Presets cover the cells people actually build with, from the workhorse Samsung 30Q to high-drain Molicel P42A.
How do you work out a battery pack’s voltage and capacity?
Cells in series add voltage; cells in parallel add capacity and current. A 13S4P pack of 3,000 mAh cells is 13 × 3.6 V = 46.8 V nominal and 4 × 3 Ah = 12 Ah, so about 561 Wh from 52 cells. Its continuous current is the per-cell rating multiplied by the parallel count.
Series sets voltage, parallel sets capacity
Cells in series add voltage and keep capacity. Cells in parallel add capacity and current capability while keeping voltage. A 13S4P pack of 3000 mAh cells is 13 × 3.6 V = 46.8 V nominal, 4 × 3 Ah = 12 Ah, so 561 Wh from 52 cells.
The standard series counts exist because chargers do. 10S is 36 V (charging to 42.0 V), 13S is 48 V (54.6 V), 16S is 60 V (67.2 V) and 20S is 72 V (84.0 V). Build to a non-standard count and you are hunting for a charger — a real constraint that decides most pack designs before capacity does.
Current capability is a parallel-count problem
A pack can safely deliver its per-cell continuous rating multiplied by the parallel count. Four Samsung 30Q cells in parallel at 15 A each gives 60 A continuous — comfortably more than most 48 V controllers ask for. Two in parallel gives 30 A, which a 1500 W controller will push against on every hill.
Design for the controller's continuous rating with headroom, not its peak. Running cells near their limit produces heat, and heat is the fastest route to capacity fade. It also drops voltage under load, which feels like a weak scooter even when the capacity is fine. A good target is keeping continuous draw under 60% of the pack's rated maximum.
Matching a BMS to the pack
The BMS must match the series count exactly — a 13S board for a 13S pack — and its continuous current rating needs about 30% headroom over the controller’s continuous draw. A 13S 40 A BMS suits a controller pulling 25–30 A; fitting a 30 A board there will trip on every hill.
| Controller continuous | Minimum BMS | Parallel count (Samsung 30Q) |
|---|---|---|
| 15 A | 20 A | 2P |
| 25 A | 35 A | 3P |
| 40 A | 55 A | 5P |
| 60 A | 80 A | 7P |
| 100 A | 130 A | 12P |
Weight, density and the cells worth choosing
An 18650 weighs about 45–48 g; a 21700 about 66–70 g with 30–60% more capacity, so 21700s usually win on energy density — high-capacity 21700s reach 250–260 Wh/kg at cell level, while most 18650s sit around 200–230 Wh/kg. Add 15–20% for nickel strip, BMS, wiring and enclosure to get real pack weight.
Choose by what the build needs. Samsung 30Q (3000 mAh, 15 A) remains the sensible default for scooters. LG M50LT gives maximum range at modest current, and Samsung 50S pairs the same capacity with real current capability. Molicel P42A (4200 mAh, 45 A) is for performance builds that pull serious amps. LiFePO4 gives up a large share of energy density — expect a pack 1.5–2 times as heavy for the same watt-hours — in exchange for roughly four times the cycle life and much better thermal safety.
The cells below have published, independently tested datasheets and a long track record in light EV packs. Prices move constantly, so the calculator asks for your supplier’s figure rather than pretending to know it.
| Cell | Format | Capacity | Continuous | Weight | Best for |
|---|---|---|---|---|---|
| Samsung INR18650-30Q | 18650 | 3000 mAh | 15 A | 48 g | The sensible default |
| Samsung INR18650-25R | 18650 | 2500 mAh | 20 A | 45 g | More current, less range |
| LG INR18650-MJ1 | 18650 | 3500 mAh | 10 A | 48 g | Maximum range, gentle loads |
| Molicel INR21700-P42A | 21700 | 4200 mAh | 45 A | 70 g | Performance builds |
| Samsung INR21700-50S | 21700 | 5000 mAh | 25 A | 69 g | Long range with real current |
| LG INR21700-M50LT | 21700 | 5000 mAh | 14.6 A | 69 g | Range and cold tolerance |
The cell comparison guide covers the choice between formats in more depth.
Building it safely
- Spot-weld, never solder. Soldering dumps heat into the can and damages the internals near the terminal.
- Capacity-match before assembly. Voltage within 0.02 V across the batch, capacity within 2%, no dents or corrosion.
- Fuse parallel groups where you can. One shorted cell should not be able to take the pack with it.
- Insulate the positive ends properly. Fish paper on every cell, and no bare nickel near an edge.
- First charge at low current, supervised. An unbalanced pack behaves unpredictably on its first full cycle.
Then size the wiring for what the finished pack can deliver, not what the old one did — a 180 A capable pack behind 12 AWG is a fire waiting for a hill.
How the calculator works
Series count multiplies the cell voltage; parallel count multiplies cell capacity and current. Watt-hours is the product of the two. Weight adds an overhead percentage to the bare cells for strip, BMS, wiring and enclosure, and the headroom check compares your controller’s continuous draw against what the parallel groups can supply.
The formula
Vcharged = S × 4.2 · Vcutoff = S × 3.0 // 3.65 and 2.5 for LiFePO4
Icontinuous = P × Acell // parallel count sets current capability
weight = S × P × gcell × ( 1 + overhead )
Related calculators
A finished pack is only safe behind the right wiring, so run your continuous current through the wire gauge calculator. Check the design hits your energy target with the watt-hours calculator, and confirm it can feed a hard climb using the hill climb calculator.
Questions riders actually ask
What does 13S4P mean?
13 cells in series, four of those series strings in parallel — 52 cells total. The 13S sets 48 V nominal, and the 4P multiplies both capacity and current capability by four. It is the most common e-bike and mid-power scooter layout.
How many watt-hours will my pack have?
Series count × nominal cell voltage × parallel count × cell capacity in Ah. For 16S5P of 21700 cells at 4000 mAh: 16 × 3.6 × 5 × 4 = 1152 Wh. That is a genuine long-range scooter pack, and about 5.6 kg of cells alone.
Can I mix cell types or old and new cells in one pack?
No. Mismatched capacity means the weakest group hits cutoff first and limits the whole pack, and mismatched internal resistance makes some groups run hotter than others. Build with cells of the same model, ideally the same batch, capacity-matched before assembly.
Why does a 48 V pack charge to 54.6 V?
Because 48 V is the nominal figure — the average voltage across the discharge — not the maximum. A 13S lithium-ion pack sits at 13 × 3.6 V = 46.8 V nominal (rounded to "48 V"), charges to 13 × 4.2 V = 54.6 V and cuts out around 13 × 3.0 V = 39 V. Always buy the charger by its output voltage, not the pack's nominal label.
Is it safe to build my own pack?
It can be, with the right equipment and respect for the failure modes — the safety checklist on this page covers the essentials, from spot-welding to a supervised first charge. If any of it is unfamiliar, buy a built pack. Lithium fires are not recoverable events.
Should I use LiFePO4 for a scooter?
Only if cycle life and safety outweigh weight. LiFePO4 gives 2000–4000 cycles against 500–800, and is far more tolerant of abuse, but at 3.2 V nominal you need more cells for the same voltage and you carry roughly 1.5–2 times the weight for the same energy. It is popular in fleets and rare in private performance builds.
Sources
Primary and reference sources for the physics, standards and figures used on this page.
- BU-302: Series and parallel battery configurationsBattery University