The higher voltage your reading isn't wrong, it's how batteries are rated. Generally each nimh cell is rated at 1.2 volts, hence 9.6v for 8 cells. However fully charged they generally read 1.5v if i'm not mistaken, which is actually 12v 100% charged up. It's the same with Lipos, each rated at 3.7 volts, but charged to 4.2v per cell. So your 7 cell battery, 8.4v will actually peak charge up to about 10.5v.
As already stated as well, if it's getting hot, turn down the charge rate. Some nimh cells can handle charge current better then others.