The short answer

Battery packs are built by connecting cells in series (S) and parallel (P). Series adds voltage (3.6–3.7V per NMC cell), parallel adds capacity at the same voltage, and the C-rate tells you how much current a pack can deliver or accept relative to its capacity. A 13S4P pack of 21700 cells is 13 cells in series (≈48V) with four parallel strings (×4 capacity). This article explains the math behind any pack spec you will ever receive.

Series (S): adds voltage

Connect the positive of one cell to the negative of the next, and voltages add while capacity stays the same. Ten 3.6V cells in series give 36V at the same Ah as one cell. The series count is written as a number followed by S: 3S, 7S, 13S, 20S and so on.

Because voltage must match the device’s controller, the series count is usually decided first. A 48V e-bike pack is 13S (13 × 3.7V ≈ 48V); a 72V e-moto pack is 20S. See our voltage selection guide for the application-by-application breakdown.

Parallel (P): adds capacity and current

Connect like poles together (positive to positive, negative to negative) and capacity adds at the same voltage. Two 5Ah cells in parallel give 10Ah. The parallel count follows the P: 1P, 2P, 3P, 4P…

Parallel groups also share current. A pack that must deliver 60A with cells rated 15A each needs four parallel strings (4P) so each cell carries 15A — which is why high-power packs are 13S4P, 20S3P and similar.

Putting them together: 13S4P as an example

Take a 13S4P pack of 21700 cells rated 3.6V / 4.8Ah / 15A each:

  • Voltage: 13 × 3.6V = 46.8V nominal (call it 48V)
  • Capacity: 4 × 4.8Ah = 19.2Ah
  • Energy: 46.8V × 19.2Ah = 898Wh
  • Max continuous current: 4 × 15A = 60A

Swap in 18650 cells (say 2.5Ah / 10A each) and the same 13S4P shape gives 10Ah and 40A — the physical pack is similar but energy and power are much lower. This is why the cell choice is as important as the configuration. Compare cell formats in our 18650 vs 21700 vs prismatic guide.

C-rate: current relative to capacity

C-rate expresses current as a multiple of capacity. A 10Ah pack discharging at 1C delivers 10A; at 2C it delivers 20A (and empties in 30 minutes); at 0.5C it delivers 5A (two hours). Every cell and pack has a max continuous C-rate and a peak C-rate (usually seconds-long).

The practical rule: size the pack so its continuous rating covers the motor’s peak draw with margin, or the BMS will cut out exactly when the device works hardest. Our maximum discharge current article goes deeper into C-rate and its effect on heat and lifespan.

Reading a pack spec sheet

A competent spec sheet tells you the cell brand and model, the S/P layout, nominal and full-charge voltage, capacity in Ah, energy in Wh, max continuous and peak current, charge current, BMS protections and dimensions. If any of these numbers is missing, ask — the missing one is usually the one that would have exposed a design problem.

Common sizing mistakes

  • Comparing packs by Ah at different voltages (always compare Wh).
  • Forgetting that LiFePO4 has a different cell voltage (3.2V), so “48V” needs 15–16S there — see Li-ion vs LiFePO4.
  • Ignoring peak current: a pack that meets continuous current can still trip on a 3-second acceleration spike.
  • Choosing a BMS rated below the pack’s own current capability, or above it — both waste money or safety.

Frequently asked questions

What does 2S2P mean?

Two cells in series, in two parallel strings: 7.2V at twice the single-cell capacity.

Does parallel increase voltage?

No — parallel keeps the same voltage and adds capacity/current. Only series adds voltage.

How many cells do I need for a 48V 20Ah pack with 21700 cells?

13S for voltage. With 4.8Ah cells, 20Ah needs 4P (19.2Ah) or 5P (24Ah) — so roughly 52–65 cells depending on the exact cell capacity and target.

Is a 1C pack suitable for a high-power e-bike?

Not usually — a 750W motor at 48V peaks well above 15A, which a 15Ah pack at 1C (15A) cannot sustain. Choose the C-rate from the real current profile, not from the label.

Let us size your pack

Share your motor power, runtime target and space via the custom battery inquiry form — we will return the S/P configuration, cell choice, BMS rating and the Wh math behind it.

Why balancing matters in series strings

Cells in a series string do not age identically, and small differences in self-discharge or temperature drift the state of charge between cells over time. That is why every series pack carries a BMS with balancing: passive balancing burns off excess energy from the highest cells as heat (simple, cheap, standard in most packs), while active balancing shuttles energy between cells (more efficient, found in high-capacity and long-cycle packs). Without balancing, the string charges only until the first cell hits its ceiling — leaving the rest under-charged — and discharges only until the first cell hits its floor, so usable capacity shrinks as the cells drift apart.

Parallel groups: matching is everything

Within a parallel group, cells share the load and the charge current, so they should be the same model, grade and age — ideally from the same production batch. Cells are matched at the factory by capacity and internal resistance; a mismatched cell in a parallel group is dragged up and down in voltage by its neighbours and fails early. The connection method matters too: nickel strip spot-welding is the standard for cylindrical cells, with strip cross-section sized for the group current, and high-current packs may use busbars or wire bonding. A 13S4P pack with 60A of continuous rating is a 60A design decision taken four cells at a time — the group current divides roughly equally only if the cells start identical.

The same matching logic applies to the charger and the BMS as the pack’s S/P configuration grows. A higher P count raises the pack’s current capability, so the BMS current rating and the charge current should scale with it — charging a 4P pack with a 1P pack’s charger is safe but slow, while discharging beyond the BMS rating trips protection at the worst moment. Write the configuration, the cell model and the BMS rating on the pack spec sheet together; a pack defined only by “48V 20Ah” is half-specified.