Sounds like a couple of things might be possible. Make sure your belts are tight - make sure you keep the vehicle turned OFF of course.
If you have play in the belts (ie able to move the belt midway between components like the water pump and alternator) of more than 1.5 inches total you may not be charging well enough. There could be some sort of electrical drain on the system, again not allowing the system to charge properly. Both of these items can be checked by a mechanic - some places will give you a quick diagnostic for free. When these items are being checked you should get the charging system checked. You mentioned 8 volts - far too low. You should be seeing around 14 volts when the vehicle is running. You may also have a battery that has gone bad. You did not say where you are from, but folks in the North East have been getting nailed by bitter cold weather. Sometimes the first good cold snap will take a marginal battery over the edge.
Good Luck!
