What is a battery pack configuration
A battery pack configuration describes how individual lithium-ion cells are wired together to achieve a desired voltage and capacity. Every battery pack — whether it powers an e-bike, an electric skateboard, a home energy storage system, or a portable power station — is built from smaller cylindrical cells arranged in a specific series and parallel pattern. The two most common cell formats used in DIY and commercial battery packs are the 18650 and the 21700, both named after their physical dimensions in millimeters.
The configuration of a battery pack is expressed using a compact notation like "10S4P." The first number followed by "S" indicates how many cells are connected in series, which determines the pack's voltage. The second number followed by "P" indicates how many cells are connected in parallel, which determines the pack's capacity in amp-hours. In a 10S4P pack, ten cells are wired end-to-end in each string to produce roughly 37 volts at nominal, and four of those strings are wired side-by-side to multiply the capacity by four.
Understanding your pack configuration is essential for selecting the right Battery Management System (BMS), choosing an appropriate charger, sizing your wiring and fuses, and ensuring safe operation. The total number of cells directly affects the pack's weight, physical size, and cost. This calculator takes your target voltage and capacity and works backward to determine exactly how many cells you need and how they should be arranged. It also computes the actual voltage and capacity the configuration will deliver, since the series and parallel counts must be whole numbers and the result may exceed your original targets slightly.
Series vs parallel explained
**Series connections** increase voltage while keeping capacity the same. When two 3.7V cells are wired in series (positive terminal of one cell connected to the negative terminal of the next), the total voltage doubles to 7.4V, but the capacity remains equal to a single cell's rating. Adding more cells in series continues to increase voltage proportionally.
**Parallel connections** increase capacity while keeping voltage the same. When two 3.7V cells with 3000 mAh each are wired in parallel (positive to positive, negative to negative), the voltage stays at 3.7V but the total capacity doubles to 6000 mAh. Parallel connections also increase the maximum discharge current the pack can deliver.
In practice, every battery pack uses a combination of both. The series count (S) is chosen to match the voltage requirement of your motor controller, inverter, or device. The parallel count (P) is chosen to deliver enough amp-hours for your desired runtime or range. The total cell count is simply S multiplied by P.
**Formula:** \`\`\` Series cells (S) = Target Voltage / 3.7V (rounded up) Parallel cells (P) = Target Capacity (mAh) / Cell Capacity (mAh) (rounded up) Total cells = S x P Pack Energy (Wh) = Actual Voltage x Actual Capacity (Ah) \`\`\`
Cell types: 18650 vs 21700
The **18650** cell measures 18mm in diameter and 65mm in length. It has been the dominant lithium-ion form factor for over a decade and is widely available from manufacturers like Samsung, LG, Sony, and Panasonic. Typical capacities range from 2500 mAh to 3500 mAh, and each cell weighs approximately 46 grams. The 18650 is a proven, affordable, and well-understood cell that remains popular for many applications.
The **21700** cell measures 21mm in diameter and 70mm in length. It offers roughly 30-50% more energy per cell compared to the 18650, with typical capacities between 4000 mAh and 5000 mAh. Each cell weighs approximately 70 grams. The 21700 format was popularized by Tesla and is increasingly used in high-performance applications because fewer cells are needed to reach the same total capacity, which reduces the number of connections and simplifies pack construction.
- **Choose 18650** when space is limited, weight per cell matters, or you need cells that are easy to source at low cost. - **Choose 21700** when you want maximum energy density, fewer total cells, and simpler pack assembly for large builds.
How to use this calculator
1. Select your cell type — choose between 18650 and 21700 based on your project needs. 2. Set the cell capacity in milliamp-hours (mAh). The default is a common mid-range value for the selected cell type, but adjust it if you have specific cells in mind. 3. Enter your target voltage in volts. Common values include 12V for small projects, 36V or 48V for e-bikes, and 52V or 72V for high-performance builds. 4. Enter your target capacity in amp-hours (Ah). This determines runtime — higher Ah means longer operation before recharging. 5. Review the results: the calculator shows your series/parallel configuration, total cell count, actual pack voltage and capacity, total energy in watt-hours, estimated weight, and a voltage table showing the pack at fully charged, nominal, and empty states.
Safety considerations
Building lithium-ion battery packs carries real risks if done improperly. Lithium cells store a large amount of energy and can catch fire, vent toxic gases, or explode if short-circuited, overcharged, over-discharged, or physically damaged.
**Essential safety practices:** - **Always use a BMS** (Battery Management System) sized for your series count and discharge current. The BMS protects against overcharge, over-discharge, short circuits, and cell imbalance. - **Use proper cell holders or spot welding** — never solder directly to cells, as the heat can damage the internal separator and cause a delayed short circuit. - **Fuse each parallel group** or use cells with built-in protection circuits for added safety. - **Match cells carefully** — all cells in a pack should be the same chemistry, brand, model, capacity, and ideally from the same production batch. Mismatched cells degrade faster and create imbalance. - **Charge with an appropriate charger** rated for your pack's series count and chemistry. - **Store and charge in a fireproof location** — use a LiPo-safe bag or metal container, especially during initial testing. - **Never exceed the cell's maximum continuous discharge rating** — this is measured in amps and varies by cell model.
FAQs
Q: Why does the calculator round up the series and parallel counts? A: You cannot use a fraction of a cell. If your target voltage of 36V divided by 3.7V per cell gives 9.73, you need 10 cells in series to meet or exceed your target voltage. The same logic applies to parallel counts. This means the actual pack voltage and capacity will always be equal to or slightly above your specified targets.
Q: What does the notation "10S4P" mean? A: The notation describes the cell arrangement. "10S" means 10 cells connected in series, giving a nominal voltage of 37V (10 times 3.7V). "4P" means 4 parallel groups, multiplying the single-cell capacity by 4. The total number of cells is 10 times 4, which equals 40 cells. This compact notation is the standard way to describe battery pack configurations in the DIY and EV community.
Q: Does this calculator account for the weight of wiring, BMS, and enclosure? A: No. The weight estimate includes only the cells themselves. A complete battery pack will weigh more once you add the BMS board, nickel strip or bus bars for connections, wiring, connectors, and the enclosure or heat-shrink wrap. As a rough guide, expect the finished pack to weigh 15-30% more than the cell-only weight shown.
Q: Can I mix 18650 and 21700 cells in the same pack? A: No. You should never mix different cell types, capacities, or brands within the same battery pack. Mismatched cells will charge and discharge unevenly, leading to premature degradation, reduced capacity, and potential safety hazards. Always use identical cells throughout your entire pack.
Q: What is the difference between nominal voltage and fully charged voltage? A: Nominal voltage (3.7V per cell) represents the average voltage during a typical discharge cycle. Fully charged voltage (4.2V per cell) is the maximum voltage when the cell is at 100% state of charge. The pack voltage will decrease from the fully charged value down through the nominal range and eventually reach the empty cutoff voltage (3.0V per cell) as the battery is depleted.
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