| Word | Description |
| I | current |
| IEC | International ElectroChemical Commission |
| Immobilized Electrolyte | An electrolyte made motionless by use of a gel additive or AGM separator. See also Gel-Cell Battery and Recombinant. |
| Impedance (Z) | The total opposition that a battery offers to the flow of alternating current. Impedance is a combination of resistance and reactance. |
| Initial Drain | The current that a cell or battery supplies when first placed on load. Also referred to as starting drain. |
| Insulator | A material — such as rubber, some plastics and glass — that is highly resistant to conducting electricity. |
| Internal Resistance (Ri) | The opposition to direct current flow within a battery, which causes a drop in closed-circuit voltage proportional to the battery's discharge rate. |
| International ElectroChemical Commission (IEC) | A worldwide organization that establishes standards in the electrical and electronic fields. |