I have a Dell Inspiron 4000 laptop. I suddenly started getting a message that my previously fully charged battery was not and was dangerously low. Unclear if the problem was battery or machine or both, I substitued a charged battery from my aunt's Inspiron 2500 (compatible) and put the dubious battery in her unit to charge. next morning the old battery was still not charged, the fresh one did not work in my laptop, and when returned to her laptop now read 14% and charging (apparently discharged in my laptop overnight). I don't want to buy a new battery if the problem is the laptop. how do I figure out the source of the problem & the solution?