How to Wire Batteries in a Series for a 24V LiFePO4 Bank

To wire batteries in a series, you connect the positive (+) terminal of the first battery to the negative (-) terminal of the second battery. This configuration doubles the system voltage (e.g., two 12V batteries become 24V) while the amp-hour (Ah) capacity remains identical to a single battery. The main positive and main negative outputs are then taken from the remaining open terminals on opposite ends of the chain. This is the standard method for building 24V or 48V off-grid solar banks and RV power systems using 12V LiFePO4 cells.

Getting the physical connections right is just as important as the topology. High-current DC systems are unforgiving of poor crimps or loose terminal hardware. Below is the complete bench-to-busbar procedure for wiring a 2-battery series bank, based on current NEC-style guidance and manufacturer specifications for lithium iron phosphate (LiFePO4) chemistry.

Tools, Materials, and Wire Sizing

Before you cut any wire, ensure your materials are rated for the continuous current your inverter will pull. For a typical 24V system running a 2000W inverter, you are looking at a continuous draw of roughly 85A, with surge peaks up to 150A.

Required Materials

• Batteries: 2x 12V 100Ah LiFePO4 batteries (matched capacity and brand, e.g., SOK or Ampere Time)
• Wire: 2 AWG fine-stranded copper wire (Class K marine tinned or THHN/THWN-2 for conduit)
• Terminals: 2 AWG to 3/8" (M8) closed-end, adhesive-lined heat shrink ring terminals
• Overcurrent Protection: 150A Class T fuse with a bolt-on fuse block
• Hardware: Stainless steel M8 flange nuts (usually included with the battery)

Wire and Breaker Sizing Table

Inverter Size (24V Nominal)	Max Continuous DC Current	Minimum Wire Gauge (AWG)	Main Fuse Rating
1000W	45A	4 AWG	70A
2000W	85A	2 AWG	125A or 150A
3000W	130A	1/0 AWG	175A

Note: Sizing assumes copper conductors in the 75°C column with a maximum 3% voltage drop over a 5-foot one-way run. Always consult your inverter manual for specific minimum gauge requirements.

Mains and High-Current DC Safety Protocol

Step-by-Step: Wiring the Series Bank

For this procedure, we are wiring two 12V batteries in series to create a 24V nominal bank. The batteries should be placed side-by-side, no more than 18 inches apart, to keep the series interconnect wire as short as possible.

1. Prepare the Interconnect Wire: Cut a length of Red 2 AWG wire just long enough to reach from the positive terminal of Battery 1 to the negative terminal of Battery 2 without pulling taut. Strip 5/8" of insulation. Slide a piece of 3:1 adhesive heat shrink over the wire, crimp the M8 ring terminal using a heavy-duty hex crimp tool, and apply heat until the adhesive oozes from the barrel.
2. Connect the Main Negative: Cut a length of Black 2 AWG wire to reach from Battery 1's negative terminal to your negative DC busbar. Crimp an M8 ring terminal on the battery end and a 5/16" or 3/8" lug for the busbar. Bolt the Black wire to the Negative (-) M8 threaded terminal of Battery 1. Torque to 5 Nm (approx. 44 in-lbs).
3. Make the Series Link: Take your prepared Red interconnect wire. Land one M8 ring terminal on the Positive (+) M8 threaded terminal of Battery 1. Land the other M8 ring terminal on the Negative (-) M8 threaded terminal of Battery 2. Torque both M8 nuts to 5 Nm.
4. Install the Main Fuse: Cut a length of Red 2 AWG wire to reach from Battery 2's positive terminal to your 150A Class T fuse block. Bolt this Red wire to the Positive (+) M8 threaded terminal of Battery 2. Torque to 5 Nm.
5. Complete the Circuit to the Busbar: Run a final Red 2 AWG wire from the output side of the 150A Class T fuse to your positive DC busbar or inverter positive terminal. Do not connect the final inverter-side lug until all battery-side torque checks are complete.

Verify and Test the Bank

Never assume the wiring is correct just because the bolts are tight. You must verify the topology and connection integrity before applying a load.

1. Voltage Topology Check: Set your multimeter to DC Volts. Place the black probe on the main negative busbar and the red probe on the output side of the Class T fuse. You should read between 25.6V and 27.2V (the normal resting voltage range for a 24V nominal LiFePO4 bank). If you read ~12.8V, you have wired them in parallel. If you read 0V, check your fuse and crimps.
2. Individual Battery Check: Measure across the terminals of Battery 1, then Battery 2. Both should read within 0.1V of each other (e.g., 13.3V and 13.4V). A massive discrepancy indicates a faulty cell or a BMS that has tripped into protection mode.
3. Torque Verification: Use an inch-pound torque wrench to verify all M8 terminal nuts are exactly at 5 Nm (44 in-lbs). LiFePO4 terminals are often soft copper or brass threaded into an aluminum busbar; overtightening will strip the threads, while undertightening causes thermal runaway at the joint.

The Most Common Botch (And Its Symptom)

The most frequent and dangerous mistake DIYers make when learning how to wire batteries in a series is hand-tightening the M8 terminal nuts and using open-barrel or stamped ring terminals instead of closed-end hex crimps.

The Symptom: The system works perfectly at low loads (lights, phone chargers). However, when you turn on the microwave or coffee maker (pulling 1500W+), the inverter immediately throws a "Low DC Voltage" error and shuts down, even though the battery monitor shows 80% State of Charge. You may also notice the plastic battery cover around the terminal post melting or smelling like burning ozone.

The Physics: A loose or poorly crimped M8 connection introduces high electrical resistance (often 0.05 ohms or more). Under a 100A load, Ohm's Law (V = I × R) dictates a 5-volt drop right at the terminal. The inverter sees the voltage sag to 20V and triggers its low-voltage disconnect to protect itself. Simultaneously, Power (P = I² × R) generates 500 watts of pure heat directly at that single loose nut, melting the battery casing and creating a severe fire hazard. Always use a calibrated torque wrench and proper adhesive-lined heat shrink crimps.

Frequently Asked Questions

Can I wire batteries in series and parallel at the same time?

Yes, this is called a series-parallel configuration (e.g., 2S2P). You first wire two strings of batteries in series to achieve your target voltage (e.g., 24V), and then wire those two strings in parallel to double your amp-hour capacity. However, modern best practice heavily favors buying a single, larger-capacity battery (like a native 24V 100Ah server-rack battery) rather than paralleling multiple 12V strings. Paralleling strings can lead to circulating currents and uneven charging if the interconnect wire lengths and resistances are not perfectly symmetrical. For deeper insights on balancing parallel strings, refer to the Victron Energy Wiring Unlimited guide.

Does wiring batteries in a series increase the total amp-hours?

No. Wiring in series only increases the voltage; the amp-hour (Ah) capacity remains exactly the same as a single battery. If you wire two 12V 100Ah batteries in series, you get a 24V 100Ah bank. However, your total Watt-hours (Wh) do increase because Watt-hours = Volts × Amp-hours. A single 12V 100Ah battery holds 1,280Wh. Two in series create a 24V 100Ah bank, which holds 2,560Wh. The total energy capacity doubles, but the Ah rating used for wire sizing and BMS limits stays at 100Ah.

What happens if I connect different battery brands or capacities in series?

You should never mix different capacities, chemistries, or ages of batteries in a series string. In a series circuit, the exact same current flows through every battery. If you mix a 100Ah battery with a 50Ah battery, the 50Ah battery will be driven into deep discharge and potentially damaged by its BMS long before the 100Ah battery is empty. During charging, the smaller battery will hit its high-voltage cutoff first, causing its BMS to open the circuit and halt charging for the entire bank. As noted by Battery University, mismatched cells in a series string lead to permanent capacity degradation and premature failure of the weakest link.