I have seen them switch about 50mV apart which is probably enough for almost any amateur application. I use them to balance two 12v batteries (for a large UPS). They are connected in series and one is slightly weaker than the other, causing an increase in voltage in one of them. One of these modules is connected to each battery and configured to close a relay if the voltage gets too high. The small resistance is the burden. The batteries are now almost perfectly balanced.