2700mAh NiMH gone bad after storage?

I bought 8 sanyo 2700mAh AA batteries 2 years ago. When tested with a maha c9000 charger, they had more or less 2700mAh capacity.

I used them on and off without problem until now. After about 6 months of storage, I put them in the charger to check their capacity. They dropped to around 2400mAh. So, I cycle them (discharge and charge) a few more times. But the more I cycle them, the worse the capacity get. Now, some of them has capacity of 1100mAh, and some are even lower.

The charging current I tried is between 500 and 900 mA (the charger says to use 0.33C to 1C).

Why do these batteries die?

I have a shoot coming up and I need 8 more rechargeable (for a flash external power pack). I wonder if I should buy 2700mAh or eneloop.

.

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