What level of imbalance in series connected cells is good?

If I have lithium battery with some cells in series (same type, same manufacturer) - how much could they disbalance after one cycle? How much is too much? If, lets say, I charge 4S pack from 12V to 16V - what is appropriate voltage difference between cells? What voltage difference could indicate that some cells are not as good as others?

The first thing you should worry about the voltage of the cells: If one of them exceeds the max allowed (or recommended) charging voltage, which is usually 4.2V, then this cell will degrade more. A 200mV (5% of max voltage) of exceed may result in 20% faster life degradation.

how much could they disbalance after one cycle?

If the cells are in good condition then you may expect less than 0.2% of imbalance depending on the temperature. It also depends on a few factors such as output impedance, state of charge (SOC), self discharge rate (SDR), etc. These are all uncontrollable parameters because even if the chemistry, capacity, manufacturer, etc. are all same it's impossible to manufacture identical cells. Generally, SDR is quite low for Li-based batteries but the output impedance may differ by 10%.

what is appropriate voltage difference between cells? What voltage difference could indicate that some cells are not as good as others?

At first, I'd like to say "I personally would rate a cell as "not good as the others" if the internal resistance differs by 15+ percent while keeping all the other parameters (discharge current, temperature, etc) the same." since the output impedance (a.k.a. internal resistance) is a crude indication of state of charge (SOC) -- the higher the internal resistance, the lower the charge level (Some Ni-based batteries may show the same resistance at 20% and at 100% but this is out of context for now). Remember that discharge current drops voltage across internal resistance. If you know the open circuit voltage (OCV), the voltage during discharge and the discharge current then you can calculate the internal resistance.

However...

This also depends on the charging/discharging scheme and the lifetime of the other cells. If there's no balancing during charging and if one cell gets higher than the max allowed charged voltage (usually around 4.2V) even if the pack voltage stays within the limit, then obviously one cell will get lower voltage.

• An exceed of 5% of max charged voltage (i.e. 200mV) can lead to a 20% faster life degradation and this will eventually show itself as a lower voltage after identical discharging.
• A cell with lower charged voltage will discharge faster so will have even lower voltage after identical discharging.

So it's hard to say.

This is only my guess but when I charged a 12v pack of 9 lithium battery I would keep the battery different voltage around 0.01 to 0.15 or 0.2 max. If I see 0.3 different voltage I would get concerned But this is still my guess and I still have to search for some based information.