hp calculators - test-drb@ccmp.vtda.org

21 Oct 2011

 On Thu, 20 Oct 2011 20:18:06 +0100 (BST), ard at p850ug1.demon.co.uk (Tony 
 Duell) wrote:
...
   years ago).
How hard is it to replace the batteries? 
 Very easy.

 The battery pack clips into the bottom of the machine, you just slide 
 it
 slightly forwards (towards the '0 key end' of the machine and lift 
 the
 rear edge to get it out.

 The battery pack contains 2 AA-size NICd cells connected in series,
 normally by a double-spring thingy (that;s a highly techncial term, 
 of
 course). The housing is made in 2 parts which were glued together. 
 You
 cna crack the glued joint, take it apart, and fit new cells.

 But actually, I don't recomend doing that. It's better to cut away 
 the
 pack housing ad the front end, where the cells touch the contacts in 
 the
 calculator. Then silde out the old cells and slide in the new ones --
 taking care of the polarity (since HP calculators were designed for 
 use
 with their own battery packs only, which only fitted the right way 
 round,
 there is no reverse-polarity protection. The cells will be held in 
 place
 when the pack is clipped into the calculator, and you can remove the
 cells easiy for charging in a normal charger, thus avoiding possible
 damage to -C models.

 There is a *much* easier way to do it, which doesn't mutilate the pack 
 anywhere as badly.

 Cut through the strip of plastic which runs along the pack between the 
 batteries at one end, using e.g. a pair of side cutters or a scalpel. 
 The strip will now flex out of the way sufficiently for you to be able 
 to prise the batteries out, one at a time. Then push the new batteries 
 in the same way. This operation doesn't weaken the pack mechanically to 
 any noticeable extent, it can be used just as before, and the batteries 
 can be replaced any number of times.

 OTOH it is harder (not very) to get the cells out to charge them in a 
 normal charger.

 Note that the contacts in the calculator are simply large rivets 
 soldered in to circuit board fingers. That solution is a bit of a 
 let-down IMO, as is using the battery as a load to reduce the voltage 
 from the charger, simply limiting the current with a whopping big 
 resistor. It could have been done a lot better, and not much more 
 expensively.

 Everybody who had an HP-25 (just about everybody who didn't have a 
 TI-57) when I was at University learnt to do this.

 /Jonas