What mAH batteries are you using?
The way it works is the mAH rating (milli-amp hours) is the amount of charge that can be stored by the battery. The higher the mAH the more power the batteries can hold but sadly the quicker they loose it.
All batteries loose charge through natural drainage but this is especially evident with rechargeables. It's a tradeoff. If you charged some 1700 mAH and 2700 mAH batteries and took them straight off the charger and left them on the shelf the 2700's would be flat first.
Which ones would last longer in you camera depends on how much current your camera draws.
For a camera like yours I would recommend going for something like four 2300 mAH as they will charge faster (the more mAH the longer the charging time). That way you have a set in the camera and a set on charge. They will loose charge slower than 2700's and shouldn't affect the "power on" time of the camera too much.
Personally I use batteries like
these. It's the charger that really dictates how long they charge and it's worth paying the £30-ish for a kit with a faster charger (at least thats my opinion anyway!

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