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.
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. Series is how you build higher-voltage packs: a 48V LiFePO4 pack needs 16 cells in series (16 x 3.2V = 51.2V).
- 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). Four 3.2V 100Ah cells in a 4S2P layout = 12.8V at 200Ah. The total cell count is 4 x 2 = 8 cells.
- 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.
Common Battery Cells for DIY Packs
| 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 |
| CATL 272Ah | LiFePO4 | 3.2V | 272Ah | EV repurpose, powerwall |
| 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 |
| Sony VTC6 | Li-ion (NMC) | 3.7V | 3.12Ah | E-bike, portable packs |
| Headway 38120S | LiFePO4 | 3.2V | 10Ah | Small solar, RC vehicles |
LiFePO4 prismatic cells (EVE, CATL) are popular for stationary and large mobile builds because of high capacity per cell. 18650/21700 cylindrical cells (Samsung, Sony) are used for e-bikes and portable electronics where weight and size matter more.
Frequently Asked Questions
Integrating your custom pack into a solar system? The <a href="/solar/solar-panel-and-battery-sizing-calculator">solar sizing calculator</a> matches panel output to your pack capacity.
A custom battery pack gives you exact specs 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.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining 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.