Here is a link to help you troubleshoot your charger. Could be that as well. There is a section for charger shutting off too soon.
http://www.lesterelectrical.com/tech...otingguide.pdf
Have you taken any voltage readings on your battery pack after the charger shuts off? Wait about an hour, then post them on here.
If you think it's the OBC, then you can bypass it and see if it fully charges the cart.