Battery characteristics

The maximum current that a battery (R_bat) can deliver depends on its internal resistance.

The internal resistance of a lead-acid battery decreases with an increase in the number and size of plates, a reduction in the distance between plates, an increase in the porosity of separators, a higher electrolyte density, and a reduction in the size of lead sulfate crystals in the active mass of the plates. The battery's internal resistance also depends on its capacity, chemical properties, age, temperature, and discharge current.

Measuring a battery’s internal resistance, along with testing current under a calibrated load, is a primary method of assessing its condition. The latter method uses portable equivalent source resistance meters, which are also employed for measuring the internal resistance of capacitors. Equivalent meters do not provide an exact value of the battery’s internal resistance but serve for condition assessment. Some chargers have a built-in equivalent internal resistance meter.

The voltage at the terminals of a non-rechargeable battery connected to a load decreases over time. This reduction is mainly associated with an increase in internal resistance rather than a drop in the equivalent source voltage. In rechargeable lithium-polymer batteries, the internal resistance is practically independent of the state of charge but increases with battery age. Thus, the internal resistance of lithium-ion batteries is an indicator of the expected battery lifespan.

Battery capacity