I picked up a v8 at the flea market for $25 with the same issue. Pried the battery apart and had the issue I suspected: A wire had been broken because it was pinched between the plastic body and a screw head where gravity pulls down on the vacuum when placed on the wall mount. Over time this disconnects the charging port from the control circuitry.
CAUTION: Taking the battery apart can be dangerous. There are exposed electrical connections everywhere once the battery cover is removed. None of these voltages can hurt you, but if you short out the battery cells (for a decent period of time) you can cause the cells to violently degas and catch fire. Also, the plastic cover is hard to disassemble without getting marred up. If you know of someone who fixes cell phone screens, they can most likely do this fix for you as it involves a lot of the same skillsets.