You might consider this mod: Easy SKY+SPA battery expansion (unofficial)
Just clip the wires off the batteries in the SPA and do the mod I reference and you should be good to go. The battery capacity doesn’t really matter too much, just that the larger the capacity, the longer it can run without sun.
More specifically, the battery in the SPA was 1000mAh and when they went to two cells the total capacity was 800mAh.