Galaxy S6 battery percentage jumps?

[CasualMember]

Old phone, but still good. One thing, though- the battery. When it's charging at around 15% I think, it jumps up to another percentage, maybe 20%. Is this normal or not?

[LongHiker]

@CasualMember This is the Galaxy S6 a 9 year old phone? I never really sat there and watched the charging percentage change. This could be an indication that the battery is failing. 

[CasualMember]

Yeah. I've never opened this up, as it's a non removable battery phone. The battery was probably sitting there for about 9 years.