Using almost new 40kwh Nissan Leaf possible battery bank for house

You'll definitely want to make sure the car still has the battery. Still runs so that implies it does, but no harm double checking

There's plenty of videos online about getting the battery modules out of a Leaf

You'll need to string them together to make 48V or whatever the battery voltage is. You'll probably have several groups in parallel then wire them all in series

Or the other way around it that's easier

You'll need your own BMS as well, preferably one that can do cell balancing to account for any difference in the battery modules

Friend of mine has done it. Personally I did what @slave1 suggested and bought Chinese Lifepo4.

The ship of cheap lithium car batteries has long sailed. It is cheaper per kWh to go the China LiFePo4 route yourself. And most importantly, no car battery is safe from a fire / explosion point of view, none should be stored in your house. On the other hand, LiFePo4 is safe.

CHAdeMO V2G charger costs $3k plus shipping plus VAT plus import duty last time I looked. Forget it. If you want home storage, get a battery inverter and do up your own battery. From about €2k all in for 10kWh

Tell your friend to sell his Leaf for scrap (if it doesn't work anymore / won't pass an NCT), these fetch big money now. Will easily pay for the total cost of the above setup.

Short answer is no. You can't just mix and match battery cells or packs. They have to be of equal capacity, equal chemistry, equal internal resistance, etc.

Battery capacity is in kWh BTW, the power of a motor or the speed at which a battery can charge or discharge are in kW

Doesn't the Leaf use LFP cells though which can't catch fire?

EDIT: Nevermind, I found out it uses NMC cells