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Replacement laptop batteries are available from a variety of sources, but the quality may be variable. In the worst case, a knock-off battery from an unscrupulous dealer and using a slightly different chemistry to the branded one could be a highly dangerous fire risk. Lithium batteries have a limited shelf life, so even a genuine branded battery might give poor performance if it's been in storage for a number of years. Wherever possible, always buy from a reputable seller, and make sure you can return it if it turns out to be faulty.