Mobile Phone Li Ion Battery minimum voltage

[LocoBlade]

I've got an old Samsung Galaxy S6 phone that died about a year ago which I'm looking to DIY fix to keep as a spare and as the phone is completely unresponsive to charging or any kind of reset I've dismantled it to have a look.

Once inside I found the battery was only at 2.5v (its a 3.85v cell) and although the phone draws current when charging the battery voltage doesn't change so assuming the battery was perhaps faulty I bought a secondhand battery taken from another phone broken for parts via eBay for a few quid, my logic being I'd prefer a secondhand genuine cell than a new battery for similar money which are obvious Chinese fakes. Anyway, battery number 2 arrives and thats' even lower at barely above 2v so my question is, regardless of whether there's other issues with the phone or not, are both of these batteries toast given their low voltage or would they recover if perhaps the power management IC has failed on the phone causing it to not charge? Some googling suggests Li Ion cells of this type shouldn't normally go below 2.7v-3v and charging them again from deep discharge can cause the battery to fail and potentially be dangerous?

[drdino]

They are toast.

[Green1man]

Yeh, depends somewhat on how long they may have been at the low voltage, sometimes you can recover cells from 2.5 (or even 2.0) volts, sometimes not.

[LocoBlade]

Thanks chaps, thought as much. I read that even if you recover them they can be damaged inside by those levels of discharge so can become dangerous to charge, is that right?

Edited by LocoBlade on Monday 4th February 20:28