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Battery Size Calculator

Find the battery capacity needed for a given load and runtime.

Load current (A)

Runtime

Usable fraction

Calculated Output

Required capacity (Ah) 12.5

Formula: capacity = current * hours / usable fraction

Step-by-Step Breakdown

1. Values used:

2. Load current = 2 A

3. Runtime = 5

4. Usable fraction = 0.8

6. Load current x Runtime = 2 x 5 = 10

7. Required capacity = (Load current x Runtime) / Usable fraction = 10 / 0.8 = 12.5Ah

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The math behind it

capacity = current x runtime / usable fraction. Don't fully discharge - 80% usable is common.

2 A for 5 hours at 80% usable needs a 12.5 Ah battery.

Why not use 100%?

Deep discharge shortens battery life, so leave headroom.