This thread would be incomplete without an endorsement of Panasonic's Eneloop batteries! Get Eneloops, and a smart charger that charges each cell individually, not in sets of two or more. The smart charger that Panasonic packages with their Eneloops is okay, but there are better analyzer/conditioning smart chargers available. Fast chargers shorten the life of rechargeable batteries (you don't get something for nothing, there is a price for quick charging).
Buy once, cry once. Eneloops can be recharged hundreds of times, saving one lots of money over "alkaleaks". BTW, never, NEVER store alkaline batteries in your detector for longer than a month, as they are notorious for leaking, and no one needs to deal with that hassle. So if you are one who only detects during certain months of the year and uses alkaline batteries, be sure you remove them before putting your machines up for the season.