####################################################################
# ATTENTION: This post is a reference to a website. The poster of #
# this Usenet article is not the author of the referenced website. #
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<URL:https://partsbox.com/blog/wireless-charging-for-a-hp-25-calculator-
05-2021.html>

The text below is a quotation from the URL above:
>
> Or, bringing a 45-year old calculator into the 21st century.
>
> The HP-25 was a revolutionary calculator for its time. Introduced in
> 1975, it was the first affordable programmable engineering calculator.
> As years passed, more advanced models were developed with more
> functionality. But the 25 struck a particularly good balance between
> features, size, and ease of use.
>
> Today we live in a very different world: we all have multiple powerful
> computing devices. No engineer would even consider doing graphing on a
> calculator today: we have computers that can do this faster and better.
> Similarly for algebraic manipulations or symbolic computing. What we
> still need, however, is a good engineering calculator - an easily
> accessible device that can perform simple calculations quickly and that
> has just the right number of functions.
>
> This is why the HP-25 is becoming attractive again. If you work with
> electronics or mechanical design, you often need to work with scientific
> or engineering notation and perform calculations for which the simple
> calculator apps on your phone are too limited. The simple
> keystroke-based programmability means that you can quickly automate
> tasks: for example, I often have the parallel resistance formula entered
> as a program, accessible using a single keystroke. Enter two
> resistances, press R/S and get the result. The HP-25 was designed by
> engineers, for engineers, and you feel it when you use it. Importantly,
> it fits well into your palm and can be used with a single hand, which
> isn't true for many otherwise excellent later HP calculators (like the
> Voyager series: HP-11C and HP-15C).
>
> The HP-25 I have was bought by my Dad, around the time I was born. I
> always enjoyed using it, but this whole series of calculators (dubbed
> "Woodstock") was limited by battery pack design. The original battery
> pack contained two sealed NiCd cells, which obviously failed many years
> ago. Most people replaced their NiCd cells with new ones, then with NiMh
> cells, or even alkaline AA batteries. This was always problematic: newer
> batteries were slightly larger and never fit well. Also, the power
> consumption of a calculator with an LED display was significant, so
> frequent battery replacements were needed.
>
> The HP-25 was shipped with a "charger" (really just a transformer), but
> the charging circuit was terrible: the charger without load supplied 10V
> AC: a higher voltage than the calculator could handle, with the
> assumption that connected NiCd battery cells would clamp the voltage.
> The entire charging circuit consisted of a diode and a resistor! If your
> cells were not making proper contact, or if you connected a charger
> without the battery pack inserted, your calculator would be toast.
>
> I decided I should do something to make my HP-25 usable every day. So, I
> set out to design a rechargeable battery pack, which replaces the
> original one, but uses a modern Li-Po battery, and has Qi/WPC wireless
> charging with a USB fallback.
>
> ...

Rich <rich@example.invalid> wrote:
> ####################################################################
> # ATTENTION: This post is a reference to a website. The poster of #
> # this Usenet article is not the author of the referenced website. #
> ####################################################################
>
> <URL:https://partsbox.com/blog/wireless-charging-for-a-hp-25-calculator-
> 05-2021.html>
>
> The text below is a quotation from the URL above:
>>
>> Or, bringing a 45-year old calculator into the 21st century.
>>
>> The HP-25 was a revolutionary calculator for its time. Introduced in
>> 1975, it was the first affordable programmable engineering calculator.
>> As years passed, more advanced models were developed with more
>> functionality. But the 25 struck a particularly good balance between
>> features, size, and ease of use.
>>
>> Today we live in a very different world: we all have multiple powerful
>> computing devices. No engineer would even consider doing graphing on a
>> calculator today: we have computers that can do this faster and better.
>> Similarly for algebraic manipulations or symbolic computing. What we
>> still need, however, is a good engineering calculator - an easily
>> accessible device that can perform simple calculations quickly and that
>> has just the right number of functions.
>>
>> This is why the HP-25 is becoming attractive again. If you work with
>> electronics or mechanical design, you often need to work with scientific
>> or engineering notation and perform calculations for which the simple
>> calculator apps on your phone are too limited. The simple
>> keystroke-based programmability means that you can quickly automate
>> tasks: for example, I often have the parallel resistance formula entered
>> as a program, accessible using a single keystroke. Enter two
>> resistances, press R/S and get the result. The HP-25 was designed by
>> engineers, for engineers, and you feel it when you use it. Importantly,
>> it fits well into your palm and can be used with a single hand, which
>> isn't true for many otherwise excellent later HP calculators (like the
>> Voyager series: HP-11C and HP-15C).
>>
>> The HP-25 I have was bought by my Dad, around the time I was born. I
>> always enjoyed using it, but this whole series of calculators (dubbed
>> "Woodstock") was limited by battery pack design. The original battery
>> pack contained two sealed NiCd cells, which obviously failed many years
>> ago. Most people replaced their NiCd cells with new ones, then with NiMh
>> cells, or even alkaline AA batteries. This was always problematic: newer
>> batteries were slightly larger and never fit well. Also, the power
>> consumption of a calculator with an LED display was significant, so
>> frequent battery replacements were needed.
>>
>> The HP-25 was shipped with a "charger" (really just a transformer), but
>> the charging circuit was terrible: the charger without load supplied 10V
>> AC: a higher voltage than the calculator could handle, with the
>> assumption that connected NiCd battery cells would clamp the voltage.
>> The entire charging circuit consisted of a diode and a resistor! If your
>> cells were not making proper contact, or if you connected a charger
>> without the battery pack inserted, your calculator would be toast.
>>
>> I decided I should do something to make my HP-25 usable every day. So, I
>> set out to design a rechargeable battery pack, which replaces the
>> original one, but uses a modern Li-Po battery, and has Qi/WPC wireless
>> charging with a USB fallback.
>>
>> ...
>

GPS is also available for the 25C now.