I purchased one of these, along with a new extended capacity battery. I charged the extended capacity battery within 2-3 hours. All fine. I then switched the batteries and left my original battery in the cradle charging, thinking that no harm could happen. When I took it out several days later, it is completely dead. If I put it in my phone, the phone is unable to start with or without the power cable connected, but the phone starts fine with the new extended life battery.
I suspect that by leaving the battery in the charger for an extended period, it has damaged / killed the battery. I would expect the charger to be able to detect that the battery is charged and that no further charging is required. That's the whole purpose of having a second battery in a desk cradle: leave it there until you need it, but then it is fully charged whenever you take it out.
I would not recommend this product. It does not appear to be of sufficient quality.