ok, i got a 81 DL and it will not hold charge. as long as i drive it every 6-10 hours, it does ok. but if it sits over night, for the duration of maybe 14 hours, theres not enough juice to turn the engine over. something is draining it possibly? or is it just time to replace the battery. the battery is nearing 4 years old. any help would be rad! thanks....