Your average name brand NiMh battery is a 300 cycle affair. That means the maker calls the battery good if it can get to 50% capacity after 300 uses. There are cheaper batteries that are worse than that. As to the time left according to the camcorder, since it's not the camcorder battery maker's battery we can't expect it to be close to accurate.
For me the big clues about battery replacements is the size and weight. If it's the same size and weight I expect close to the same as a new maker battery. And by close I mean within 50%.
Bob
Hello,
I own a Sony HDR CX230 camcorder and wanted to extend the battery life. The battery it came with was 500mah and gave me about 1 hour and 20 minutes of battery. I checked on Amazon and found a pair of extended FV100 batteries for 34.99 from Big Mike's Electronics. They said 4500mah, but the 2 batteries I got were 5900mah (I hope that's not a sign...) Anyways, I've had no compatibility issues with it; the batteries run just fine on my camcorder. When I first tried them, they said about 9 hours of battery and I was excited. But when I started using them, that number dropped significantly and after an hour of total use (based on how much memory card space I had used up.) it was already down to one bar and saying I had about an hour of battery left. This happened on both batteries. I'm asking if this is an issue with the camcorder not recognizing all of the juice this battery can hold, or if I got ripped off and these batteries are almost no bigger than the included battery (the battery itself is way bigger and heavier than the included battery.) Any help is appreciated. Thanks!
Tommy

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic