rebuild them, or have them rebuilt. They normally use sub-C flattopped cells, soldered together in series. My experience is that it's cheaper to get a specialist to do it, as they charge about what it costs me to get just the cells, or less. (You can get substantial discounts on cells if you buy them in large quantities.) and I don't have to solder them, or figure out how to non-destructively open the case. If the battery packs are more than a couple years old, they'll be better than new. NiMH battery technology continues to improve, and higher capacity cells are available.