How to use this battery pack calculator
- Chemistry, pick the cell type: Li-ion (NMC/NCA), Li-Po, LiFePO4, sodium-ion or NiMH. This sets the per-cell nominal, full and empty voltages.
- Cell format, choose 18650, 21700, 26650, 32650 or AA to auto-fill a typical capacity and weight, then correct both to match your actual cell's datasheet.
- Series (S), how many cells are chained to add voltage.
- Parallel (P), how many series strings sit side by side to add capacity.
- Depth of discharge, how much of the pack you actually use per cycle: 80-90% is typical for lithium.
- Converter efficiency, the loss in your boost/buck converter or BMS output stage, usually 85-92%.
- Discharge rate (C), your cell's continuous rating, to see the pack's max amps and watts.
- Your load, optionally enter a wattage to get an estimated runtime.
Every result updates live as you type.
Series adds voltage, parallel adds capacity
The whole trick of pack design is two multiplications. Wiring cells in series stacks their voltages: three Li-ion cells at 3.6 V nominal make a 3S pack of about 10.8 V. Wiring strings inparallel stacks their capacity: two parallel strings of 3,000 mAh cells give 6,000 mAh. A3S2P pack therefore uses six cells for roughly 10.8 V and 6,000 mAh. Energy in watt-hours ties the two together: Wh = nominal volts × amp-hours, about 65 Wh for that example. Wh is the number that matters when comparing packs of different voltages, and it is also the number airlines regulate.
The power-bank mAh number, and the real one
Commercial power banks print their capacity in mAh at the 3.7 V of the cells inside, not at the 5 V coming out of the USB port. Stepping 3.7 V up to 5 V costs energy in the converter as well, typically 10-15%. That is why this calculator shows both figures: the marketing-style rating at 3.7 V, and thedeliverable mAh at 5 V after efficiency, which is what your phone actually receives. If you have ever wondered why a 20,000 mAh power bank charges a 5,000 mAh phone about three times rather than four, this gap is the answer.
Choosing a chemistry
Li-ion NMC/NCA (most 18650/21700 cells) offers the best energy per gram, ideal for power banks, e-bikes and portable builds. LiFePO4 trades weight for lifespan and thermal stability, which is why it rules home solar storage: a 4S LiFePO4 pack lands at 12.8 V nominal, a near drop-in for old 12 V lead-acid systems. Sodium-ion is the emerging low-cost option with good cold-weather behaviour, and NiMH remains the safe choice for low-power devices. The calculator applies each chemistry's real per-cell voltages, so the same S count gives honestly different pack voltages.
Balancing mismatched cells (salvaged 18650s)
Cells recovered from old laptop packs or bought second-hand never test the same: three "2,000 mAh" cells might really hold 1,450, 1,720 and 1,980 mAh. The Mixed cells tab takes your real tested capacities and distributes the cells into parallel groups whose totals come out as equal as possible, largest cells placed first, each into the currently weakest group. That matters because a pack is only as strong as its weakest parallel group: the BMS cuts off when that group runs empty, no matter how much capacity the other groups still hold. The build sheet tells you exactly which numbered cell goes into which group, shows the imbalance percentage (under about 3% is a healthy pack), and flags a weak outlier cell when leaving it out would clearly improve the balance.
Sizing a pack from your requirement
The What do I need? tab works in reverse: tell it the system voltage you are targeting (a 12 V UPS, a 36 V e-bike, a USB power bank) and the load you want to run for how long, and it returns the series count that hits your voltage, the parallel count that covers your energy and current needs, the total cell count, and the practical shopping line most guides skip: which BMS to buy and what voltage your charger must output.
Related tools: size a whole backup system with theBattery Backup Calculator, plan storage with theSolar Battery Bank Calculator, or convert betweenwatts and amps.
Frequently asked questions
What does 3S2P mean for a battery pack?
The S number is how many cells are wired in series and the P number is how many of those series strings sit in parallel. Series adds voltage, parallel adds capacity. A 3S2P pack of 18650 Li-ion cells uses 6 cells: 3 in series gives about 10.8 V nominal, and 2 strings in parallel doubles the mAh of a single cell.
Why is my power bank rated in mAh at 3.7 V?
The mAh printed on a power bank is quoted at the 3.7 V nominal voltage of its lithium cells, not at the 5 V of the USB output. Converting 3.7 V energy up to 5 V, and losing a little in the converter, means a "20,000 mAh" power bank realistically delivers about 13,000 to 14,000 mAh into your phone.
How is pack energy in Wh calculated?
Watt-hours equal the pack nominal voltage times its amp-hour capacity: Wh = V x Ah. A 3S2P pack of 3,000 mAh Li-ion cells is 10.8 V x 6 Ah, about 65 Wh. Wh is the honest way to compare packs of different voltages, because mAh alone ignores voltage.
Can I take my battery pack on a plane?
Most airlines follow the IATA guidance: lithium packs up to 100 Wh are allowed in carry-on baggage, 100 to 160 Wh packs need airline approval, and anything over 160 Wh is generally not allowed on passenger aircraft. The calculator shows your pack Wh so you can check which band it falls in.
What is the difference between Li-ion and LiFePO4 for a pack?
Standard Li-ion (NMC/NCA) cells run at 3.6-3.7 V nominal and store more energy per gram, which suits power banks and portable gear. LiFePO4 runs at 3.2 V nominal, is heavier for the same energy, but survives far more charge cycles and is more thermally stable, which is why it dominates solar storage. The same S count therefore gives a different pack voltage: 4S LiFePO4 is about 12.8 V, close to a lead-acid 12 V system, while 3S Li-ion gives about 10.8 V.
Is it safe to build my own lithium pack?
Lithium cells can catch fire if over-charged, over-discharged, shorted or physically damaged, so a proper BMS (battery management system), matched cells from the same batch, and correct spot-welded connections are essential. This calculator only does the electrical arithmetic. If you are not confident in safe assembly, buy a finished pack instead.
How do I balance mismatched 18650 cells into a pack?
Test each cell's real capacity, then distribute the cells so every parallel group adds up to roughly the same total mAh: place the largest cells first, each into whichever group is currently weakest. The Mixed cells tab of this calculator does that distribution for you and prints a build sheet showing which cell goes in which group. Aim for under about 3% spread between group totals.
Why does the weakest parallel group limit the whole pack?
All parallel groups in a series pack carry the same current, so the group with the least capacity empties first. When it hits the low-voltage cutoff the BMS shuts the whole pack down, leaving unused energy stranded in the stronger groups. That is why balanced groups matter more than total capacity when building from mismatched cells.
How many cells do I need for a 12 V battery pack?
It depends on chemistry: 4 LiFePO4 cells in series give 12.8 V nominal, the standard drop-in for 12 V systems, while 3 Li-ion cells in series give 10.8 V. The parallel count then comes from your runtime: divide the watt-hours you need by the pack voltage to get amp-hours, and divide that by one cell's amp-hours. The What do I need tab does this sizing automatically and also tells you which BMS and charger voltage to buy.