Manipulation charge does not make economic sense
For the past two years, the value of the yuan has been closely linked to that of the euro, so the accusation that China is manipulating its currency to stimulate exports is not supported by the data - unless the European Central Bank is also being accused of currency manipulation. What has actually been happening is that the US dollar has been rising against other world currencies.
Furthermore, a policy of driving down the value of the yuan would make no sense from the point of view of Chinese economic strategy. Hypothetical currency manipulation would, if implemented, raise inflation in China by raising the price of its imports, force China to buy more very low-yield US Treasury bills, and would be an ineffective way to boost exports.
The accusation of currency manipulation is vague, with no clearly stated time period. So, let's examine the last few years.