A battery's internal resistance is made up of two factors: electronic and ionic resistance. Electronic resistance encompasses the resistivity of the actual materials that make up the individual cells such as the cell cover, can, and current collectors, as well as the welded interconnection links between cells and the battery-level elements such as wiring, FETs, fuses and sense resistors.
Ionic resistance is the resistance to current flow within the cells due to electrochemical factors such as electrolyte conductivity, ion mobility and electrode surface area. The combination of these factors makes the total effective resistance, which results in a voltage drop once the battery is placed under load.
Effective internal resistance is usually calculated by placing a fully charged battery under a low current load, typically 0.2CmA, for 10 seconds. Once this has elapsed, the current is immediately increased to a higher level, typically 1.0CmA, and this is held for 1 second. Ohms law is then used to calculate the resistance based on the difference between the two on load voltages and the two currents.
Figures one and two each show the discharge voltage of a 14.4V Lithium ion battery, from two different suppliers, consisting of eight 18650 cells in a 4-series, 2-parallel array. The load is switched between 110W x 200ms, 90W x 300ms and 40W x 1000ms, resulting in a voltage drop and a "thick" voltage trace.
Supplier A has a low effective resistance, calculated at 69mΩ, which results in a minimal voltage drop and a temperature rise of only 16.0 degrees Celsius. Supplier B has a far higher effective resistance, calculated at 209mΩ, which results in a severe voltage drop and 37.6 degrees Celsius temperature rise.