When you wire batteries in series, the total voltage is the sum of the individual cell voltages, while the amp-hour (Ah) capacity remains identical to a single cell. For example, wiring four 3.2V 100Ah LiFePO4 cells in series yields 12.8V at 100Ah. The current path forces electrons through every single cell in the chain, meaning the string is only as strong as its weakest link. Understanding how this topology behaves under load, how it fails, and how to prototype it safely is the difference between a reliable 24V solar bank and a melted busbar.
The Series Topology: Node Labels and Voltage Stacking
In a pure series circuit, the positive terminal of one cell connects directly to the negative terminal of the next. There are no branching paths. To design and troubleshoot effectively, we map the circuit using sequential node labels. Let us look at a standard 8-cell string used for a 24V nominal (25.6V actual) LiFePO4 system.
- Node 0 (Cell 1 Negative): System GND (0V reference).
- Node 1 (Cell 1 Pos / Cell 2 Neg): 3.2V nominal.
- Node 2 (Cell 2 Pos / Cell 3 Neg): 6.4V nominal.
- Node 3 (Cell 3 Pos / Cell 4 Neg): 9.6V nominal.
- Node 4 (Cell 4 Pos / Cell 5 Neg): 12.8V nominal.
- Node 5 (Cell 5 Pos / Cell 6 Neg): 16.0V nominal.
- Node 6 (Cell 6 Pos / Cell 7 Neg): 19.2V nominal.
- Node 7 (Cell 7 Pos / Cell 8 Neg): 22.4V nominal.
- Node 8 (Cell 8 Positive): System VCC (25.6V nominal, up to 29.2V fully charged).
Series vs. Parallel: Why Stack Voltage and Failure Contrasts
Why choose series over parallel? The physics of power transmission dictates the answer. Power equals Voltage times Current ($P = V \times I$). If you need 2400W from a 12V parallel bank, you must push 200A through your cables. That requires massive 2/0 AWG copper, expensive Class-T fuses, and generates significant $I^2R$ heat loss. By wiring in series to create a 24V system, the current drops to 100A, allowing you to use cheaper, more flexible 2 AWG wire and smaller breakers. For a 48V system, it drops to just 50A. Higher voltage means lower current, thinner wires, and higher efficiency.
However, the trade-off lies in the failure modes. According to Victron Energy's engineering guidelines, understanding how series and parallel strings break at the extremes is critical for system safety.
Failure Mode Contrast
| Topology | Failure Type | What Breaks at the Extremes | System Consequence |
|---|---|---|---|
| Series | Open Circuit | A busbar snaps, a fuse blows, or a cell dies open. The single current path is broken. | Output drops to 0V. The entire system is completely dead until the break is found. |
| Series | Short Circuit | One cell internally shorts. The string loses one cell's voltage (e.g., 25.6V drops to 22.4V). | The system may limp along at a lower voltage, but the BMS will flag severe cell imbalance and likely disconnect to prevent over-discharging the remaining cells. |
| Parallel | Open Circuit | One cell or parallel group disconnects. | Total Ah capacity drops, but voltage remains unchanged. The system keeps running, but runtime is reduced. |
| Parallel | Short Circuit | One cell internally shorts. | Catastrophic. The remaining parallel cells dump hundreds of amps into the shorted cell, causing immediate thermal runaway and fire. |
Design Walkthrough: Prototyping and Scaling a 24V Pack
Let us design a 24V nominal (25.6V) 105Ah battery bank for an off-grid solar inverter. Before you torque M8 terminal bolts to 12 Nm on heavy prismatic cells, you must prototype the BMS logic and verify your node mapping on the bench.
Phase 1: The Breadboard Bench Test
We will use small 18650 cells to simulate the 8S topology and test the BMS sense wiring without the risk of high-current arcs.
- Components: 8x Panasonic NCR18650B (3.6V nominal, 3400mAh) and an 8-slot series battery holder.
- Load Cells: Insert the 18650s into the holder, ensuring alternating polarity matches the series trace on the holder.
- BMS Wiring: Connect a Daly 8S 20A Smart BMS. Connect the thick black B- wire to Node 0. Connect the 9-pin sense harness sequentially from Node 0 up to Node 8.
- Verify Nodes: Set your multimeter to DC Volts. Place the black probe on Node 0. Step the red probe through Nodes 1 through 8. You should read roughly 3.6V, 7.2V, 10.8V, up to 28.8V.
- Test Faults: Intentionally remove one 18650 cell to simulate an open circuit. Verify that the BMS triggers an open-wire alarm via Bluetooth and opens the discharge MOSFETs.
Phase 2: Scaling to the Final Prismatic Build
Once the bench test confirms your BMS configuration, scale up to the real components.
- Cells: 8x EVE LF105 LiFePO4 prismatic cells (3.2V, 105Ah each).
- Busbars: 2mm thick copper busbars, M8 holes.
- Main Cables: 2 AWG THHN stranded copper for the main positive and negative feeds to the busbar.
- Overcurrent Protection: 150A Class-T fuse on the main positive Node 8 feed, placed as close to the terminal as possible.
- Assembly: Compress the cells in a steel battery box to 10-12 PSI (crucial for LiFePO4 cycle life). Torque all M8 busbar nuts to exactly 10-12 Nm using a calibrated torque wrench. Overtightening will strip the aluminum terminal threads; undertightening will cause high resistance and melting under load.
Behavior Matrix: What Happens When Elements Change
Series circuits are unforgiving of mismatched components. Here is exactly how the system behaves when variables change in the string.
| Event / Change | Effect on Total Voltage | Effect on Total Ah Capacity | System Consequence |
|---|---|---|---|
| Add 1 identical cell in series | Increases by one cell's nominal voltage (e.g., +3.2V). | No change (remains 105Ah). | System voltage rises. If the inverter or BMS is not rated for the new voltage, components will blow. |
| Remove 1 cell from the string | Decreases by one cell's nominal voltage. | No change. | Inverter may trigger an under-voltage disconnect (LVD) and shut down the AC loads. |
| One cell degrades (high ESR) | Total voltage sags heavily under load. | Usable capacity drops drastically. | The degraded cell hits the BMS low-voltage cutoff first, shutting down the entire pack prematurely. |
| One cell installed backwards | Total voltage drops by double that cell's voltage (e.g., -6.4V). | No change. | The reversed cell will be violently overcharged and destroyed during the first charge cycle. |
Frequently Asked Questions
Does wiring batteries in series voltage increase the amp-hours (Ah)?
No. Wiring batteries in series strictly increases the voltage while the amp-hour capacity remains exactly the same as a single cell. If you wire four 12V 100Ah batteries in series, you get 48V at 100Ah. The total energy (Watt-hours) increases because Watt-hours = Voltage × Amp-hours (48V × 100Ah = 4800Wh), but the Ah rating itself does not stack. To increase Ah, you must wire batteries in parallel.
Can I mix different battery capacities in a series string?
Never mix different capacities, chemistries, or ages in a series string. Because the exact same current flows through every cell in a series circuit, a smaller capacity cell (e.g., 50Ah) will be fully discharged and driven into a dangerous deep-discharge state long before the larger cells (e.g., 100Ah) are empty. The BMS will eventually trip on low voltage, but the smaller cell will suffer severe degradation or internal damage from being forced to pass current it cannot handle.
What happens to the voltage if I wire two 12V batteries in series?
If you wire two standard 12V nominal lead-acid or LiFePO4 batteries in series, the total voltage becomes 24V nominal. During charging, this voltage will peak higher; for two 12V AGM lead-acid batteries, the absorption charging voltage will reach roughly 28.8V to 29.2V. You must ensure your inverter and charge controller are configured for a 24V battery bank profile before applying a charge.
Do I need a special charger for batteries wired in series?
Yes, your charger must match the total series voltage, and ideally, you should use a BMS with cell-balancing capabilities. When you charge an 8S (25.6V) LiFePO4 string, you apply roughly 29.2V to the main terminals. However, due to slight manufacturing variances, some cells will reach the 3.65V maximum limit faster than others. Without a BMS to bleed off excess current from the full cells (passive balancing) or redistribute it (active balancing), the leading cells will overvolt and trigger a fault, leaving the rest of the string undercharged.
