When you need to wire 4 batteries in series and parallel, you are building a 2S2P (2-series, 2-parallel) matrix. This configuration doubles both the voltage and the amp-hour capacity of a single cell. For standard 3.7V nominal lithium-ion cells, a 2S2P pack yields 7.4V nominal (8.4V fully charged) and doubles your capacity. If you are using 12V LiFePO4 blocks, it yields a 24V nominal bank with double the amp-hours.
This guide breaks down the exact topology, real-world component selection, catastrophic failure modes, and how to safely prototype the circuit on your bench.
The 2S2P Topology: Node Labels and Circuit Logic
To combine 4 batteries in series and parallel, you create two parallel pairs, then wire those pairs in series (or vice versa; electrically, the matrix is identical).
When connecting this to a Battery Management System (BMS) or a load, you must identify three critical nodes:
- B+ (Pack Positive): The positive terminal of the first series string.
- BM (Balance/Mid-Node): The electrical midpoint. This is the physical wire connecting the negative side of the first series string to the positive side of the second series string.
- B- (Pack Negative): The negative terminal of the second series string.
Wiring 4 cells in pure series (4S) yields 14.8V nominal, which requires complex step-down conversion for standard 5V logic and exceeds basic USB-C PD input thresholds without active negotiation. Wiring 4 in pure parallel (4P) keeps voltage at 3.7V but demands massive current (e.g., 40A for a 150W load), which will melt standard PCB traces and connectors. The 2S2P topology hits the 7.4V sweet spot: it easily steps down to 5V via a standard buck converter and keeps discharge currents within the limits of standard 18 AWG wiring and XT60 connectors.
Component Selection and Design Walkthrough
Let us design a portable 2S2P power bank for a 12W embedded sensor node. We will use 18650 lithium-ion cells.
The Bill of Materials
| Component | Specification | Reasoning |
|---|---|---|
| Cells (x4) | Molicel P28A 18650 | 2800mAh capacity, 35A max continuous discharge. High cycle life and reliable chemistry. |
| BMS | Daly 2S 15A Common Port | Handles up to 15A continuous. Common port means charge and discharge share the same P+ and P- pads. |
| Interconnect Wire | 18 AWG Silicone | Rated for ~10A in free air. Silicone insulation won't melt if your soldering iron lingers too long. |
| Cell Holder | Keystone 2-slot x2 | Spring-loaded contacts rated for 10A. Avoid cheap plastic holders with thin nickel strips. |
| Load Regulator | LM2596 Buck Module | Steps the 7.4V-8.4V pack down to a stable 5.0V for the microcontroller. |
The Math: Four Molicel P28A cells in 2S2P yield a pack capacity of 5600mAh (5.6Ah) at 7.4V nominal. Total energy is roughly 41.4Wh. At a 12W continuous load, the buck converter will draw about 1.8A from the battery (accounting for 85% converter efficiency). This is well within the 15A limit of the Daly BMS and the 10A ampacity of the 18 AWG wire.
Failure Mode Contrast: What Breaks at the Extremes?
Understanding what happens when a single element fails is the difference between a safe battery pack and a fire hazard. According to OSHA lithium-ion safety guidelines, internal shorts and thermal runaway are the primary risks in multi-cell configurations.
| Failure Event | Electrical Result | Physical Consequence |
|---|---|---|
| 1 Cell Open (Series Leg) | Pack voltage drops to 0V. Circuit is broken. | System shuts down safely. BMS registers fault. No fire risk. |
| 1 Cell Open (Parallel Leg) | Pack voltage remains 7.4V. Capacity halves to 2.8Ah. | The remaining cell in that parallel group must now supply 100% of the string's current. It will experience 2x the discharge stress, accelerating degradation and risking BMS over-current tripping. |
| 1 Cell Shorted (Internal) | Pack voltage drops to ~3.7V. | Critical Hazard: The parallel cell will dump its entire charge into the shorted cell at maximum current (limited only by internal resistance). This causes rapid heating, venting, and thermal runaway. A BMS cannot prevent parallel cross-current. |
| BMS Balance Wire (BM) Broken | Pack functions normally under load. | BMS cannot monitor individual string voltages. During charging, one string may overcharge past 4.25V while the BMS thinks the pack is fine, leading to cell venting. |
Step-by-Step Breadboard and Prototyping Test
A critical note on terminology: Do not push 18650 cells into a standard solderless breadboard. Standard breadboard spring clips are rated for roughly 1A and have high contact resistance. Pushing 5A through them will melt the plastic housing and cause a short. For power prototyping, we use a screw-terminal prototyping board or a dedicated battery holder with alligator clips.
- Prep the Cells: Measure all four Molicel P28A cells with a multimeter. Ensure they are all within 0.05V of each other (ideally all at 3.8V storage voltage). If not, charge/discharge them individually to match.
- Wire the Parallel Pairs: Place two cells in the first Keystone holder. Solder 18 AWG wire across the positive terminals, and another across the negative terminals. Repeat for the second pair.
- Create the Series Link: Solder a wire from the negative terminal of Pair 1 to the positive terminal of Pair 2. This is your BM (Mid-Node). Leave this wire loose at the BMS end for now.
- Connect the BMS Sense Wires:
- Solder the BMS B- wire to the negative terminal of Pair 2.
- Solder the BMS BM wire to the series link you created in Step 3.
- Solder the BMS B+ wire to the positive terminal of Pair 1.
- Verify Voltages (Crucial Step): Before connecting the main P+ and P- pads, use your multimeter. Measure B- to BM (should read ~3.8V). Measure BM to B+ (should read ~3.8V). Measure B- to B+ (should read ~7.6V). If these are wrong, stop and re-check your wiring.
- Test the Load: Connect your LM2596 buck converter to the BMS P+ and P- pads. Adjust the potentiometer on the buck module until the output reads exactly 5.00V. Connect your microcontroller or a 10-ohm dummy load resistor to verify current draw.
Frequently Asked Questions
Can I mix different capacity cells when wiring 4 batteries in series and parallel?
No. Mixing capacities (e.g., two 2800mAh cells and two 3500mAh cells) in a 2S2P matrix creates severe imbalance issues. During discharge, the lower-capacity parallel group will hit the BMS low-voltage cutoff before the higher-capacity group is empty. During charging, the lower-capacity group will hit 4.2V and trigger the BMS over-voltage protection while the other group is still undercharged. Always use identical cells from the same manufacturing batch.
Do I need a BMS for a 2S2P Li-ion configuration?
Yes, absolutely. While you can technically wire 4 batteries in series and parallel without a BMS for a crude DIY project, it is a massive safety risk. Lithium-ion cells cannot tolerate over-discharge (below 2.5V) or overcharge (above 4.25V). A 2S BMS monitors the voltage of both series strings independently via the BM node and disconnects the pack if either string drifts outside the safe operating area. As noted by researchers at Argonne National Laboratory, proper electronic management is mandatory for preventing lithium plating and thermal runaway in multi-cell packs.
How do I balance charge 4 batteries in series and parallel without a BMS?
If you are strictly prototyping and refuse to use a BMS, you must use a balanced power supply that limits the maximum voltage to exactly 8.40V (4.20V per series string) and relies on the parallel connections to naturally balance the cells within each pair. However, this offers zero over-discharge protection. If your load drains the pack to 6.0V, the weaker series string might drop to 2.0V, permanently damaging the cells. For any deployment outside of a supervised bench test, a BMS is non-negotiable.
