From what I can tell from the service manuals, the HD1 and HD3 use the same battery. I recently replaced my HD3 battery with a Chinese-made one for the HD1 that I bought on eBay. It works very well and I'm getting slightly longer battery life than I had from the unit when it was new. Time will tell how reliable the battery is in the long term though The other thing to bear in mind is when dismantling the unit, you do need to carefully follow the disassembly steps in the service manual. HTH, Reuben0
