Voltage recorder for 12V batteries connected in series.

[marcingebus]

I'd like to build a set of data loggers that would measure the voltage of 12V batteries connected in series to a UPS in a single string. The goal is to detect the weakest batteries in the string. The strings contain 30-50 batteries, so the only solution is to power the battery locally and connect to the data logger wirelessly or via an isolated link. I'm thinking of one voltmeter per battery, or one voltmeter per multiple batteries (two or four?). Has anyone implemented this approach and have experience? Will the ADCs built into the ESP32 suffice, or is it definitely better to use an external ADC? I need devices for three strings, so about 150 devices, so price is a factor.