use identical batteries purchased at the same time.
Do I need a BMS for series parallel LiFePO4 batteries?
Yes, but it depends on the battery type. If you are using drop-in 12V LiFePO4 batteries (like Renogy or Ampere Time), they already have an internal BMS. You do not need an external BMS; the internal units will handle cell balancing and high/low voltage disconnects. However, if you are building a bank from raw, unmanaged prismatic LiFePO4 cells, you must install a single, high-quality external BMS (like a JK or Daly BMS) on the main negative busbar lead, with active balancers on each series string to manage top-end cell drift.
What happens if I cross the series and parallel connections?
If you accidentally wire the series links incorrectly (e.g., Positive to Positive instead of Positive to Negative), you are creating a dead short across that specific battery. When you then tie the parallel strings together, the 12V potential difference between the incorrectly wired string and the correctly wired string will cause a massive current spike. The internal BMS of the batteries will likely trip into protection mode, or the wires will melt and catch fire before the BMS can react. This is why the Verify and Test step with a multimeter is non-negotiable before applying a load.
How do I balance the strings before connecting them in parallel?
Before you connect the final parallel links to the busbars, the two series strings must be at the exact same voltage. Charge each 24V string individually using a 24V lithium charger (or charge them as 12V batteries before wiring them in series). Once charged, let them rest for 2 to 4 hours. Measure the voltage of String A and String B. If they are more than 0.1V apart, connect them in parallel with a current-limiting resistor or a small gauge wire (like 10 AWG) for 30 minutes to allow them to equalize slowly without triggering the BMS over-current protection, then swap to your final 2/0 AWG busbar cables. For comprehensive safety standards regarding stationary battery installations, refer to NFPA 70 (National Electrical Code) Article 480.
