High quality rechargables such as Panasonic Eneloops are all that I use when I need AA batteries for my Fisher, Tesoro, or Whites machines. The best feature of the NMh batteries is that they DO NOT leak.
Also, the eneloops will hold their charge for long periods of time when unused, so you can charge them and they will maintain the charge in storage, with about 70% charge remaining after a year in storage.
As far as runtime: I have never had an issue with these batteries not lasting at least a full hunting day, and usually a couple of full hunting days. I always carry a spare set in a plastic box in my pouch, and really don't worry about having to remember to charge, but if I change a set out in the field, I just put them on the charger when I get home. They are well worth the cost, and you will save money in the long run over the cost of non-rechargable batteries.
And..... if you forget to take them out of the detector and leave them over the winter, you won't find a corroded mess in your machine in the spring when you head out to use it.
I personally have made that mistake with a Tesoro Stingray, and ruined an expensive detector. It does not take long for a bad battery to destroy electronics.