On your mini-c it could be that your rear u-joint could be binding one way or another against the transom or outdrive. I would check that first and adjust accordingly if need be. The other thing could be the prop itself . If it has any nicks or bumps that will also affect it being able to get up on plane. I would always sharpen and smooth up any rough edges or you could upgrade to a replaceable prop by getting a prop shaft and prop for the shockwave 26. Makes all the difference in the world. I'm not sure on your battery question.