I saw that this questions was previously asked from other version of ubuntu, and no one seems to have an answer.
How can I fix this?. How can I check also if is a bettery problem (the battery does not charge anymore also)
This could be one of several things:
Do you have another charger to test with? Does the battery itself have a charging indicator (a number of the Dell batteries have this)?
I dont think its that simple. I am having the same issue, and i was experiencing the same in Ubuntu 11.10. If the AC is on while booting, ubuntu also shows a charging sign. If however, its un-plugged and re-plugged while ubuntu is running, it does not change back to battery charging mode. I have windows 7, and it does work ok on it. So I dont have a battery issue at all.
I think there is no clear solution right now, i hope there will be one soon.