HiLiKuS,
You should be able to replace the Ni-MH, with the Li-Ion battery as long as the Li-Ion has all the propietary connections needed for the VERY propietary Compaq Presario 12XL 500.
Here is some general info on the differences between the to batteries.
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Nickel Metal Hydride (NiMH)
These batteries were introduced in the early 1990's as an improvement over older chemistries like Nickel Cadmium (NiCd). Although they may have a very slight memory effect, NiMH batteries are much more reliable than previous chemistries. Compared to a NiCd battery of equal size, a NiMh battery will run for 30-40% longer on each charge. NiMH batteries can also be recharged 60% in just 15 minutes. The unique NiMH chemistry packs much more power than NiCd. Additionally, NiMH batteries often have twice the lifespan of NiCds.
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Lithium Ion (LiION) :
The most popular batteries available. Li-ION have replaced NiMH batteries for many laptops because they provide 4 distinct advantages :
1) 40% longer Standby and uptime than NiMh.
2) Lighter and more compact than NiMh batteries.
3) LiIon batteries do not suffer from memory effect .
4) The battery lifespan will be over 30% longer than NiMh batteries because the cells can accept more charge cycles.
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Go here to buy, they are the best to order from online that I've seen and offer both NiHh and LiION batteries for your laptop.
I would go with the 14.4 Volt Li-Ion Laptop Battery personally, but double check with ebatts to make sure it fits.
Good Luck,
-Mike