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Joined: Jan 1970
To All, The batteries I have found are normally used in Cordless handset phones, and I obtained them in the UK from RS Components, (www.rswww.com) stock No.595-491 and cost £7-20 plus local tax. They are Ni-Cad 3.6V/700mAh, and I have used them in three of my HP35's and a HP45 with no problems.
They come as a unit with two tags and I just solder on flying leads (very thin wire is best) which I then solder to the battery terminals in the back of the calculator, I solder them halfway down the terminal so as not to modify the ends that might want to be used in the future. As they are shrink wrapped with insulator I just let them rest in the battery compartment.
It might be possible to take the three cells and incorporate them into the HP battery pack after cutting around the joint on the plastic.
The other thing I do is to take 3 AA Alkaline cells and put them into a HP battery pack again after cutting around the joint to separate the two halves.
This is a solution I have used very succesfully in my HP25, just take 2AA's and place them in the battery pack from your HP25,21,27,29 etc.
I have some pictures to show you what I have done.
RS Components has a place in the USA called "Allied Electronics in Texas, their web address is www.alliedelec.com, I found this from the RS catalogue I receive regularly.
I hope this is of some help, let me know if it's helpful or makes sense, please feel free to let others know and call me if you want more info if I can provide it.
Regards Richard.