Monitoring discharge of multiple batteries

I'm building a device that is supposed to discharge a couple of (aiming for 16) lithium batteries and monitor their voltage individually using a mega 2560.

The way I've connected it is that the batteries are shorted through a power resistor and a relay that is controlled by the arduino. so when a fresh battery is inserted the arduino flips the relay on and monitor the battery discharge until it hits 3V and then flips it off and clock the total discharge time. So far everything kinda works except for the fact that each battery I add to the monitoring process it reads a higher input value of roughly +0,1V on that analog input.

For for example if I'm already monitoring a battery and the voltage is 3.92V it will read 4.02 when I start to monitor another battery in parallel on another analog input.

any ideas why and how to fix this?