New battery: Stuck on Apple logo

Hello everyone,

A friend of mine told me his iPhone didn't boot, sometimes it did but he had to be lucky. So what I did is I replaced the battery with an OEM battery and that worked, it booted fine.

The OEM battery had to go back into the other phone, so I ordered a new battery. Now when I put in the new battery it's stuck on the Apple logo. Now, I don't want to restore because the whole issue is that he wants a backup so he can put his business partners on his new phone.

What I finally did is I placed his old battery in that didn't boot.. and guess what, it booted.. but it showed RED battery.

Now, 20 minutes later, I tried to boot the phone thinking it needed a charge.. but it still shows the red battery when I press the button and the screen goes back to black in 2 secs.

What can I do to preserve his information but get the phone booted so we can backup everything.

Thanks!