What is the test for determining whether a confession is voluntary?

"A statement is involuntary if it is not the product of '"a rational intellect and free will."' [Citation.] The test for determining whether a confession is voluntary is whether the defendant's 'will was overborne at the time he confessed.' [Citation.] '"The question posed by the due process clause in cases of claimed psychological coercion is whether the influences brought to bear upon the accused were 'such as to overbear [the defendant's] will to resist and bring about confessions not freely self-determined.' [Citation.]" [Citation.] In determining whether or not an accused's will was overborne, "an examination must be made of 'all the surrounding circumstances - both the characteristics of the accused and the details of the interrogation.' [Citation.]" [Citation.]' [Citation.]" (People v. Maury (2003) 30 Cal.4th 342, 404; accord, Colorado v. Connelly (1986) 479 U.S. 157.)