[Q] Battery voltage fluctuation?

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Hi all,

Faced a weird issue today, after playing Real Racing 2 for about 30 minutes, my phones battery was left at 1%, and it's voltage around ~3.43v.

Within the next couple of minutes the battery voltage upped itself to about ~3.51v without me charging the phone. And after some more usage of ~4-5 minutes (calling + browsing) the voltage levels dropped again and phone switched itself off! All this while battery percentage was 1%

So two questions here:

1 - Whats with the marginal voltage gain on the battery post a stressful activity. This is the first time I've noticed this, hence will try to replicate this again tomorrow.

2 - I thought the maximum(100%) and minimum(0%) voltages of Lithium-Ion battery are 4.2v and 3.0v respectively. How come my phone switched itself off at ~3.4-3.5v?

Please advice..
TIA