We have an Electrolux cordless vacuum that works perfectly for our small apartment. We only have area rugs so we don't require (or want) a large corded vacuum. Unfortunately, the last few months the suction has been noticeably weaker and it was not staying charged long enough to vacuum everything at once. I suspected the batteries had lost capacity, so I opened it up. I found that the 10 battery cells used were AA sized, so I decided to replace them.
The replacement batteries have been great! The vacuum is back to its original power (or better) and definitely lasts longer than it used to.
Hope this helps someone else too!