Belt tensioning device comprising: a base member that has an attachment portion for being attached to an accessory, and an opening for a drive shaft of the accessory; a first tensioning arm that is pivotably supported on the base member around a first pivot axis by a first bearing and has a first tensioning roller that is rotatable around a first axis of rotation; a second tensioning arm that is pivotably supported on the base member around a second pivot axis by a second bearing and has a second tensioning roller that is rotatable around a second axis of rotation; a spring arrangement that is arranged between the first tensioning arm and the second tensioning arm such, that the first tensioning arm and the second tensioning arm are pretensioned in circumferential direction relative to each other by the spring arrangement; wherein the spring arrangement comprises at least one bow spring (25, 25') that has a circumferential extension of less than 360° around the first and the second pivot axis (A4, A6), wherein the at least one bow spring (25, 25') has a first support portion (26, 26') that is supported on the first tensioning arm, and a second support portion (27, 27') that is supported on the second tensioning arm, as well as a spring portion (28, 28') extending between the first support portion (26, 26') and the second support portion (27, 27'), wherein the at least one bow spring (25, 25') has an axial length (L26, L27) in the area of the first and the second support portion (26, 27; 26', 27'), which axial length (L26, L27) is shorter than a total axial length of the bow spring (25, 25'). a base member that has an attachment portion for being attached to an accessory, and an opening for a drive shaft of the accessory; a first tensioning arm that is pivotably supported on the base member around a first pivot axis by a first bearing and has a first tensioning roller that is rotatable around a first axis of rotation; a second tensioning arm that is pivotably supported on the base member around a second pivot axis by a second bearing and has a second tensioning roller that is rotatable around a second axis of rotation; a spring arrangement that is arranged between the first tensioning arm and the second tensioning arm such, that the first tensioning arm and the second tensioning arm are pretensioned in circumferential direction relative to each other by the spring arrangement; wherein the spring arrangement comprises at least one bow spring (25, 25') that has a circumferential extension of less than 360° around the first and the second pivot axis (A4, A6), wherein the at least one bow spring (25, 25') has a first support portion (26, 26') that is supported on the first tensioning arm, and a second support portion (27, 27') that is supported on the second tensioning arm, as well as a spring portion (28, 28') extending between the first support portion (26, 26') and the second support portion (27, 27'), wherein the at least one bow spring (25, 25') has an axial length (L26, L27) in the area of the first and the second support portion (26, 27; 26', 27'), which axial length (L26, L27) is shorter than a total axial length of the bow spring (25, 25').