Dremel battery charge

I have a 7.2 V battery powered Dremel which gets lots of use. One of the batteries would no longer accept a charge so I bought a replacement battery. The instructions said to charge the new battery for at least three hours. I let the battery charge over night. After about 15 minutes of use it ran out of juice. I charged it again for around four hours ( the time it took for my other battery to run down ), and again it ran for about 15 minutes. I charged it up again for six hours and got less than ten minutes of use out of it.

I returned the battery where I purchased it and got a replacement. I charged it up for four hours. It operated my Dremel for less than five ninutes. I called the Dremel customer service and explained the problem to them. Two days later I got two new batteries delivered to me at no charge. Both accepted a full charge and operates my Dremel for at least four to five hours. Now I have three batteries to use in my Dremel. Talk about customer satisfaction.