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Unfortunately, sourcing replacement batteries is something of a minefield. Genuine manufacturer's batteries tend to be outrageously expensive, and if they have been sitting on a shelf for several years they may already be losing capacity. After market batteries may or may not be worth the money and may even be pulled from scrap laptops. Some are re-manufactured using fresh cells, which is fine so long as the smart controller within them has been reset. Other things being equal, a mid-price battery could be a good choice. To protect yourself:

• Check the vendor's warranty period and returns policy.
• Check the vendor's reputation and other purchasers' experience on independent review sites such as Trustpilot. Be aware that favourable reviews can be bought.
• When the battery arrives, check whether you are required to register a warranty for it to be valid, though under consumer law this should not be necessary.