Why do the internal resistance sometimes drop in the end of the profile?

Battery profiler discharges the battery with a high discharge pulse and a low discharge pulse. When the switch from high to low happens, the voltage (VH) and current (IH) is measured . Then when the switch back from low to high happens, the voltage (VL) and current (IL) is measured. The internal resistance is then calculated by Ohms law Rint = (VL-VH)/(IL-IH) The currents are negative. When the battery is getting to the end of the profiling, the voltage during the low discharge pulse will drop more and more. If the high discharge pulse is very short compared to the low discharge pulse, then the voltage drop in the end of the battery discharge might drop more during the low pulse and thus, the calculation of internal resistance will look like the internal resistance is getting less and less and can even be zero or negative.

To avoid this do the following:

• Have a shorter low discharge pulse
• Have a higher difference between high and low current