Not much magic to it.
Just weighted a 2 KWH 48 volt battery module out of chevy volt at 32 LBS.
they are all over e-bay for $300. Better look around and call local junkyards. In a pre 2016 volt pack are 7 pieces 48 volt and 2 pieces 24 volt. It is not adviseable to split these units into 12 volt.
48 volt is anyhow the standard that high power and high efficiency inverters work with.
So technically these batteries are 8x2kwh=16kwh.
To give them a long life Chevy and you derate them and thus the capacity is about 12 - 14 kwh.
In reality Do NOT charge higher then 4 volt / cell , I do 3.9 volt and do not discharge below 2.7 volt per cell.
You need to read up on this to keep it safe. There are other consideration as cell balancing, temperature etc.
All has been solved with relative cheap solutions.
But as with any DIY project you have to be comfortable so read up.
look up JDontheGO lithium set up. he does not use chevy volt or nissan leaf but it is all very very similar. By the time you read about the differences between these batteries you are on expert level
It is hard to say but I think at least 50% is available in worst case situation. That would reduce the chevy volts range to less then usefull.
Together with a friend we bought about 5 packs over the last 2 years ranging from $1500 to $800, you can do the math, it is really good.
A nice thing about these batteries is that the AH ids indeed there . lead acid is limited to 50% discharge if you want to get some life out of them.