Battery discharge test is the ohmic measurement which is one of the oldest and most reliable test methods. The battery receives a brief discharge for a second or longer. The load current for a small battery is 1A or less; for a starter battery it might be 50A or more. A voltmeter measures the open circuit voltage (OCV) with no load, followed by the second reading with a load; Ohm’s law calculates the resistance value (voltage difference divided by current equals resistance).
Battery discharge test is also called DC load measurements work, well to check large stationary batteries, and the ohmic readings of the device are very accurate and repeatable. High-end test instruments claim resistance readings in the 10 micro-ohm range. Many garages use the carbon pile to measure starter batteries and an experienced mechanic gets a reasonably good assessment of the battery.
The discharge method has limitations in that it blends R1 and R2 of the Randles model into one combined resistor and ignores the capacitor. “C” is an important component of a battery that represents 1.5 farads per 100Ah capacity. In essence, the DC method sees the battery as a resistor and can only provide ohmic references. In addition, the DC load method gets similar readings from a good battery that is partially charged and a marginal battery that is fully charged. State-of-charge and capacity estimations are not possible.
The two-tier DC load method offers an alternative method by applying two sequential discharge loads of different currents and time durations. The battery first discharges at a low current for 10 seconds, followed by a higher current for 3 seconds; the Ohm’s law calculates the resistance values. Evaluating the voltage signature under the two load conditions offers additional information about the battery, but the values are strictly resistive and do not reveal SoC or capacity estimations. The load test is the preferred method for batteries that power DC loads.