I recall the imbalance of fresh cells is less than 1% worst case ( usually much better). Since LiPo cells have a useful range of 300mV to 600mV (500mV typ) depending on chemistry with an average in the range of (3~3.9 for most types
Li-SOCl2 Li/SO2 Li/MnO2 bobbin, spiral etc. )
If the discharge range of voltage is about 1/7th of the 100% SoC voltage a 1% no load mismatch translates into 7% mismatch in ESR.
Similar to LEDs all White LEDs have the same threshold voltage ~ 1mA as LiPos may have the same 4V no load voltage after charging @4.2V.
In both cases LEDs & batteries have an ESR inverse to power capacity and ability to dissipate heat for a given small temperature rise.
As pulse discharge in LiPo and pulse drive in LEDs the Vf is totally dependent on the ESR and chemistry of the device.
- The RC time constant for the battery can determined over the life span to know what pulse width to use for this measurement.
- Capacitance, C = I *dt/dV for a DC charge or discharge for slope e.g. 2.5Ah with 1A load drops 0.5V in 2.5h= 18ksec thus C=36kF equivalent capacitance
- ESR= ΔV/ΔI for initial response due to a pulse or step. e.g. 5mΩ for a nominal cell
- thus ESR*C= is the time constant of the cell where... 5mΩ*36kF= 180 seconds, if I worked this out ok.
- This time constant reduces with aging since ESR rises faster than the drop in C.
- Two perfectly matched voltage cells can be tested on some LCR meters if put in series with opposite polarity.
We know when source ESR = Cell ESR maximum power transfer occurs but only 50% efficiency, so this is often compromised to 75% efficiency or limited by the temperature rise in the cell. Although capacity of the cell increases with temperature, cell life-time also degrades much more rapidly.
In the consumer area, Li/CFX (poly carbon monofluoride) and Li/MN02 (manganese dioxide) are found in cameras, calculators and watches. In the military area, Li/SO2 (sulfur dioxide) batteries are used in high power radios. Li/SOCl2 (thionyl chloride) and LiI2 (lithium iodine) are commonly utilized in industrial and medical applications. ref. Tadiran
What I suggest is consider your slow charger but rather than monitor individual cell ESR, monitor temperature with LM35's mux the signals to a serial composite signal and regulate load balance to equalize temperature rise with a setpoint 35~45'C. With forced air to improve response time and cell isolation.
What cell array sizes are you looking at? Ser/Par. Are they bridged?