Jon,
I've actually taken one apart to confirm the cells and replaceability. If it's still within the 1 year warranty, you should contact us at 888-200-5129 and we'll replace it under warranty. If you are out of the warranty, you can open her up as you choose. It's very simple, but as with all electronics be sure to take proper precautions. You might also make sure the cable going to the battery input is solid and is not shorting. Will it charge from the 12V DC but not the 110V AC, or vis versa?
There is a black circular sticker on the top near the on/off switch. Remove that sticker to expose the phillips lock screw. Once you remove this screw you should be able to slide the case open. There you should find the premium l-ion cells inside. It's a great construction, no cutting or cracking glued cases open. I hope this helps.
Please keep in mind if you have not had it for a year, call us first for a replacement. Thanks again for your business and support of our product and company.