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I work for SafeBatteries.com which specializes in making and selling laptop batteries that perform better than OEM. We currently use only the advanced Panasonic NNP Li-ion cells in all our laptop batteries and these cells are widely considered to be the best performing cell currently made. The offer about %30 more power capacity and should easily last longer than the typical OEM batteries that use the typical standard cells.
A typical 6 cell HP OEM battery rates at 4400mAh or 48Wh of total power capacity and last roughly 400-600 charging cycles until they are completely used up.
The "Max Capacity" brand of battery that we make use only the newer Panasonic NNP type cells which provides about 30% more power for the same 6 cell laptop battery (5800mAh or 63Wh of total power). As for service life, our battery retains about 60%-70% of its capacity even after 500 charging cycles. This means the cells are considered to be long lasting (3 years or more depending how quickly you go through a charging cycles).
All the replacement batteries models we carry use these newer/advanced cells from Panasonic and have an average selling price around ~$75 for a 6 cell and $109 for extended.
Please feel free to let us know what you think, or if we can provide more details about us vs. other laptop batteries.
John from SafeBatteries.com (A US based company)
It really varies. I wish I could give a better answer. Some are absolute trash; I've seen new replacement batteries that wouldn't hold a charge, and the output was below specs. On the other hand, I've seen replacements that were just as good as OEM. It really varies by manufacturer. If you get a cheap, generic Chinese replacement, it could go either way. Buy something from a reputable battery manufacturer (it could even be the company that made it for Compaq), and you will probably be just as satisfied with the result as you would with OEM replacement. Lenmar batteries are widely available and usually ok, but they may vary by model - I don't have a large of enough sample to endorse them with confidence.