Coulombic Efficiency Calculator

Coulombic efficiency is the ratio of the electric charge (capacity) a battery delivers on discharge to the charge it received on the preceding charge, expressed as a percentage: CE = Qdischarge / Qcharge x 100. It measures how much of the charge put into a cell comes back out, a value of exactly 100 percent would mean no charge is lost to side reactions.

Because capacity retention compounds multiplicatively: after N cycles at a constant Coulombic efficiency CE, retained capacity is CE^N, not CE times N. A 99.9 percent efficiency retains about 90 percent after 100 cycles but only about 61 percent after 500 cycles and about 37 percent after 1000, the small daily loss snowballs.

Irreversible side reactions consume charge without contributing to reversible capacity: solid electrolyte interphase growth, electrolyte decomposition, lithium plating, or active material loss and dissolution. Each of these consumes some of the charge put in during charging that never comes back out during discharge.

Coulombic efficiency only compares charge in coulombs or amp-hours, ignoring voltage. Energy efficiency compares the actual energy in watt-hours, which is always lower than Coulombic efficiency because discharge voltage is always somewhat below charge voltage due to internal resistance and overpotential, even when Coulombic efficiency itself is close to 100 percent.

Commercial lithium-ion cells with a stable solid electrolyte interphase typically run 99.9 to 99.99 percent Coulombic efficiency per cycle after initial formation. Research cells with new electrode materials often report first-cycle efficiency as a key metric, since low first-cycle Coulombic efficiency (from forming the interphase) signals how much irreversible capacity loss to expect before the cell settles into steady cycling.

No, it assumes a single constant Coulombic efficiency applies to every cycle, which is a simplification. Real cells often show lower efficiency during early formation cycles, a stable plateau in the middle of life, and declining efficiency again near end of life as degradation accelerates, so treat the long-cycle projection as an idealized best case at the entered efficiency.

In a single well-behaved cycle, no, discharge capacity cannot exceed charge capacity since charge cannot be created from nothing. Values reported above 100 percent in real testing usually indicate a measurement artifact, a change in test conditions between charge and discharge, or a small amount of capacity carried over from a previous cycle's incomplete discharge.

A battery cycler charges the cell to a cutoff voltage or capacity, integrates the current over time to get charge capacity, then discharges to a cutoff voltage while integrating current again to get discharge capacity. Dividing discharge capacity by charge capacity for that cycle gives the Coulombic efficiency, repeated automatically every cycle throughout a cycling test.

Yes, temperature strongly affects Coulombic efficiency. Higher temperatures generally accelerate unwanted side reactions like electrolyte decomposition and solid electrolyte interphase growth, lowering Coulombic efficiency and accelerating capacity fade, while very low temperatures can promote lithium plating during charging in lithium-ion cells, which also lowers efficiency.

Coulombic efficiency measures how much of a cycle's charge is recovered and therefore predicts cycle life and capacity fade over time. Energy density measures how much energy a cell packs per unit mass or volume in a single cycle, a completely separate property. A cell can have excellent energy density but poor Coulombic efficiency, or the reverse, they answer different design questions.