What makes a laptop battery specific to a model?

What makes a laptop battery specific to a model? I have an old Toshiba laptop (pentium 4) whose battery just crapped out. Looking at the battery, it says it's a PA3251U. Looking online, this thing is going for about $100! I don't want to spend probably 50% of this machine's value on a battery replacement!
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Guest [Entry]

It's about the physical shape of the battery pack. Other than than, it's the voltage that the back can provide (and the maximum amps as well I suppose). Inside the battery pack are standard battery cells that can be purchased cheaply. It is possible to open up the pack and replace the cells, but it's not easy or completely safe. See this question.

Guest [Entry]

"Laptop batteries will not just be a collection of cells. In general they also have at least some intelligence, from an overcharge protection circuit to a usage counter.

You might be able to replace the cells in the battery, but just soldering together cells to match the voltage won't generally work.

There may be third-party batteries available for popular laptops at a somewhat lower price than OEM."