Dead battery - Dell Latitude 3340.

Hi my laptop's battery won't work at all. Initially, the battery couldn't charge only and that happened for a day. The next day, the battery went totally dead. Not only couldn't it charge, the laptop won't even turn on when the battery is inserted (and is already charged at 70%) unless the AC adapter is plugged in. When i pull out the AC adapter the laptop shuts down. I rarely use my battery. I usually take it out and frequently use it directly with the AC adapter and I have had the laptop for only a year plus. Can I get some advise on what's going on? Thanks.