Linking batteries in series is the standard method for scaling voltage to match 24V or 48V inverters without multiplying current. When you wire the positive terminal of one cell to the negative of the next, the string's total voltage is the sum of the individual cell voltages, while the amp-hour (Ah) capacity remains identical to a single cell. For a 24V nominal solar or RV system, this means stacking eight 3.2V LiFePO4 cells to achieve 25.6V nominal, allowing you to push 2,000W of power at a manageable 78A rather than the 166A required by a 12V system.
The Series Topology: Node Labels and Voltage Stacking
To design or troubleshoot a battery management system (BMS), you must think in terms of electrical nodes rather than just physical terminals. In a series topology, every junction between two cells creates a new voltage reference point. According to Kirchhoff’s Voltage Law, the sum of the potential differences across these nodes equals the total pack voltage.
For an 8-series (8S) LiFePO4 pack, the topology is defined by nine distinct nodes:
- Node 0 (B-): The main pack negative. This connects to the load negative and the BMS P- (or C-) terminal.
- Node 1 (B1): The junction between Cell 1 (+) and Cell 2 (-). Nominal voltage relative to B- is 3.2V.
- Node 2 (B2): The junction between Cell 2 (+) and Cell 3 (-). Nominal voltage relative to B- is 6.4V.
- Nodes 3 through 7: Sequential interconnects, each adding 3.2V to the cumulative stack.
- Node 8 (B+): The positive terminal of the final cell (Cell 8). This is the main pack positive, measuring 25.6V nominal relative to Node 0.
Component Selection: Designing a 24V 8S LiFePO4 Pack
Let’s walk through a real-world 24V 100Ah build, a common size for off-grid cabins and marine house banks. We are selecting the EVE LF100LA 3.2V 100Ah prismatic LiFePO4 cells. These are Grade A, 100Ah capacity cells with M8 threaded terminals.
Below is the spec-sheet-table mapping the single-cell parameters to the fully assembled 8S series string. Notice how the energy capacity (Wh) scales, but the Ah rating does not.
| Parameter | Single EVE LF100LA Cell | 8S Series Pack (24V Nominal) |
|---|---|---|
| Nominal Voltage | 3.2V | 25.6V |
| Max Charge Voltage | 3.65V | 29.2V |
| Min Discharge Cutoff | 2.5V | 20.0V |
| Capacity (Ah) | 100Ah | 100Ah |
| Total Energy (Wh) | 320Wh | 2,560Wh |
| Max Continuous Discharge | 100A (1C) | 100A |
| Interconnect Hardware | N/A | M8 Copper Busbars, 5-6 Nm torque |
For the BMS, a JK-BMS 8S 200A active balancer is the current benchmark for this size. It monitors all nine nodes (B- through B+) and uses capacitive active balancing to shuffle up to 2A of current from high-voltage nodes to low-voltage nodes during the charging phase.
Failure Modes: What Breaks When a Single Cell Fails
Series strings are only as strong as their weakest node. Because the exact same current flows through every cell, a single anomaly changes the behavior of the entire pack. Here is the behavior table detailing what happens at the extremes.
| Failure Mode | Physical Cause | Pack Behavior & BMS Response |
|---|---|---|
| Open Circuit | Snapped busbar, loose M8 terminal, or internal cell tab fracture. | Current flow drops to 0A immediately. The pack goes completely dead. The BMS will read 0V across the broken node and trigger a wire-break or cell-missing fault. |
| Internal Short | Dendrite formation or separator collapse inside one cell. | The shorted cell drops to ~0V. The remaining 7 cells must absorb the full charger voltage. If charging at 29.2V, the 7 cells see 4.17V each, triggering the BMS Over-Voltage Protection (OVP) in seconds. If the BMS fails, the 7 cells vent or catch fire. |
| High Impedance (ESR) | Corroded busbar, under-torqued terminal, or degraded cell chemistry. | Under load, the high-ESR node experiences severe voltage sag. The BMS sees this node hitting the 2.5V low-voltage cutoff prematurely, shutting down the inverter even though the other 7 cells are at 50% State of Charge (SoC). |
| Capacity Mismatch | One cell is 90Ah while the rest are 100Ah (mixed batches). | During discharge, the 90Ah cell hits 2.5V first, cutting off the whole pack at 90% usable capacity. During charge, it hits 3.65V first, stalling the charge cycle. |
Series vs. Parallel: Why Choose Voltage Scaling?
A common alternative to linking batteries in series for higher voltage is wiring 12V pre-packaged batteries in parallel to increase Ah. Why do we prefer series voltage scaling for systems over 1,000W? The answer lies in Ohm's Law and I²R (heat) losses.
| Criteria | Series Topology (24V 8S) | Parallel Topology (12V 2P) |
|---|---|---|
| Current for 2,000W Load | ~83A | ~166A |
| Required Wire Gauge (Main Feed) | 2 AWG THHN | 2/0 AWG THHN (or dual 2 AWG) |
| BMS Architecture | Single 8S BMS (monitors all nodes) | Requires matched internal BMS in each 12V box; risk of circulating currents if voltages drift. |
| Cell Matching Requirement | High (must top-balance before assembly) | Extreme (parallel strings will fight each other if SoC differs by even 0.05V upon connection) |
According to Battery University, parallel strings require rigorous voltage matching before connection to prevent high inrush currents between the batteries. Series strings avoid this circulating current issue entirely, making them vastly superior for DIY bench assembly.
Bench Testing: Step-by-Step Verification Before Final Assembly
Do not bolt your pack together and immediately connect the inverter. Follow this bench-test sequence to verify the series links and BMS sense wires. This assumes you have already top-balanced your cells (charging them in parallel to exactly 3.65V before assembly).
- Assemble the Physical Stack: Clamp the 8 cells together using end plates and threaded rod. Ensure the insulation tape between the aluminum casings is intact to prevent casing-to-casing shorts.
- Install Busbars: Place the copper busbars across the alternating positive and negative terminals. Torque the M8 nuts to 5 Nm using a calibrated torque wrench. Do not overtighten; stripping the aluminum terminal is a fatal error.
- Verify Node Voltages (No BMS): Set your multimeter to DC Volts. Place the black probe on Node 0 (B-). Step the red probe through Nodes 1 through 8. Because you top-balanced, every step should read exactly 3.65V ± 0.01V. Node 8 should read 29.20V.
- Wire the BMS Sense Harness: Plug the 9-pin sense harness into the BMS. Connect the black wire to Node 0, the first red to Node 1, and so on. Do not plug the harness into the BMS board until all ring terminals are bolted to the nodes. Plugging it in while wiring can blow the BMS microcontroller.
- Verify via BMS Bluetooth/App: Power on the JK-BMS by connecting the main B- and P- wires momentarily. Open the Bluetooth app. Verify that the app reads 9 distinct cell voltages, all within 0.030V of each other.
- Load Test: Connect a 24V DC load (like a 24V to 12V step-down converter powering a 50W light). Watch the BMS app. The voltages at all nodes should sag evenly. If Node 4 drops 0.1V while the others drop 0.02V, you have a high-resistance connection at Node 4. Re-torque that busbar.
Properly linking batteries in series requires respecting the physics of the stack. By selecting matched cells, torquing interconnects to spec, and verifying every node voltage before closing the enclosure, you build a 24V pack capable of delivering a decade of reliable off-grid service. For full installation safety standards, always cross-reference your build with NFPA 855 guidelines for Energy Storage Systems regarding clearance, fire separation, and ventilation.
