A batteries in series diagram connects the positive terminal of one cell to the negative terminal of the next, summing their voltages while keeping the amp-hour (Ah) capacity identical to a single cell. If you wire four 3.2V 100Ah LiFePO4 cells in series, you get a 12.8V 100Ah pack. The total energy (Wh) remains the same, but the higher voltage allows you to push that energy through thinner, cheaper copper wire.

This topology is the backbone of every 12V, 24V, and 48V solar and EV battery bank. Below is the exact blueprint for designing, testing, and protecting a series string, moving from bench-top 18650 prototypes to full-scale prismatic solar arrays.

Topology and Node Labels in a Series String

In a pure series topology, current has only one path to flow. To design or troubleshoot these packs, you must label your nodes sequentially from the negative busbar to the positive busbar. This node mapping is critical when wiring a Battery Management System (BMS) sense harness.

  • Node 0 (B-): The main negative terminal of Cell 1. This connects to the load’s negative busbar and the BMS main B- pad.
  • Node 1 (B1): The junction between Cell 1 Positive and Cell 2 Negative.
  • Node 2 (B2): The junction between Cell 2 Positive and Cell 3 Negative.
  • Node N (B+): The main positive terminal of the final cell in the string. This connects to the load’s positive busbar (often via a fuse and the BMS P+ charge/discharge pads).

Why Series Over Parallel?

The decision to use a series diagram instead of a parallel one is driven by current reduction. According to Joule's heating law and basic power equations ($P = IV$), doubling your voltage halves your current for the same wattage.

Consider running a 2,000W inverter. On a 12V parallel bank, you will pull roughly 166A of continuous current, requiring expensive 2/0 AWG welding cable and massive busbars to prevent voltage drop and melting. By using a series diagram to build a 24V pack, the current drops to 83A. You can safely use 4 AWG THHN wire and standard Anderson connectors. Higher voltage means thinner copper, lower $I^2R$ heat losses, and cheaper infrastructure.

Behavior Matrix: What Changes When One Element Shifts

Series circuits are unforgiving. Because there is only one path for current, a single cell anomaly dictates the behavior of the entire pack. Here is how the string reacts to element changes and extreme failures.

Element Change / Event Pack Voltage Pack Capacity (Ah) Internal Resistance System Result & BMS Action
Add 1 identical cell in series Increases by $V_{nom}$ Unchanged Increases by $R_{cell}$ Higher voltage threshold; BMS must be reconfigured for new cell count.
One cell ages (capacity fades) Unchanged (nominal) Drops to match weakest cell Increases locally Weakest cell hits Low Voltage Disconnect (LVD) first, stranding usable energy in healthy cells.
Extreme: Open Circuit Drops to 0V at output 0 Ah Infinite Pack is completely dead. Like a broken filament in a series Christmas light strand. No current flows.
Extreme: Internal Short Drops by $V_{cell}$ Unchanged (theoretically) Drops to near 0Ω locally Remaining cells dump massive current into the shorted cell. High risk of thermal runaway if BMS does not trigger short-circuit protection.
Safety Warning: An internal short in a series string is the primary cause of lithium battery fires. The healthy cells in the series string will forcefully drive current backward through the shorted cell, generating extreme heat. Never wire cells in series without a BMS that features hardware-level short-circuit and over-current protection.

Design Walkthrough: 3S 18650 Prototype Pack (11.1V Nominal)

Before welding a 16-cell 48V solar bank, you should prototype your BMS logic and series wiring on the bench. We will design a 3S (3-series) 18650 pack to power a 12V LED strip or Arduino relay module.

Component Selection

  • Cells: 3x Samsung INR18650-35E (3.5Ah capacity, 8A continuous discharge). Total pack: 11.1V nominal, 12.6V fully charged, 3.5Ah.
  • BMS: Daly 3S 12V 100A Smart BMS (Common port).
  • Wiring: 18 AWG silicone wire for sense leads; 14 AWG for main discharge leads.

Wiring the Diagram

The physical wiring follows the node map strictly.

  1. Solder the negative of Cell 1 to the BMS B- pad.
  2. Solder the positive of Cell 1 to the negative of Cell 2. Attach a sense wire here (Node 1 / B1).
  3. Solder the positive of Cell 2 to the negative of Cell 3. Attach a sense wire here (Node 2 / B2).
  4. Attach the final sense wire to the positive of Cell 3 (Node 3 / B+).
Bench Tip: When prototyping with 18650 battery holders, the cheap spring-loaded contacts often have high resistance (up to 0.5Ω per contact). This will cause false voltage drops under load. For final assembly, always use nickel strip spot-welded directly to the cell terminals.

Step-by-Step Breadboard and Multimeter Verification

The most common way to destroy a new BMS is plugging in the sense harness when the wires are in the wrong order. If B1 and B2 are swapped, the BMS will see a negative voltage differential and instantly fry its internal balancing MOSFETs. Follow this exact verification sequence before connecting the sense plug to the BMS board.

  1. Insert and Measure Base Cells: Place the cells in the holder. Set your multimeter to DC Volts. Measure each cell individually. All three should read between 3.0V and 4.2V. If a cell reads below 2.5V, charge it before proceeding.
  2. Verify Junction Voltages: Place the black probe on the main negative (Node 0). Place the red probe on Node 1. It should read the exact voltage of Cell 1 (e.g., 3.7V). Move the red probe to Node 2. It should read the sum of Cell 1 + Cell 2 (e.g., 7.4V). Move to Node 3. It should read the sum of all three (e.g., 11.1V).
  3. Wire the Sense Harness: Crimp your 18 AWG sense wires to the JST connector. Do not plug it into the BMS yet.
  4. Back-Probe the Connector: Turn your multimeter back on. Insert the black probe into the first pin of the JST connector (B-). Insert the red probe into the second pin (B1). Verify it reads ~3.7V. Move the red probe to the third pin (B2). Verify it reads ~7.4V. Move to the fourth pin (B+). Verify it reads ~11.1V.
  5. Connect and Balance: Once the connector pins are mathematically verified, plug the harness into the Daly BMS. Connect a load. Open the Daly Smart BMS Bluetooth app on your phone and verify that all three cell voltage readings match your multimeter within 0.02V.

Decision Tree: Sizing Your Series String and BMS

Choosing the right series count and BMS rating depends entirely on your inverter voltage threshold and continuous wattage requirements. Use this decision matrix to lock in your final component list.

Your Target Application Inverter / Load Voltage Max Continuous Wattage Required Series Count (LiFePO4) Minimum BMS Current Rating
Camper Van / Small Marine 12V Nominal < 1,500W 4S (12.8V) 150A
Off-Grid Cabin / Standard Solar 24V Nominal 1,500W - 3,500W 8S (25.6V) 150A
Whole Home Backup / EV Conversion 48V Nominal 3,500W - 8,000W+ 16S (51.2V) 200A - 300A

The Default Recommendation

If you are building a standard off-grid solar or backup system, do not build a 12V bank. The copper costs and inefficiencies at high currents will ruin your budget.

The Concrete Pick: Default to a 24V architecture. Buy eight EVE LF105 3.2V 105Ah prismatic LiFePO4 cells (typically $45–$55 per cell in 2026) and wire them in an 8S series configuration. Protect the pack with a Daly 8S 250A Smart BMS (Part # D-8S250A). This specific combination safely supports a 3,000W 24V inverter, keeps your current under 125A (allowing the use of standard 2 AWG battery cables), and provides Bluetooth cell-level telemetry for less than $500 total in raw components.

For deeper reading on series/parallel DC theory, refer to the All About Circuits DC textbook chapter on batteries. For physical wiring safety standards, consult the Electronics Tutorials DC Circuits guide to ensure your busbar sizing matches your series current limits.