Originally Posted by mushroomed

To figure this out, start by discharging (not just storage) your 2S pack while running from the power supply. Then fully charge your 4S pack also from the power supply.

Then switch to powering the charger from your 4S pack, note the input voltage and charge your 2S pack. Note the input voltage again the difference between your two voltage values will give you the approximate reduction or gain in your 4S pack over a charge/discharge cycle. If I were doing this I’d be aiming to keep the 4S input pack between 15-16V at all times through the cycle. This requires the 4S pack to be much higher in capacity than the 2S pack you’ll be charging.

When regenerative charging like this you’re charging the 4S pack without any balance connection, so there is potential for over or under charging if you’re not careful, and could ruin your battery or cause a fire with some minor lapse of attention. Set up the voltage limits in the power source settings menu, and be sure to switch to this profile.

Really though it is a better and safer idea to build or buy a discharge bank for fast discharging.