Originally Posted by LittleBigMan
Don't be fooled by the humble look of a battery pack. It might look like just a bunch of flashlight batteries covered in plastic shrink wrap, but it's not about looks, it's about performance.
It's really not that hard, as long as you use similar cell chemistry, operate within the battery specs and use a charger that's appropriate for the cells (!!!) . But, if you don't remember your physics class it should be no problem to get a replacement made or off-the-shelf.