Understanding NiMH Theoretical Capacity
A nickel metal hydride cell stores charge through two active electrodes. The positive electrode uses nickel hydroxide. The negative electrode uses a hydrogen absorbing alloy. The ideal capacity comes from electrochemistry, not from a label. This calculator estimates that ideal value first. Then it applies practical factors that reduce usable output.
Why the Limiting Electrode Matters
A cell can only deliver charge until one electrode is exhausted. Extra material on the other side does not add capacity. It only improves balance, aging margin, or safety. The calculator compares the positive electrode capacity with the negative electrode capacity. The smaller value becomes the limiting cell capacity.
Positive Electrode Method
Nickel hydroxide capacity is calculated from Faraday’s law. One mole transfers one mole of electrons during the main reaction. The molar mass, active mass, purity, and utilization define the available milliamp hours. You can edit these values for custom paste blends.
Negative Electrode Method
The metal hydride electrode is often easier to model with specific capacity. Alloy grades may differ. AB5 and AB2 materials may not share the same rating. Enter the tested or supplier stated value in milliamp hours per gram. The calculator adjusts it for purity and utilization.
Practical Pack Output
Real cells lose capacity through internal resistance, temperature, aging, separator limits, and discharge cutoff. The discharge efficiency, temperature derating, and aging derating fields reduce the theoretical limit. A safety reserve can also be removed. This helps estimate usable pack capacity.
Runtime and Energy
Pack energy depends on usable amp hours and nominal voltage. Series cells raise voltage. Parallel strings raise capacity. Runtime divides usable amp hours by the load current. The result is only an estimate. Pulsed loads, high current, poor cooling, and old cells can reduce runtime.
Design Use
Use this tool during early cell design, repair checks, or pack planning. It helps compare electrode balance before building a pack. It also shows whether a target runtime is realistic. Always verify the final design with measured discharge tests under the actual load.
Limits and Checks
This model assumes uniform paste, clean tabs, and stable voltage. Treat results as planning numbers. Keep charge rates conservative. Review heat, venting, and manufacturer limits before final assembly work.