For three years in college I worked at Batteries Plus, and other than installing car batteries, 4.5 million watches, I was the main tech guy at our store. My part time hours I'd probably do about 10-20 (sometimes more) cordless drill packs a week. Basically, we open up the pack take the contact connectors, temp sensors, or other resistors off the pack. Glue up the new batteries to the same shape, spot weld the connections together and finish the final connection.
Skill packs were always a POS. Once you get them open, they were cheaply made and the cells were sub par quality with lower capacities. When I was done rebuilding them, my quality was much better than their China factories. But that's where every brand makes them. But some brands are much better than others.
Black and Decker chargers were always POS as well. My manager and a customer ordered replacement 14.4V chargers (direct from B&D) within a 1 week timeline. The first two chargers my manager received already had burnt circuit boards in them prior to use, and the same happened to the customer. Finally they both got working ones but you could tell that these chargers that they charged $20 for (8-hour chargers) were made for $0.10. This is their old style batteries like this one:
Anyways there are brands that do make good quality batteries (and chargers). But they match their price tags.
Good:
Dewalt (occassionally prone to breaks in connections between the neck/top cell and the bottom, overall good batteries)
Bosch
Average:
Craftsmen (actually these were pretty good, but they use lower capacity cells and often use 4/5 Sub-C's and made them fit in full Sub-C cases)
B&D (similar construction to Dewalt, though not quite as good, used lower capacity and often 4/5 sub-c cells like Craftsmen)
Milwaukee (though they are constructed well, a couple years ago they recalled about a million batteries)
Porter Cable (hit and miss sometimes, their 2.0Ah batteries marked on the outside are actually 1.9Ah internally)
Bad Design/Overall:
Skill
Lincoln Grease Guns (I laugh at what they charger for OEM batteries, I've seen more of these batteries melted by internal short circuits than anything)
Any $10-$20 18V no-name brand. Good luck having a battery last more than 6-12 months.
I can go on for hours about this, and I'll field any questions!