Rechargeables dischargeables?

[RayB]

Is it normal for rechargable batteries (AA) to just lose power after a while even if not in a device?

[Marsupial]

yes

[Ed_McCarron]

For NiMH:

The self-discharge is 5–10% on the first day and stabilizes around 0.5–1% per day at room temperature.

[MonMotha]

Yup, very common.  In fact, it's worse on the newer batteries.  There's something of a trade-off between absolute capacity (mAh rating) and self-discharge rate for Ni-MH chemistries.  Since Ni-MH batteries are sold mostly based upon the capacity figure, development has been driven in that direction at the expense of self-discharge for the stuff you normally see advertised and at retail.

It's possible to make the trade-off the other way.  These are made and sold under various trade names with "Infinium" and "eneloop" being some of the more common.  They have slightly (~10%, typically) lower capacity than the top-of-the-line batteries you normally see, and they cost a little more, but they'll hold most of their charge for weeks or even months. There's a Wikipedia article that lists some of the common trade names for these.

[EwJ]

The Sanyo Eneloops keep a decent charge for months on the shelf.  They won't be full capacity but enough to still be usable while you're charging another set.  I think there's some Sony's out there that are rebranded eneloops .

[RayB]

Well then, lesson learned. Don't buy Energizers.