I use a Maha AA/AAA charger with Imedion batteries. The Imedions are similar to Eneloops as they come with a charge and retain the charge long term. Regular high power NiMH batteries can also be used, but when not in use i recommend putting a charge to them every month or two as they lose their charge while sitting much faster. These batteries are available in higher current ratings than for those from Minelab and will last much longer and with a quality fast charger you can be up and running in just a few hours. I don't recommend you operating the detector with rechargeable NiCD or NiMH batteries where the batteries are allowed to run down too low, like 1v per cell. This is apt to occur very quickly as the batteries are used near depleted or the single bar on a Safari battery indicator. I haven't any experience with the lithium ion types, but I do know they store a great deal of power for their size.
Edit to add; I paid about $24 for an 8 pack of 2400ma Imedion batteries and the charger I bought was $60 for the Maha C800S8AA27 with 8 2700ma Powerx AA batteries from B & H Photo. The charger can be bought alone at $39 so this is way cheaper than going with Minelab. Mods-If this isn't allowed, please edit.