Plan a DIY battery pack from individual cells. Pick a cell type such as 18650, 21700 or LiFePO4 (or enter your own specs), set the number of cells in series and in parallel, and get the pack's nominal, full and empty voltage, capacity in Ah and Wh, maximum continuous current, weight and runtime.
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Cell specs (leave blank for the typical values above)
mAh
A
V
V
V
g
Pack and load
%
%
W
for a runtime estimate
Enter your values to see the result.
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How to use the Battery Pack Calculator
Choose a cell type. Typical specs are filled in; type your cell's datasheet values in the override boxes if they differ.
Enter the series (S) and parallel (P) counts, e.g. 13S4P for a 48 V e-bike pack.
Optionally set a load in watts and a depth of discharge to estimate runtime.
Press Calculate to see voltage, Ah, Wh, current and weight.
Check the table to see how many cells in series other common system voltages need.
Typical cell specifications
Cell
Chemistry
Nominal
Full / empty
Typical capacity
Weight
Energy density
18650
Li-ion NMC/NCA
3.6–3.7 V
4.2 / 2.5–3.0 V
2,500–3,500 mAh
45–48 g
~220–260 Wh/kg
21700
Li-ion NMC/NCA
3.6–3.7 V
4.2 / 2.5–3.0 V
4,000–5,000 mAh
66–72 g
~230–270 Wh/kg
18650 / 26650 / 32700
LiFePO4
3.2 V
3.65 / 2.5 V
1,500 / 3,300 / 6,000 mAh
40 / 85 / 145 g
~110–135 Wh/kg
Prismatic
LiFePO4
3.2 V
3.65 / 2.5 V
50–300 Ah
1–5.5 kg
~150–165 Wh/kg
Pouch
LiPo
3.7 V
4.2 / 3.0–3.3 V
varies
varies
~150–200 Wh/kg
AA
NiMH
1.2 V
~1.45 / 1.0 V
1,900–2,500 mAh
26–31 g
~80–100 Wh/kg
These are typical ranges for planning. Real cells differ by model, so use the exact datasheet values (especially the continuous discharge rating) for your final design.
Common pack layouts
System
Li-ion (3.6 V)
Full charge
LiFePO4 (3.2 V)
Full charge
12 V
3S (10.8 V)
12.6 V
4S (12.8 V)
14.6 V
24 V
7S (25.2 V)
29.4 V
8S (25.6 V)
29.2 V
36 V
10S (36 V)
42 V
12S (38.4 V)
43.8 V
48 V
13S (46.8 V)
54.6 V
16S (51.2 V)
58.4 V
52 V
14S (50.4 V)
58.8 V
16S (51.2 V)
58.4 V
72 V
20S (72 V)
84 V
24S (76.8 V)
87.6 V
Pack-building tips
Match cells: use cells of the same model, age and capacity in every parallel group so the groups stay balanced.
Size by current, not just energy: if your motor or inverter needs 30 A and each cell is rated 10 A, you need at least 3P even if fewer cells would hold enough Wh.
BMS matches S: a 13S pack needs a 13S BMS, and the charger voltage must equal S × full-charge voltage (54.6 V for 13S Li-ion).
Plan for real-world capacity: high current, cold weather and ageing all reduce usable capacity; the depth-of-discharge setting and a 10–20% margin help keep the estimate realistic.
Frequently asked questions
How do series and parallel cells change a battery pack?
Cells in series add their voltages while the capacity stays that of one cell; cells in parallel add their capacities (Ah) and current capability while the voltage stays that of one cell. A 13S4P pack of 3.6 V, 3,000 mAh cells is 13 × 3.6 = 46.8 V and 4 × 3 = 12 Ah.
How do I calculate battery pack Wh?
Multiply the nominal pack voltage by the capacity in amp-hours: Wh = V × Ah. It is the same as the number of cells times the energy of one cell. A 13S4P pack of 3.6 V, 3 Ah cells holds 46.8 V × 12 Ah ≈ 562 Wh.
What does 13S4P mean?
13S4P describes a pack with 13 cell groups connected in series, each group made of 4 cells in parallel, for 52 cells in total. With lithium-ion cells, 10S is the usual 36 V nominal layout and 13S is the usual 48 V nominal layout (54.6 V when full).
What voltage is a full 18650 or LiFePO4 cell?
Typical lithium-ion 18650 and 21700 cells are 3.6–3.7 V nominal, 4.2 V when fully charged and are usually cut off around 2.5–3.0 V. LiFePO4 cells are 3.2 V nominal, about 3.65 V full and 2.5 V empty. Always use your cell's datasheet values for the charger and BMS settings.
How much current can my battery pack deliver?
Roughly the continuous discharge rating of one cell times the number of cells in parallel. Ten-amp cells in a 4P pack give about 40 A. Running cells at their limit shortens their life and increases heat, so many builders design for 50–70% of the rating and choose a BMS rated at least for the expected current.
How accurate is the pack weight estimate?
Cell weight is exact if the cell weight is right; the rest of the pack (BMS, nickel strip, wiring, insulation, case) varies. The default 15% overhead suits a simple shrink-wrapped pack; hard cases and large connectors can add 25–40%.
Do I need a BMS for a lithium battery pack?
Yes. Multi-cell lithium packs need a battery management system (or at least a protection board) that matches the series count, to prevent over-charge, over-discharge and over-current and to balance the cell groups. Only build packs if you know how to handle lithium cells safely; matched, genuine cells and correct fusing matter.
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