The Rio Volt CD player I bought my sister came missing the rechargable batteries. I put some NiMH in, and found it wouldn't charge them. With some help here, I found that the supplied batteries expose the bottom metal of the battery for a second contact to sense. Scraping the label off the bottom of the rechargable batteries worked as a substite to the missing ones.

Not sure how her newer one does it. The Volt eventually broke, so she got a new one under the protection plan at Best Buy. She is rough on electronics, and is bound to break anything given time. The Volt still outlasted any other CD players she had, so it was worth the money.