Battery Pack Calculator
Plan and size a custom 18650 or 21700 lithium battery pack. Live cell lookup from 431 cells, series/parallel calculations, runtime estimates, and BMS recommendations. 249 cells with Mooch-tested discharge ratings.
Not sure what to put here?
Browse our database of 431 lithium cells to find the right one for your build. Start with the highest capacity 18650 cells for runtime-focused builds or the highest discharge 18650 cells for high-power applications.
If your exact cell isn't listed, enter the specs from the datasheet manually. The voltage, capacity, and discharge fields below will drive all the calculations.
Not sure which cell to pick? Step 2 in the guide below walks through how to choose based on your build's requirements.
Not sure what to put here?
These values come from your cell's datasheet and drive everything below. If you picked a cell from the database, they're already filled in.
The C-rate tells you how fast the cell can safely discharge relative to its capacity. A 1C rate on a 3000mAh cell means 3A of discharge. Most cells are rated between 0.5C and 3C for continuous use.
Not sure about discharge current? Step 4 in the guide below explains how to check if your pack can handle the load.
Not sure what to put here?
Series sets voltage. Parallel sets capacity and current. Pick one of the preset configurations above or enter your own numbers.
Not sure what configuration you need? The guide below has a table of common setups for different applications. Or read our full series vs parallel explainer for a deeper look at how the wiring works.
If this is your first build, our step-by-step 18650 build guide walks through the whole process from cell selection to final testing.
Not sure what to put here?
Enter the wattage your device pulls and get a rough runtime estimate. For example, a 500Wh pack running a 250W load lasts about 2 hours in theory.
Real runtime is about 80 to 90% of the number shown. Voltage sag under load, temperature, and cell age all cut into the ideal number. Use this as a sanity check, not a guarantee.
Want to understand the difference between Ah and Wh? Our capacity guide breaks it down.
39.2 V – 58.8 V
Pack can deliver 6.0 A but load demands 25.5 A.
~36 min at 1000 W draw.
Target a 14S 30A BMS for this pack.
No matching BMS products found for this configuration.
Search "14S 30A BMS" on AliExpress →Not sure what to put here?
This recommends a BMS based on your pack's series count and current output. The results pull live products from AliExpress matched to your exact configuration.
Always size your BMS 20 to 30% above your max expected draw. If your load pulls 20A, get at least a 25A BMS. This gives you headroom for startup surges and keeps the BMS from cutting out under normal use.
Need more options? Our BMS Picker lets you filter by chemistry, port type, and current rating. Or read our BMS selection guide for the full breakdown on what to look for.
Not sure what to put here?
This is cells only. The real cost of a finished pack adds about 20 to 30% on top for the BMS, nickel strip, wire, connectors, and an enclosure.
Weight is the same story. Cell weight is the minimum. Add 15 to 25% for the BMS, structural materials, and wiring to get a more realistic total pack weight.
Check our recommended supplies page for everything you need beyond the cells, with links to buy.
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Complete Guide to Designing a Lithium Battery Pack
Step 1: Define your requirements Back to calculator ↑
What voltage does your device need? What current will it draw? How long does it need to run? Those three numbers determine everything else about your pack.
Start with voltage. Most devices specify a nominal voltage: 12V, 24V, 36V, 48V, and so on. Your pack needs to match that. Then figure out how much current the device pulls at peak and at average load. Finally, decide how long you need it to run. Runtime times power equals energy, which tells you the minimum watt-hours your pack needs.
| Application | Voltage | Current Draw | Typical Config | Dedicated Tool |
|---|---|---|---|---|
| Ebike 36V | 36V | 10–20A | 10S 3–5P | Ebike Calculator |
| Ebike 48V | 48V | 10–25A | 13S 3–5P | Ebike Calculator |
| Ebike 52V | 52V | 10–30A | 14S 3–5P | Ebike Calculator |
| DIY Powerwall | 48–58V | 5–20A | 16S 4–8P | Powerwall Planner |
| Car Audio | 12–14V | 50–200A | 4S 6–10P | Car Audio Calculator |
| Power Tools | 18–20V | 20–40A | 5S 2–3P | Use this planner |
| RC / Drone | 11–22V | 30–100A | 3–6S 1–2P | Use this planner |
| Portable Bank | 12–25V | 2–10A | 3–7S 2–4P | Use this planner |
For ebikes, powerwalls, and car audio, the dedicated tools linked above include application-specific features like range estimation and solar integration. This planner is best for power tools, RC and drone builds, portable stations, and any custom configuration.
Step 2: Choose your cells Back to calculator ↑
Pick your cell based on what matters more for your build: capacity for runtime, or discharge rate for power. You generally cannot max out both in the same cell.
High-capacity cells (3000 to 3500mAh) are best for builds that prioritize runtime. High-discharge cells (20 to 30A continuous) are best for power tools, RC vehicles, and anything with high startup current.
Browse our cell database to compare options:
- Highest capacity 18650 cells
- Highest capacity 21700 cells
- Highest discharge 18650 cells
- Mooch-tested cells (independently verified discharge ratings)
Our database has 431 cells with specs from manufacturer datasheets and 249 with Mooch-tested continuous discharge ratings.
Step 3: Calculate your series and parallel configuration Back to calculator ↑
Series cells set voltage. Parallel cells set capacity and current. These two numbers work independently.
Series count = Target voltage divided by cell nominal voltage. Parallel count = Target capacity divided by cell capacity.
Example: You want a 48V pack with 15Ah using 3.6V, 3000mAh cells.
- Series: 48V / 3.6V = 13.3, round to 13S
- Parallel: 15Ah / 3Ah = 5P
- Total cells: 13 x 5 = 65 cells
The calculator above does this math automatically once you enter cell voltage and capacity.
Step 4: Check your current requirements Back to calculator ↑
This is where most first-time builders run into problems. Your pack needs to safely deliver the current your load demands.
Max pack current = Cell max discharge x Parallel count.
If your motor draws 20A and your cells are rated for 5A each, you need at least 4P (4 x 5A = 20A). Always add headroom. A 5P configuration gives you 25A capability and 25% margin. The calculator's "Current headroom" check flags this for you.
Step 5: Select a BMS Back to calculator ↑
Every lithium pack needs a BMS (Battery Management System) for safety. It prevents overcharge, over-discharge, and short circuits. Without one, a small wiring mistake can start a fire.
Match your BMS to three things:
- Series count: a 13S pack needs a 13S BMS.
- Maximum current: size the BMS above your peak draw, not average draw.
- Chemistry: most BMS are set for NMC/NCA at 4.2V per cell. LiFePO4 uses 3.65V per cell and needs a different BMS.
Use our BMS Picker to find matching products. The pack planner also shows live BMS recommendations once you enter your configuration.
Step 6: Estimate cost and weight Back to calculator ↑
Cell cost is the biggest factor. Multiply cell count by price per cell. Add 20 to 30% for the BMS, nickel strip, wire, connectors, and enclosure.
Weight works the same way. Cell weight times cell count is your minimum. Real pack weight runs 15 to 25% higher with structure and wiring included.
Budget 18650 cells run $3 to $5 each. Name-brand 21700 cells run $5 to $8. A 13S4P ebike pack (52 cells) costs roughly $150 to $250 in cells plus $30 to $60 for a BMS and hardware. That is $200 to $350 total, compared to $500 to $1000 for a comparable pre-built pack.
Series vs. parallel explained
Series (S) connects cells positive-to-negative. Each cell adds its voltage to the total. Capacity stays the same as a single cell. Four 3.6V cells in series gives you 14.4V at whatever the single-cell capacity is.
Parallel (P) connects cells positive-to-positive and negative-to-negative. Each cell adds its capacity to the total. Voltage stays the same as a single cell. Four 3.6V, 3Ah cells in parallel gives you 3.6V at 12Ah.
Most packs use both. A 4S4P pack of 3.6V, 3Ah cells gives you 14.4V and 12Ah. That is 16 cells total. You can learn more in our series vs. parallel guide.
Understanding capacity: Ah vs. Wh
Amp-hours (Ah) tells you how many amps the pack can deliver for one hour. A 10Ah pack can deliver 10A for one hour, or 1A for ten hours, or 20A for 30 minutes.
Watt-hours (Wh) is a better number for comparing packs. It accounts for voltage. A 36V, 10Ah pack holds 360Wh. A 12V, 30Ah pack also holds 360Wh. Same energy. Same runtime at the same load wattage. Ah alone would make the 30Ah pack look three times bigger.
Wh = Nominal voltage x Ah. The calculator shows both numbers in the pack configuration section. Read more about this in our Ah vs. Wh guide.
Safety considerations
Lithium cells store a lot of energy in a small package. That is what makes them useful. It is also what makes them dangerous when mishandled.
- Always use a BMS. No exceptions for lithium packs.
- Use nickel strip for cell connections, not solder. Soldering applies too much heat to the cell terminals.
- Match cells by capacity and internal resistance before building. Mixed cells age unevenly.
- Never charge unattended until you have confirmed the pack is working correctly.
- Store lithium cells at 40 to 60% charge if not using them for an extended period.
- Use cells from reputable sources. Counterfeit cells lack safety features and have inaccurate ratings.
Read our step-by-step 18650 build guide before your first build. It covers cell selection, spot welding, BMS wiring, and testing the finished pack.
Battery Pack Calculator FAQ
How many cells do I need for my battery pack?
Multiply your series count by your parallel count. Series cells set the voltage (each lithium cell is about 3.6 to 3.7V nominal) and parallel cells set the capacity. Enter your target voltage and capacity above and the calculator works out the series and parallel counts plus total cell count.
What is the difference between series and parallel?
Series (S) adds voltage while capacity stays the same. Parallel (P) adds capacity while voltage stays the same. A 4S1P pack of 3.7V, 3Ah cells is 14.8V and 3Ah. A 1S4P pack of the same cells is 3.7V and 12Ah. Most real packs combine both to hit a target voltage and capacity.
How do I calculate battery pack capacity in Ah and Wh?
Amp-hours (Ah) is the single-cell Ah rating times the number of parallel cells. Watt-hours (Wh), the better figure for comparing packs, is nominal pack voltage times pack Ah. A 36V pack at 15Ah is about 540Wh. The calculator reports both.
What voltage will my battery pack be?
Nominal voltage is your series count times the cell nominal voltage (about 3.6 to 3.7V for most lithium-ion). The pack charges to 4.2V per cell and should not be discharged below about 2.5 to 3.0V per cell. A 10S pack is nominally about 36V, charges to 42V, and should not drop below about 30V.
How do I choose the right BMS for my pack?
Match the BMS to your series count (a 10S pack needs a 10S BMS) and size its continuous current rating above the maximum current your load will draw, not the average. The BMS section above pulls live products matched to your exact series count and current. Our BMS Picker goes deeper on chemistry and port type.
How accurate is the runtime estimate?
Treat it as a sanity check, not a guarantee. Real runtime is lower than the ideal figure because of temperature, cell age, voltage sag under high current, and not fully draining lithium cells. Expect around 80 to 90% of the ideal figure for a healthy pack.
What is the best 18650 cell for a high-drain battery pack?
For high-drain builds, look for cells with at least 20A continuous discharge. The Samsung 25R (2500mAh, 20A) and Molicel P26A (2600mAh, 25A) are popular choices. Check our highest discharge 18650 cells list for more options.
How much does it cost to build a DIY battery pack?
Cell cost varies by brand and source. Budget about $3 to $5 per 18650 cell or $5 to $8 per 21700 cell at retail. A 13S4P pack (52 cells) costs roughly $150 to $250 in cells plus $30 to $60 for a BMS, nickel strip, and wire. Total is usually $200 to $350, compared to $500 to $1000 for a comparable pre-built pack.
Can I mix different 18650 cells in one pack?
You can mix cells of different capacities between parallel groups, but every cell within a parallel group should be the same model and close in capacity. Mixing different models within a group ages the pack unevenly. Our Pack Builder tool sorts your cells into balanced groups automatically.
What BMS size do I need for my battery pack?
Match the BMS to your series count and current. A 13S pack needs a 13S BMS. Size the current rating 20 to 30% above your max expected draw. If your motor pulls 20A peak, get at least a 25 to 30A BMS. Our BMS Picker matches your specs to products you can buy.
How do I calculate battery pack runtime?
Divide pack energy (Wh) by load power (W). A 500Wh pack running a 250W load lasts about 2 hours in theory. Real runtime is 80 to 90% of that due to voltage sag, temperature, and efficiency losses. The calculator above does this math automatically.
What is the difference between 18650 and 21700 cells?
Size and capacity. 18650 cells are 18mm x 65mm and typically 2500 to 3500mAh. 21700 cells are 21mm x 70mm and typically 3000 to 5000mAh. 21700 cells give more energy per cell but are physically larger. Use 21700 when you have the space and want fewer cells for the same capacity.
Is it safe to build your own battery pack?
It can be, if you follow proper procedures. Always use a BMS, match cells by capacity and internal resistance, use nickel strip rather than solder, and never charge unattended until you have verified the pack is working correctly. Read our lithium battery build guide before your first build.
How do I connect 18650 cells in series and parallel?
Series connections go positive-to-negative between cells or groups, adding voltage. Parallel connections go positive-to-positive and negative-to-negative within a group, adding capacity. Most packs use both. Our Pack Designer shows the exact physical layout and wiring for your configuration.
What is a good battery configuration for a portable power station?
Most portable stations use a 7S configuration (25.2V nominal). Use high-capacity cells like the Samsung 35E or LG MJ1 since portable stations prioritize runtime over discharge rate. A 7S3P pack gives about 90Wh. Good for camping, emergency backup, or charging devices off-grid.
Do I need to balance 18650 cells before building a pack?
Yes. Charge all cells to the same voltage (within 10mV) before assembly. The BMS handles balancing during use, but starting with mismatched cells puts extra strain on it from day one. A cheap cell analyzer can measure capacity and sort cells into matched groups before you build.
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