[ 18650 & 21700 Battery Pack Calculator ]

Two numbers decide what a pack is. Series (S) stacks cells to add their
voltage; parallel (P) joins cells to add their capacity and their current. A
4S3P pack is four cells in series, three of those in parallel, twelve cells
total. Pick those two counts and the cell, and everything else follows.

This calculator takes a cell format (18650, 21700, 26650 or 32700), a
chemistry, and your S and P, and gives back the pack voltage, capacity, energy,
the most current the cells can deliver, a charge-current estimate, the weight in
cells, and an optional runtime at a load you type in. It all runs in your
browser; nothing is sent anywhere.

It sizes the cells. It does not pick your BMS, your charger, or your wire for
you, but it gives you the numbers those choices hang off. When you are ready to
actually build and wire one, the Daly BMS guide and the 5S BMS guide cover that
end, and the safety notes below cover the parts that start fires.

Start from a cell format and it fills in typical numbers; edit any of them for
the exact cell in your hand (the cell's datasheet, or lygte-info.dk, has real
figures). Leave Load at 0 if you only want the pack ratings.

The two counts do different jobs:

  series (S)     cells stacked + to −, which adds their voltages. S sets
                 the pack voltage. A 4S Li-ion pack is ~14.8V nominal.
  parallel (P)   cells joined + to + and − to −, which adds their
                 capacity and their current. P sets runtime and how hard
                 you can pull.

Everything the calculator shows falls out of those two and the cell you picked:

  pack voltage       = S × cell voltage     (full / nominal / empty)
  capacity (Ah)      = P × cell mAh / 1000
  energy (Wh)        = nominal volts × capacity
  max continuous A   = P × the cell's max continuous current
  charge current     = capacity × charge-rate C
  usable energy      = energy × usable depth %

A few notes on the outputs:

  — Empty (pack floor) is where the BMS should cut off, not zero volts.
    Running a cell below it is what kills it.
  — Usable energy trims the full figure to the depth you actually cycle.
    Draining to 100% every time shortens life, so 80% is a sane default for
    planning runtime.
  — Max continuous is what the cells can give. Your BMS and wiring have to
    be rated for your real peak draw, which is usually far lower.
  — Weight is cells only. Add roughly 10–20% for nickel, wiring, the BMS,
    and the enclosure.

The number is just the size in millimetres: an 18650 is 18mm across and 65mm
long, a 21700 is 21 × 70, and so on. Bigger can means more capacity and
usually more current, at more weight and volume.

  format   size mm    typical cap    typical max    rough weight
  18650    18 x 65    2500–3500      8–30 A         45–48 g
  21700    21 x 70    4000–5000      10–35 A        68–70 g
  26650    26 x 65    4000–5000      20–50 A        85–95 g
  32700    32 x 70    5000–6000      ~10 A           140–150 g

A real 18650 tops out near 3500mAh; a Samsung INR18650-35E (3500mAh, 8A)
is about as much as the chemistry gives in that size. Anything on the wrapper
claiming 9900mAh is a re-wrapped 800mAh cell with a sticker. lygte-info.dk has
independent discharge curves for most cells worth buying, and plenty that are
not.

26650 and 32700 are most common as LiFePO4 (3.2V nominal), the chemistry
you want for a flat discharge curve and a long cycle life. Switch the Chemistry
selector and the pack voltages change to match: LiFePO4 is 3.65V full and 2.5V
empty per cell, against 4.2V and 3.0V for Li-ion.

A shorted lithium cell dumps tens of amps into whatever is across it and gets
hot enough to start a fire. The arithmetic above is the easy part; the build is
where people get hurt. The short version:

  — Match cells in a parallel group. Same model, same capacity, same
    voltage before you join them. Mismatched cells in parallel surge current
    into each other the instant you connect them.
  — Use a BMS that matches the series count exactly, rated above your real
    peak current. It gives per-cell cutoff, over-current, and balancing. It is
    not a fuse.
  — Fuse the pack output on the positive leg, as close to the pack as it
    physically goes, sized just above your real peak draw. The wire between the
    cell and the fuse is the only unprotected copper in the build.
  — Charge with a CC/CV charger set to the full-charge voltage the
    calculator shows (16.8V for a 4S Li-ion pack, for example). Never a
    nominal-voltage supply, never a lead-acid or wrong-chemistry charger.
  — Never charge lithium-ion below freezing. It plates metallic lithium
    inside the cell, which is permanent and dangerous. That is what the BMS
    temperature probe is for.

The wiring, the balance-tap order, and the meter checks that catch a bad build
live in the BMS guides, not here. Read one before you pick up a soldering iron:
the Daly BMS guide (12V / 3S) and the 5S BMS guide (20V, tool-battery
voltage) walk a pack from loose cells to a sealed enclosure.

18650 or 21700 for a new pack?
21700 if the space and weight allow it. It holds roughly 50% more energy than an
18650 and usually gives more current, so you reach the same pack with fewer
cells, fewer welds, and fewer balance taps. 18650 still wins for tight or
odd-shaped enclosures, or when you are reusing cells you already have. The math
is identical either way.

Can I mix different cells or brands in one pack?
No. A series pack is only as good as its weakest cell, and cells in a parallel
group must match closely or they fight each other. Same brand, model, capacity,
and roughly the same age across the whole pack. Capacity-test and charge every
cell to the same voltage before you join them.

What charge rate should I use?
0.5C is the comfortable default: a 9Ah pack charges at about 4.5A and fills in
roughly two hours plus a CV taper. Faster works up to the cell's rated charge
current but runs warmer and shortens life. Above 1C is hard on most
cells. Always use a CC/CV charger set to the pack's full-charge voltage.

How many cells for a target energy or runtime?
Work backwards from watt-hours. Energy is nominal voltage times capacity in
amp-hours, so a 100W load for 3 hours needs about 300Wh. Pick S for the
voltage you want, then add parallel cells until the usable-energy row clears
your target.

Series or parallel first when I assemble it?
Parallel first. Charge every cell to the same voltage, build the parallel
groups, let them settle, then wire the groups in series. A BMS then sees each
group as one cell. Building series first and paralleling afterwards risks a
large surge between mismatched groups.