I managed to source a couple of new batteries for my trusty Dell Latitude x200 recently. With them came clear instructions that the batteries must be 'conditioned' with several full charge/discharge cycles when first used.
So I did that - and the battery now keeps going for over 2 hours whereas the old one died after about 15 minutes after being disconnected from the charger.
A good idea to do a complete charge/discharge/charge every week, I reckon.