A term providing that one party or his successor in interest may accelerate payment or performance or require collateral or additional collateral "at will" or "when he deems himself insecure" or in words of similar import shall be construed to mean that he shall have power to do so only if he in good faith believes that the prospect of payment or performance is impaired. The burden of establishing lack of good faith is on the party against whom the power has been exercised.