I think to use it, you need to re-install Battery & performance "com.miui.powerkeeper" and Power Detector "com.xiaomi.powerchecker", I am not sure if you will need both or just one of them, you can try.
Thanks reinstalling powerkeeper fixed ' App Battery Saver ' , although ultra battery saver still doesnt work apparently searching around it gets fixed with factory update which I dont want to do so ill live without it I guess