My understanding is that used laptop batteries are particularly good for this sort of thing, because what happens is that one cell (out of six or nine) dies and the laptop battery becomes unusable since they are wired in series, and then you have five (or eight) used but relatively healthy cells to reuse.

I am not sure whether the same idea could be applied to a car battery with much more advanced battery management systems, since in those situations dead cells don't cripple the battery, so the other cells keep getting used until they lose efficacy. So I suspect cells scavenged from car batteries will be of lesser quality than those scavenged from laptops.

Not totally sure if I have it right.