Switched Capacitor Circuit for Measuring Battery Cell Voltage Difference in LTspice Simulation

Hi, I am new at this forum. I need help with an analog design that I have in mind but I am not sure how it will work. I am trying to create a way to measure the voltage of battery cells through trespassing the voltage of the battery cell to a capacitor and then reading the voltage from the capacitor; something like it is used to actively balance battery cells using four switches and a capacitor between two battery cells but instead of trespassing the voltage to the adjacent cell, reading the voltage out. Here I attach my idea. I already simulated in ltspice but it gives me a weird error (it is not what I expected). In the waveform you can see how the output voltage ranges from 6 to 8.4 V, which is not what I was expecting (0-4.2 V). In my mind, this circuit should behave in the following way: when M1 and M2 are closed and M5 and M6 open, the capacitor C1 should charge up to V2-V1. Then, M1 and M2 are open and M5 and M6 are closed, so the Vout will be this difference. What is wrong with my reasoning and why it is not working? Thank you very much for your help, I will appreciate it a lot!

Leandro my advice is, don't trust simulations, especially with out-of-the-ordinary situations like this.  I had a case recently where diodes were treated as ideal diodes, ie no forward voltage when conducting.  It's a simple enough circuit - build it and see what it does.

You need to use four-phase non-overlapping clocks. The four phases are successive time intervals in one clock cycle:Phase 1: M1 M2 M5 M6 all off.Phase 2: M1 M2 on, M5 M6 off.Phase 3: M1 M2 M5 M6 all off.Phase 4: M1 M2 off, M5 M6 on.The four phases ensure that C1 is never connected to both input and output, even during the switching times.
Also, connect high-resistance resistors from both ends of C1 to ground. Use high-enough resistances (10 megohms or more) so the resistors do not significantly discharge C1 over one clock cycle. These resistors are needed because simulators don't like truly floating nodes.