The battery reading in G5 sometimes crazy.. When i charge my phone full charge (overnight) and then used it for a few minutes, it goes down to 70%.. but that 70% lasts a long time..
Sometimes its like that, i use always on 3g, and can last for about 15 hours before battery bar goes red, (check facebook using fb app, read tweets, read rss feeds, get emails, gtalk, facebook and skype chat, few sms)

So in my case, even though the battery shows 20%, it doesnt really mean 20%..