Will a laptop battery lose capacity over time if it is NOT used.

I can get an extra 9 cell battery with a new laptop for $59. They cost $160 when purchased separately.

I wanted to store the extra one and use it only when the orig one goes bad - maybe 2 yrs?

Thanks.

Yes a Li-ion battery loses some 20% of its capacity per year (depends on storage temperature).

There's absolutely no point in buying the battery in advance. Over time, they will come cheaper and with more capacity. Just buy a new one when you need it.

In short, yes, but the rate depends on the type of battery.