Battery 12 V, 4 Ah, type YT4-B in a slim case, Atlant brand. The name says "for Japanese scooters", meaning scooters of the Japanese type. The designation YT4-B refers to the case size and terminal layout, which you should compare with your original battery.
Replace the battery if it does not hold a charge well, the starter turns sluggishly, or the voltage drops when starting. Before buying, compare the dimensions and terminal layout with your original battery.
