Battery Calculators & Tools
Three calculators and a finder that answer the four questions every battery decision starts with — and each one carries the honest adjustments that make the number worth trusting.
Which tool answers which question?
| Question | Tool | What it adjusts for |
|---|---|---|
| How long will this battery run this load? | Runtime calculator | Usable depth of discharge, Peukert rate effect |
| How much battery capacity do I need? | Bank sizing calculator | Depth of discharge, autonomy days, system voltage |
| How long until the battery is charged? | Charging time calculator | The absorption tail, charger sizing |
| Which battery group fits this vehicle or case? | Battery finder | Group code, case size, voltage — with the CCA standard |
The assumptions behind the tools
All three calculators assume the nameplate capacity at its rated rate (C20 for lead-acid) and room temperature. Three real-world conditions bend the results: heavy loads deliver less than the nameplate (the Peukert effect), cold reduces available capacity, and an aging battery delivers less than its label. Each calculator's page states where its estimate stops being reliable — the honesty is part of the tool.
The methods behind the tools
- How to calculate battery runtime — the full method with worked examples
- How much battery capacity do I need? — sizing in depth
- How long does it take to charge a battery? — the charging-time method
Our Read
My read: The best calculator is the one that tells you when it is wrong — and the battery arithmetic is honest only when its three adjustments (depth, rate, temperature) are visible on the same screen as the answer.
Why: A tool that hides its assumptions produces confident errors; a tool that shows them produces decisions. These calculators exist to be the second kind — the number, the method, and the boundary, all on one page.
My editorial view, not a purchasing guarantee.
Related: lead-acid knowledge · lithium knowledge · testing methods · Last reviewed: 2026-09-17