This "trick" is pretty much as old as the HT. But, I think it's worth repeating here since I hardly ever hear about it.
If you're like me, you keep an HT around just in case you need it to talk to your buddies at a hamfest, but it's not your primary radio. Being a cheap ham, I have no interest in sinking money in my HT, but over a few hundred charge cycles and a couple of re-conditioning runs, the batteries do wear out. So, you can shell-out $50 for a replacement...or you can replace the cells for about $10, depending on your particular needs:
- Remove battery from HT.
- Pry battery open and determine what kind of cells are in the pack.
- Select similar [or better] cells from a large electronics distributor. [Digi-Key is my favorite for this]
- When you order the cells, have them built into a pack of the same configuration as your HT's battery. [Not all vendors will do this]
- Carefully desolder and remove the old cells from your pack, being careful not to disturb an thermal fuses.
- When you install the new pack, return the thermal fuse to the same location amongst the cells. It is ok to cut a hole in the plastic pack.
- Re-assemble the mechanical parts of the pack. Use epoxy for a professional look and permanent enclosure. I just used black electrical tape on mine to facilitate future replacements.
- Charge the battery for the first time in a safe place, especially a shop or garage, to avoid significant damage if something goes awry.
- Enjoy your new battery life.
Be careful substituting different (eg NiMH for NiCd) battery types, especially if you have an internal charging circuit. Not all batteries have the same charging requirements. Check your charger for information on what batteries it can charge, keeping in mind that you may have to do major rewiring to your pack or charger in order to accomodate a different kind of cell.