Do I need a BMS for a custom battery pack?

Yes, always. A BMS (Battery Management System) monitors individual cell voltages, prevents overcharge and over-discharge, balances cells during charging, and protects against short circuits and overcurrent. Running lithium cells without a BMS risks thermal runaway (fire) if any cell is overcharged or over-discharged. Even lead-acid packs benefit from monitoring. A BMS sized for a 4S LiFePO4 pack is one of the cheapest parts of the build and the one part you cannot leave out. Price it at the same shop as your cells, and match its continuous current rating to the highest load your pack will see.

What happens if cells in a pack are mismatched?

Mismatched cells (different capacity, internal resistance, or age) create imbalanced packs. During discharge, the weakest cell hits empty first and gets reverse-charged by the other cells. This damages it permanently. During charging, the strongest cell reaches full first and gets overcharged while the others catch up. Both scenarios reduce pack life and create safety hazards. Always buy cells from the same batch and test them before assembly. Capacity-test and measure the internal resistance of every cell, then group the closest readings together and set aside any that sit apart from the rest.

How do I calculate the total energy of a custom battery pack?

Total energy (Wh) = Pack voltage x Pack capacity in Ah. A 4S LiFePO4 pack (12.8V) with 280Ah cells = 12.8 x 280 = 3,584Wh (3.58kWh). For a 4S2P configuration with 100Ah cells: voltage is still 12.8V, capacity is 200Ah, energy is 12.8 x 200 = 2,560Wh. Total cell count = series count x parallel count. Total cost = cell count x price per cell, plus BMS, busbars, fuses, and wiring. Once your pack is assembled, estimate its real-world runtime with our LiFePO4 runtime calculator using the final pack specs.

Is building a battery pack cheaper than buying a pre-built battery?

Often yes, and the gap widens as the pack gets bigger. A DIY 12V 280Ah LiFePO4 pack is four EVE LF280K cells, a BMS, busbars, and hardware; the cells are most of the cost, and you are not paying for a case, a warranty or an assembly line. Price four cells plus a BMS against a finished 12V battery of the same capacity on the day you buy, because cell prices move faster than retail prices do. The catch is real: DIY needs electrical-safety knowledge, compression hardware or a spot welder, and time. An incorrectly assembled pack is a fire hazard.

Battery Pack Calculator - Series and Parallel

6 min read

Building a custom battery pack from individual cells lets you match exact voltage and capacity requirements that off-the-shelf batteries cannot. This calculator tells you how many cells you need and how to arrange them: series strings for voltage, parallel groups for capacity. It is the starting point for DIY powerwall builds, e-bike batteries, and custom LiFePO4 packs.

Battery pack wiring concepts showing series, parallel, and mixed configuration options.

Series vs Parallel: How to Configure Your Pack

Series for voltage. Cells wired in series add their voltages. Four 3.2V LiFePO4 cells in series = 12.8V. Eight in series = 25.6V. The capacity stays the same as a single cell.
Parallel for capacity. Cells wired in parallel add their amp-hours. Two 100Ah cells in parallel = 200Ah at the same voltage. Parallel is how you increase runtime without changing voltage.
Combine both. A "4S2P" configuration means 4 cells in series (for voltage) with 2 parallel groups (for capacity). Eight 3.2V 100Ah cells in a 4S2P layout give 12.8V at 200Ah: four cells per group, two groups. The total cell count is 4 x 2 = 8 cells.
Both counts round up. A pack cannot hold part of a cell, so the calculator rounds each count up to the next whole cell. That is the only choice that meets or exceeds both targets: ask for 100Ah from 105Ah cells and you get 1 string at 105Ah, not 0.95 of one. The result panel prints the voltage and capacity you actually get beside the ones you asked for.
12V, 24V, 36V and 48V are class names, not voltages. Makers label packs by class and build them at the nominal figure: Victron builds its "12V" LiFePO4 battery from 4 cells at 12.8V and its "24V" from 8 at 25.6V, LiTech Power sells its 36V packs as 38.4V 12S, and Gobel Power's "48V" server rack is 16S at 51.2V. Enter the nominal figure. Type 48V against a 3.2V cell and the calculator correctly returns 15 cells, because 48 divided by 3.2 is exactly 15 and nothing needs rounding up.
Always match cells. Every cell in a pack must be the same chemistry, voltage, capacity, and ideally the same manufacturing batch. Mismatched cells create weak links that reduce pack performance and can be dangerous. Use a BMS (battery management system) to monitor and balance individual cell voltages. If you are building a pack to power a motor, our HP to watts converter gives its rated output in watts, but the pack must deliver the input power the motor draws.

Series vs parallel battery wiring showing voltage addition and capacity addition paths. — Series wiring adds voltage (4 cells × 3.6V = 14.4V) while parallel wiring adds capacity. Choose based on your target voltage and amp-hour needs.

Common Battery Cells for DIY Packs

These are the cells DIY builders actually buy, with the figures each maker prints on its own datasheet. Nominal voltage sets how many you need in series; cell capacity sets how many strings in parallel.

Cell	Chemistry	Nominal V	Capacity	Typical Use
EVE LF280K	LiFePO4	3.2V	280Ah	DIY powerwall, solar storage
EVE LF105	LiFePO4	3.2V	105Ah	RV, marine, off-grid
Headway 38120S	LiFePO4	3.2V	10Ah	Small solar, RC vehicles
Samsung 30Q	Li-ion (NMC)	3.6V	3Ah	E-bike, power tools, EV
Samsung 40T	Li-ion (NMC)	3.6V	4Ah	High-drain e-bike, EV
Murata VTC6 (ex-Sony)	Li-ion (NMC)	3.6V	3.12Ah	E-bike, portable packs

Every figure in the table above is the one its maker prints: EVE for the LF280K and LF105, Headway for the 38120S, Samsung SDI for the 30Q and 40T, and Murata for the US18650VTC6, which took over Sony's cylindrical cell business and rates it at 3.6V, not the 3.7V it is often sold as. Prismatic LiFePO4 cells suit stationary and large mobile builds because of the capacity in one can. Cylindrical 18650 and 21700 cells suit e-bikes and portable packs where weight and shape matter more.

Worked Examples

Building a 51.2V LiFePO4 Battery from Prismatic Cells

Context

You want a "48V" LiFePO4 pack for a home solar system. Makers build that class at 51.2V nominal, 16 cells in series. You have EVE LF105 prismatic cells (3.2V, 105Ah each) and need at least 200Ah. How many cells, and how are they wired?

Calculation

Series cells for voltage: 51.2V / 3.2V = 16 cells in series

Parallel strings for capacity: 200Ah / 105Ah = 1.9, rounded up to 2 parallel strings

Total cells: 16 x 2 = 32 cells

Capacity you actually get: 2 x 105Ah = 210Ah, and 51.2V x 210Ah = 10,752Wh (10.8kWh)

Interpretation

32 cells arranged in 16S2P (16 series, 2 parallel), giving 51.2V at 210Ah. EVE prints 1,980g per LF105 cell, so the cells alone weigh 63kg (140 lb) before the BMS, busbars and enclosure. The 10Ah of headroom over your 200Ah target is the round-up: two strings is the smallest whole number that reaches 200Ah, and there is no half-cell option.

Takeaway

After building the pack, size the solar array to recharge it daily with our solar battery charge time calculator. Enter 210Ah at 48V with your panel wattage.

Designing a 7S Li-ion E-Bike Pack from 18650 Cells

Context

You want a 25.2V nominal (7S) e-bike battery with 15Ah capacity using Samsung 30Q cells (3.6V nominal, 3Ah each).

Calculation

Series: 7 cells (7 x 3.6V = 25.2V nominal)

Parallel: 15Ah / 3Ah = 5 cells in parallel per series group

Total: 7 x 5 = 35 cells in 7S5P configuration

Energy: 25.2V x 15Ah = 378 Wh

Interpretation

35 cells in a 7S5P pack giving 378 Wh, with both divisions landing whole so nothing rounds up. How far that goes depends on rider weight, terrain and assist level, which swing it more than the pack size does. Put your own figures into the e-bike battery range calculator for a range you can plan around.

Takeaway

To convert this pack's capacity between Wh and Ah at different voltages, use our Wh to Ah calculator.

Frequently Asked Questions

Glossary

Series Configuration

Connecting cells positive-to-negative in a chain. Each cell adds its voltage while amp-hours stay the same. Four 3.2V 100Ah cells in series = 12.8V 100Ah. Series connections set the pack voltage.

Parallel Configuration

Connecting cells positive-to-positive and negative-to-negative. Each group adds its amp-hours while voltage stays the same. Two 3.2V 100Ah cells in parallel = 3.2V 200Ah. Parallel connections set the pack capacity.

Cell Balancing

The process of equalizing voltage across all series cells in a pack. Without balancing, individual cells drift apart over charge cycles, causing premature capacity loss and potential safety issues. A BMS handles this automatically.

Integrating your custom pack into a solar system? The solar sizing calculator matches panel output to your pack capacity.

A custom battery pack gives you exact specs, often at a lower cost, if you build it correctly. Start with matched cells from the same batch, use a proper BMS, fuse every connection point, and never skip cell-level testing before assembly. The calculator above gives you the cell count and configuration. The rest is careful execution. If you are building a pack for an e-bike, our e-bike battery range guide explains how pack size translates to real-world miles. For electric scooter packs, the scooter range calculator shows how your pack specs translate to miles. For drone-specific builds, the drone flight time guide covers LiPo safety and C-rating requirements.

More Battery calculators

Browse all battery calculators: LiFePO4, lead-acid, AGM, and lithium-ion runtime, charge time, and capacity sizing.

Last updated: October 27, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, wind power, and EV ownership/driving, all from building, maintaining, and operating his own systems.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Damir Topić, Assistant Professor, FERIT Osijek.