It seems you're referencing a specific battery model for the Vivo Y31 from 2015, which is an OEM B-77 Li-ion polymer rechargeable battery. Here are the details based on what you've provided:
Model: B-77
Compatible Phones: Vivo Y31, Vivo Y28, Vivo Y928, Vivo Y628
Type: Li-ion polymer rechargeable battery
Capacity: 2100mAh
Condition: 100% Original (OEM)
If you're looking to replace the battery or purchase one, it's important to ensure that the seller provides an original OEM product to maintain the phone’s performance and safety. Would you like help finding a source to purchase this battery or more details on its compatibility?