Does regen improve any over stock "new" battery state? Regen seems to be limited by reduced Hx values, so I wonder if the converse is true?
Shouldn't be possible get better regen than with new vehicle.
Maximum regen Leaf supports (with brake pedal) is exactly 50kW.
Without brake pedal, around 30-35kW in B mode and high speed.
Regen is limited by software. Input for calculations are (at least): Hx value, battery temperature, SOC and maximum cell voltage.
When vehicle is started, regen limit is estimated by latest battery resistance value (Hx), temperature and SOC. Sometimes this limit is too pessimistic and during first regen event, regen can change a lot. Example, I charge vehicle to 90%, temperature around +10C, regen shows 2 bubbles. The first time I decelerate (or accelerate hard) regen adds 1-2 bubbles. It can be felt as well. It starts with 10kW regen and within 1-2 seconds it grows to 20-25kW. Clearly, voltage doesn't spike enough and more is allowed. Up to ~4.12V max cellpair.The easiest way to fix regen is to find somebody who can access vehicle module software and remaps that regen capability.
Hx value is likely measured indirectly. It takes discharge/charge current (hall sensor), battery temperature (must be above +15C) and finds out, how voltage spikes/dips at those loads. If two batteries are in parallel, computer doesn't know that. As voltage dips/spikes are extremely small, regen capability map will adjust regen accordingly. But not more than 50kW with brake pedal and likely up to 30-35kW without (brake lights requirement).
My Hx on 4yo Leaf is jumping between 92% and 98%, depending on the season. Regen is almost always fully available (down to 4 Temp bars, aka +5C).
Easiest (cheapest) way to prolong range on a Leaf with worn battery is to find another worn 24kWh pack, connect them in parallel.
I bet safest connection point would be BEFORE contactors. So two batteries are ALWAYS in parallel. So there will never be arcing. Cable between two packs must be routed safely and reliably and also protected from possible damage. Secondary Leaf pack (in the trunk) must have DIY BMS that will balance out those 48 modules. Maybe original BMS inside secondary battery will work on it's own, but that must be manually verified. Balancing between two batteries will happen on its own.