2S 18650 Li-ion battery pack conundrum
My 2 cents coz had the same problem with my grandson's rc. The stock battery died after 4-5 charges.
Simply, get new 2 pieces 18650 battery. Unwrap the stock battery pack, unsolder the wirings with the balance connector. . Use that same wire  exactly on the new batteries. Bingo, you just save couple of dollars for stock replacement battery! Test using the balance connector, will read cell 1, cell 2 and total charge.