Battery Pack Calculator
Plan and size a custom 18650 (or any cell) battery pack with live cell lookup, series/parallel calculations, runtime estimates, and BMS recommendations.
Not sure what to put here?
Search by manufacturer or part number to prefill the core voltage and capacity fields. If your exact cell is missing, enter the datasheet numbers manually and continue with the planner.
This list is by no means exhaustive. If you have cells you'd like added, or find missing data for cells we do have, please email us or fill out a contact form. Cell database made possible and initially compiled by Wolf @ the SLS.com forums.
Not sure what to put here?
These values drive everything downstream. Pull them from the datasheet or your own tested cells. The C-rate will be used down below at pack level calculations, so make sure you fill this section out.
Not sure what to put here?
Set the series and parallel counts, then confirm voltage, capacity, and current before moving on. You can learn more about wiring batteries in series & parallel configurations in the context of a battery pack by visiting these posts.
If this is your first time planning out a battery pack, check out our step-by-step 18650 build guide, which walks through cell selection, welding, BMS wiring, and testing the finished pack.
Not sure what to put here?
Estimate runtime at a fixed load to get a rough sanity check before you build anything. For now this calculates battery life at a fixed load amount. For example, if you plan on running something that consumes 1000 W and your battery pack is 1 kWh you can now estimate the life of the battery at a given load in watts.
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?
Based on your current planner inputs, this recommends a BMS and shows matching products you can buy. Keep in mind that you'll also need to add a BMS to your pack, so that will add to the overall cost and weight. Check out this post about choosing a BMS for your lithium ion battery pack.
Not sure what to put here?
This estimates only cell weight and cell cost. BMS, structure, nickel, wire, and enclosure still add overhead. Keep in mind that you'll also need to add a BMS to your pack, so that will add to the overall cost and weight.
How to Use Our Battery Pack Calculator and Planner
Getting the most from this planner is straightforward. Start by selecting your cell from the database or entering specs manually — the fields auto-populate from our cell data. Understanding capacity (Ah or Wh) is key to interpreting the results correctly.
For the highest energy density, start with the highest-capacity 18650 cells we have tested; if the slightly larger 21700 format fits your enclosure, the highest-capacity 21700 cells deliver more watt-hours per cell.
The pack configuration section lets you set how many cells in series and parallel. Series cells add voltage; parallel cells add capacity. Use the quick presets as a starting point if you're not sure.
Runtime is estimated at a fixed load — real-world performance varies based on temperature, cell age, and discharge curve. Use it as a rough sanity check, not a guarantee.
The BMS section pulls live product data from AliExpress matched to your exact series count and current requirements. Always size your BMS to handle the maximum current your pack can deliver under load, not just the average draw.
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 3.6–3.7V nominal) and parallel cells set the capacity. A 36V e-bike pack is typically 10S; if you want ~15Ah from 3Ah cells you'd use 5P, so 10 × 5 = 50 cells. Enter your target voltage and capacity above and the calculator works out the series/parallel counts and total cell count for you.
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 (for example 4S4P) 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's nominal voltage (~3.6–3.7V for most lithium-ion). The pack also has a higher fully-charged voltage (4.2V per cell) and a lower empty voltage (~2.5–3.0V per cell). A 10S pack is nominally ~36V, charges to 42V, and should not be discharged 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. It assumes a fixed load and ideal conditions; real runtime is lower because of temperature, cell age, voltage sag under high current, and the fact that you should not fully drain lithium cells. Expect real-world runtime around 80–90% of the ideal figure for a healthy pack.
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