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Using 4/5 AA cells as power source for spice calculators with broken battery clips
06-03-2020, 04:19 PM (This post was last modified: 06-03-2020 04:43 PM by Sadsilence.)
Post: #1
Using 4/5 AA cells as power source for spice calculators with broken battery clips
Modern AA NiMH cells are no perfect battery pack replacement for spice calculators. They are a little bit too long, often leading to accidentally or incidentally broken battery clips or even to broken housing (which is not HP's best calculator innovation anyway). Sometimes battery leakage does a similar job.

Here's my solution for such a case. One HP-34C in all other parts in nearly mint condition "teased" me. I used 4/5 AA NiMH flat top cells, which are available from a couple of manufacturers (Panasonic, XCell, OEM). Capacity varies between 1100 and 1500 mAh. That is enough to run a spice calculator for around 10 hours, more than doubling run time of original 500 mAh NiCD cells. You can use HP's charger for recharging. Charging current is around 120 mA. Empty cells are completely recharged in 12 to 16 hours (a "long overnight charging session"). As charging current is around 1/10th of cell capacity there is no real danger of overcharging, e.g. when recharging cells before being completely discharged or forget to quit charge session for several days. Only drawback: There are no LSD cells available in 4/5 AA size. So a discharge/recharge cycle is recommended every 3 months.
   

4/5 AA cells of course are too small (42x14mm). AA cells have 50x14mm. Ideal spice cells would be 47x14 mm.

As spacer I used typical AA battery springs "with tail" by cannibalizing an aa battery holder. Simply because they are cheap and available everywhere.
   

"Tails", bowed and cut correctly to fit in remaining parts of spice battery clips can be easily soldered like a through hole solder component of "good old times", leading to a mechanically stable battery holder. Durable and reliable electricity contact is guaranteed by spring effect. (Same springs can be used for repairing woodstock battery packs, by the way. AAA size is better for HP-41C battery holder.) Take care to avoid short curcuits on battery clip PCB board of calculator, as springs on base have a greater diameter than original battery clips. One small piece of insulation tape will do.
   

I would strongly recommend to connect other end of battery cells using a solder tail. When no spot welder is available: There are 4/5 AA cells with preconfigured solder fans available, which can be easily soldered together. Of course existing metal clip of original spice battery pack can be used for closing curcuit instead. In my example below metal clip of original battery pack was missing. So I had no choice. Nevertheless a soldered or welded connection is more reliable anyway.
   

Generally (not the case with mentioned HP-34C), when there are remains of battery leakage I disassemble calculator before, clean affected parts in vinegar essence bath (citric acid, part of many descalers does same job), wash away acid with demineralised water and dry parts for a half an hour at 60 degrees celcius in our oven.

   

One advantage of described procedure: When true collector's soul at some time can not live with that modification: With some help of desoldering braid it can be reversed completely.
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06-03-2020, 04:48 PM
Post: #2
RE: Using 4/5 AA cells as power source for spice calculators with broken battery clips
Excellent post and pictures! I will try that on my 33c.

I also have a set of 2/3 AAA batteries for my HP28s. They are just a bit shorter than N-cells and take pressure from the brittle battery door.
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06-03-2020, 06:42 PM
Post: #3
RE: Using 4/5 AA cells as power source for spice calculators with broken battery clips
That is exactly what I was looking for! So many SPICE cases are broken by using full size AA batteries, Now this has an end. Thanks for this idea.

Bernhard

That's one small step for a man - one giant leap for mankind.
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06-04-2020, 12:21 AM
Post: #4
RE:Using 4/5 AA cells as power source for spice calculators with broken battery clips
Very neat idea. If the battery contacts are not broken, you might be able to use a double-spring AA contact at the opposite end to bridge the two batteries, like this

[Image: 5228_tn.jpg]

https://www.mouser.com/ProductDetail/Key...4Mpg%3D%3D

Who decides?
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06-04-2020, 10:04 AM
Post: #5
RE: Using 4/5 AA cells as power source for spice calculators with broken battery clips
Perfect. I hesitated to give an advice when battery clips are still ok, as it is difficult to find an durable and reliable mechanical spring solution on the opposite end.

But with mentioned plate design it is easy to glue springs in original battery pack handle or in empty battery compartment. Thanks for sharing this resource.

(06-04-2020 12:21 AM)mfleming Wrote:  Very neat idea. If the battery contacts are not broken, you might be able to use a double-spring AA contact at the opposite end to bridge the two batteries, like this

[Image: 5228_tn.jpg]

https://www.mouser.com/ProductDetail/Key...4Mpg%3D%3D
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