Why is my back up battery coffin not charging my battery fully anymore?

[AC Question]

In the past when my battery runs low, I've always had the back up battery ready to roll. This week it had started blinking differently, and the coffin is only chatting batteries to halfway. Not sure why this is happening but I use my phone daily throughout my work day and this is getting to be a pain. There's not always a place to charge my phone so the coffin had been a great add in from Lg