Ascend G7 wont turn on (battery problem?)

I am fixing a friends G7 and basically it just wont turn on. Pressing and holding the power button provides no response from the phone what so ever. The screen stays black and there is no vibration or anything. The phone doesnt seem to charge properly also. When i plug it in a solid red light appears (i think it informs that the phone is charging) but after about 5 minutes of charging it goes off. I have taken the back of the phone and measured the battery voltage with a voltmeter. The battery at first read 2.50V. After trying to charge the phone multiple times yesterday it has raised up to about 2.8V but the phone still wouldnt turn on. So i plugged it back into the mains and left it charging for about an hour. I came back measured the battery voltage and it was 2.6V. This morning i left it in the mains for a good 9 hours and when i came back to check the battery voltage it is only 2.67V. I think that the battery might be a problem here but i want to get any second opinions before i buy a battery. Also is there any way of turning the phone on with the battery unplugged? Thanks for any responses