HP 19BII running on SR-44s

[Chasfield]

I have seen the HP 19BII battery door problem solved in a couple of ways. Somebody made a clip style battery retainer out of metal strip, then installed SR-44s in the single cell half of the battery compartment with the help of foil packing pieces to take up the unused spaces. The other method was braver. The owner cemented the original battery door shut, then cut out a new one in the rear case to allow easy lifting in and out of batteries. A very neat job it was.

My HP 19BII's battery door was just about to give out so I decided on a fix midway between the above two. I cut a small access door for SR-44s over the single cell side of the battery compartment. In order to reduce its length and give firm support to the positive base of the button cells, I cemented in a conductive spacer, made from a piece of scrap copper pipe. I soldered a wire onto the spacer and ran it back to the contact spring of the double cell side of the battery compartment. An access hole was drilled near the spring to facilitate soldering. The original battery door was then cemented permanently with two part epoxy - it is now mechanically and electrically redundant, and sits nice and flat.

A couple of pictures are attached to show the progress so far. I still need to knock up some kind of battery cover. The soldering access hole will be covered with some thin rubber strip. A square of insulation tape is doing the job for now.

The calculator is working fine. Inserting the cells is fairly easy but is helped considerably if the first two to go in are Sellotaped together.

This is the first HP calculator I have hacked holes in on the same day that I received it!

[Chasfield]

I have tidied up the battery modification by making a flush fitting cover out of some scrap ABS sheet, which will be retained by a square of adhesive tape. This will suffice, as the SR-44s are at least as secure with no cover than the original N cells were when restrained by the stock item. I also cemented a blanking plate over the soldering access aperture. This was fashioned from a piece of a defunct Rockwell 18R rear case. The textured brown plastic is similar to the stuff HP used on the 19BII.

Some slightly deeper than stock adhesive rubber feet will finish the job.

[Chasfield]

I decided to find out the likely run time for the SR-44s, given that they only have about one fifth the capacity of N sized alkaline batteries.

I made a test rig out of two strips of aluminium foil, separated by double sided adhesive tape. This I inserted into the SR-44 stack so that I could apply multi-meter-probes in series with the cells in order to measure current draw.

With the calculator in the off state, the current draw was 13.5 micro-amps. In the on state, while pressing keys, the current swung around 1 milli-amp.

With 175 milli-amp hours available from the SR-44s, it would be reasonable to expect 150 hours of actual use, or around 500 days of standby time in the off state. This battery life doesn't seem too bad and is rather better than I had anticipated.