Putting voltage sources in series adds their electromotive force (EMF) while keeping the current capacity (mAh) identical to a single cell. If you need 12V from 3.7V Li-ion cells, you wire three in series (3S). The direct answer for most DIY robotics and embedded projects requiring 11V to 14V is to build a 3S lithium-ion pack rather than boosting a single cell, because series topologies handle high-current transients with vastly lower thermal losses and input ripple.
The Series Voltage Topology: Node Labels and Core Math
When you connect voltage sources in series, the cathode (negative terminal) of one source connects to the anode (positive terminal) of the next. To analyze this mathematically using Kirchhoff’s Voltage Law (KVL), we assign specific node labels to the junction points.
Consider a 3-cell series string:
- Node 0 (System GND): The cathode of Cell 1. This is your 0V reference.
- Node 1 (Intermediate V1): The anode of Cell 1 connected to the cathode of Cell 2. Voltage relative to Node 0 is $V_{cell1}$.
- Node 2 (Intermediate V2): The anode of Cell 2 connected to the cathode of Cell 3. Voltage relative to Node 0 is $V_{cell1} + V_{cell2}$.
- Node 3 (System V+): The anode of Cell 3. This is your total output voltage ($V_{total}$).
The total voltage is the algebraic sum: $V_{total} = V_1 + V_2 + V_3$. However, the maximum continuous current ($I_{max}$) of the series string is strictly limited by the cell with the lowest current rating. If you mix a 35A cell with a 10A cell in series, the entire string is bottlenecked to 10A.
Series vs. Parallel: The Decision Tree for Power Design
Why choose series over parallel? Parallel configurations (connecting anode-to-anode, cathode-to-cathode) keep voltage constant but multiply current capacity (mAh). You choose series when your load's voltage requirement exceeds the nominal voltage of your available chemical cell, and you want to avoid the inefficiencies of DC-DC step-up (boost) converters.
| Design Constraint | Topology Choice | Concrete Component Pick |
|---|---|---|
| Need 12V nominal, load draws < 500mA | 1S Cell + Boost Converter | 1x 18650 + MT3608 Boost Module |
| Need 5V nominal, high runtime (days) | Parallel (1P to 4P) | 4x Molicel P28A in parallel |
| Need 11.1V-12.6V, load draws 5A-20A (Stepper motors, high-power LEDs) | Series (3S) | 3x Molicel P28A in series (Default Pick) |
The Default Recommendation: For any mobile robotics, CNC, or high-power LED project requiring 12V, choose a 3S series Li-ion topology. Boosting a single 3.7V cell to 12V at 10A requires the input side to pull over 32A, which will melt standard PCB traces and generate massive heat. A 3S series pack delivers 12.6V natively, keeping the current draw on the cells equal to the load current (10A).
Failure Modes at the Extremes: Opens, Shorts, and Reversals
Understanding what breaks when a single element fails is the starkest contrast between series and parallel topologies. According to Battery University, series strings are highly vulnerable to single-point failures because the same current must flow through every cell.
1. The Open Circuit Failure
If Cell 2 develops an internal open (e.g., a blown internal fuse or a severed tab weld), the entire series string goes dead. $V_{total}$ at Node 3 drops to 0V relative to the load, because the circuit path is broken. In a parallel topology, an open cell merely reduces total capacity; in series, it kills the system.
2. The Internal Short Circuit
If Cell 2 shorts internally, its EMF drops to ~0V. The total pack voltage drops by one cell's nominal voltage (e.g., from 12.6V down to 8.4V). The danger here is secondary: if the load continues to draw power, the remaining two cells will be forced to discharge deeper to compensate, rapidly driving them below their safe 2.5V cutoff and causing copper dendrite growth.
3. Cell Reversal (The Lethal Edge Case)
If one cell in a series string has a lower capacity (say, 2000mAh mixed with 2800mAh cells), it will deplete first under load. If the load keeps pulling current, the stronger cells will force current backward through the depleted cell. This 'cell reversal' drives the weak cell into negative voltage, causing rapid outgassing, venting, and in lithium chemistries, thermal runaway. This is why a BMS with low-voltage cutoff is non-negotiable for series lithium packs.
Behavior Matrix: How the Circuit Reacts to Element Changes
Use this matrix to predict circuit behavior when modifying or troubleshooting a series string.
| Parameter Changed | Effect on $V_{total}$ (Node 3) | Effect on $I_{max}$ Capacity | Effect on Internal Resistance ($R_{int}$) |
|---|---|---|---|
| Add 1 identical cell in series | Increases by $V_{cell}$ | No change | Increases by $R_{cell}$ |
| Replace 1 cell with higher mAh rating | No change (if same chemistry) | No change (bottlenecked by smaller cells) | Decreases slightly |
| Load resistance decreases (draws more current) | Drops due to $I \times R_{int}$ sag | No change (limit is fixed) | No change |
| One cell drops to 0V (internal short) | Drops by $V_{cell}$ | Severely compromised (risk of reversal) | Drops by $R_{cell}$ |
Design Walkthrough: Building a 3S 18650 Pack for Stepper Drivers
Let’s design a concrete power source for a TB6600 stepper motor driver powering a NEMA 23 motor on a DIY CNC router. The driver requires 12V-24V DC and can peak at 6A per phase. We will use voltage sources in series to build a 3S pack.
Component Selection
- Cells: 3x Molicel P28A (INR18650-P28A). Nominal 3.6V, 2800mAh, 35A max continuous discharge. (Total pack: 10.8V nominal, 12.6V max, 2800mAh).
- BMS: HX-3S-JH20. A 3-series Li-ion BMS rated for 20A continuous discharge, featuring over-charge, over-discharge, and over-current protection, plus passive cell balancing.
- Interconnects: 0.15mm x 27mm pure nickel strips (spot welded). Do not use solder directly on 18650 terminals; the heat degrades the internal separator.
Wiring the Topology
- Cell Array: Arrange the three Molicel cells in a 1x3 row, all positive terminals facing the same direction.
- Series Jumps: Spot weld a nickel strip from the positive terminal of Cell 1 to the negative terminal of Cell 2. Weld another from Cell 2 positive to Cell 3 negative.
- BMS Connections:
- Solder the BMS 'B-' pad to the negative terminal of Cell 1 (Node 0).
- Solder the BMS 'BM1' (balance 1) wire to the positive of Cell 1 (Node 1).
- Solder the BMS 'BM2' (balance 2) wire to the positive of Cell 2 (Node 2).
- Solder the BMS 'B+' pad to the positive of Cell 3 (Node 3).
- Output: The main charge/discharge leads connect to the BMS 'P+' and 'P-' pads, not directly to the cells. The BMS acts as a high-side/low-side MOSFET switch to protect the series string.
Step-by-Step Breadboard Verification Protocol
You should never test raw 18650 series strings on a solderless breadboard. The breadboard's internal phosphor-bronze clips are rated for roughly 1A to 2A max; an 18650 can dump 35A, melting the plastic housing and causing a fire. To verify your series topology logic and measure node voltages safely, we step down to AA NiMH cells (like Eneloops), which are current-limited by their higher internal resistance (~30mΩ per cell) and lower voltage (1.2V nominal).
The Safe Breadboard Test
- Prep the Sources: Insert 3x AA batteries into three separate 2-slot AA battery holders. Each holder outputs 2.4V to 3.0V depending on charge state.
- Place and Jumper: Place the red (positive) wire of Holder 1 into Breadboard Row 10. Place the black (negative) wire of Holder 1 into Row 15. Jumper Row 15 to Row 20. Place Holder 2's red wire into Row 20, and black into Row 25. Jumper Row 25 to Row 30. Place Holder 3's red wire into Row 30, and black into Row 35.
- Verify Nodes with a Multimeter:
- Set your DMM to DC Volts. Place the black probe on Row 10 (Node 0).
- Place the red probe on Row 15 (Node 1). Read ~2.4V.
- Move red probe to Row 25 (Node 2). Read ~4.8V.
- Move red probe to Row 35 (Node 3). Read ~7.2V.
- Apply a Load: Insert a standard 5mm red LED with a 330Ω current-limiting resistor across Row 10 (GND) and Row 35 (V+). The LED should illuminate brightly, proving the series string can source current through the load.
- Simulate an Open Failure: Pull the jumper wire between Row 15 and Row 20. The LED will instantly extinguish. Measure Node 3 again; it will read 0V relative to Node 0, confirming the open-circuit failure mode of series topologies.
By validating the node math and failure modes at low power on the breadboard, you build the intuition required to safely spot-weld and deploy high-current 3S lithium packs on the bench. When your load demands high voltage and high current, series topology is the only physics-compliant path forward.
