Sure, he's been saving on gas money for 3+ years, driving around on inexpensive electricity :D, or something like that 8)
I don't know what this page says, I think selected Nederlands for language.
Can search for similar - 5V, 250ohm coil (20mA to turn on), 30V, 1A.
Maybe look around for similar spec part but at lower cost.
Need a relay per battery.
NO side of relay connects to battery. Call that pins 4 to B+s & 7 to B-s.
Then all pin 3s get connected together and go to Analog in.
All pin 8s get connected together and to arduino Gnd.
Then the multplex circuitry turns on 1 relay at a time, connecting B+ and B- to Analog In & Gnd.
Make the measurement, turn off the relay, on to the next device.