Wis. Stat. § 411.109: Option to accelerate at will.

(1) A term providing that one party or his or her successor in interest may accelerate payment or performance or require collateral or additional collateral “at will” or “when he or she considers himself or herself insecure” or in words of similar import means that he or she has power to do so only if he or she in good faith believes that the prospect of payment or performance is impaired.

(2) With respect to a consumer lease, the burden of establishing good faith under sub. (1) is on the party who exercised the power. With respect to a lease other than a consumer lease, the burden of establishing lack of good faith under sub. (1) is on the party against whom the power has been exercised.