Phone only turns on when it's plugged in but then works normally

[Tigrowski]

My grandma uses a cheap 5.1-based Android phone, manufactured by a company named Manta and for the past few days, there have been a problem with turning it on after it randomly shut off (which I described in detail in the previous thread if the specifics are of any importance). Suddenly, I found a way to turn it on yesterday - namely, if you try to turn it on unplugged, you may leave it for a few hours and it won't go past the producer's splash screen. Try to turn it on when plugged in the charger, though - and it'll power up within 30 seconds to the home screen.

Normally I would think that it's caused by something battery related - but the thing is, it doesn't seem so. First, if you try to turn it on unplugged and leave it on that never ending splash screen, it'll hold its charge alright, not turn off after a minute. And more importantly, second - if you plug it in, turn it on and unplug it - it'll keep working perfectly until you have to turn it off or restart it. What may be the problem, then? Is there anything I may try to do to fix the issue and allow it to turn on when unplugged as well?

 

[pkcable]

Sounds like maybe the battery is bad. Is it still under warranty? I THINK I found the support page....

Customer Support - Manta S.A.