HP25 Replacement Batteriy Type

[Ignatz]

Hi, all...



I’ve got an HP25 on its way to me, so was wondering about replacement batteries... I’d want rechargeables, flat-top, but does the battery chemistry matter? NiMh, Li-on, NiCad? Will they charge using the 25's charger, or will I have to use a dedicated charger?

I also have a 3D-printed battery compartment on its way to me, so I won’t have to butcher the original...

[Maximilian Hohmann]

Hello!

(11-20-2019 11:33 AM)Ignatz Wrote:  I also have a 3D-printed battery compartment on its way to me, so I won’t have to butcher the original...

What do you want to do with an original battery compartment that has corroded batteries in it? They will slowly "contaminte" everything else that is stored in the same box, like the proverbial bad apple... Eventually the old batteries will even swell and break their holder by themselves, I have seen that in differen calculators. So I think it is best to "butcher" the battery holder and get rid of the old (toxic!) NiCd batteries ASAP.

(11-20-2019 11:33 AM)Ignatz Wrote:  I’d want rechargeables, flat-top, but does the battery chemistry matter? NiMh, Li-on, NiCad?

Li-Ions are no good as they have the wrong voltage. You can either use NiCd or NiMh, they both work. Simple ordinary cells from the supermarket (they sometimes have rechargeable batteries in my part of the world) or electronic store. Personally I would stay away from NiCd because they contain very toxic Cadmium. Also, NiMH cells have a larger capacity. 2000 or 2500mAh work fine.

Regarding the charger: As already written in 100+ threads here, make a habit of not pluging that charger into the calculator. Ever. The biggest part of the Woodstock series calculators were (and are!) killed by their own chargers. Charge your batteries in a standard external charger instead. Since they have five times the capacity of the originals (which were 500mAh NiCd cells) the original charger is too "weak" for charging them anyway. Especially NiCd cells suffer from low current charging and can lose part of their capacity in the process ("memory effect"). This does not apply to NiMh cells, so another reason to use those!

Regards
Max

[Ignatz]

Excellent!!! Thanks for the comprehensive reply, Max!!! Vielen Dank für die umfangreiche Antwort, Max!!!

[Maximilian Hohmann]

Here are some pictures for illustration:

This is an HP-25 with a "butchered" battery holder in place. You really can't tell from the outside:



This is what the battery holder looks from the other side. All you have to do is remove the central bar between the batteries with a sharp knife. The spring is required and must be preserved. These springs are made from non-corrosive material and survive battery leakage. If it is encrusted with oxidation products just pour vinegar over it and it will become like new. The picture shows a set of very cheap NiMh Batteries from supermarket "Aldi" or "Lidl" which have been powering this HP-25 for ten years or so.



If you are careful with your knife it is possible to cut the bar on one side only and still be able to remove and insert batteries. From a little distance one couldn't even tell that the battery holder has been "butchered" in the process. On this picture you can see normal disposable batteries in the holder which work equally fine. I do not buy disposable batteries myself (as I fully agree with Greta on these issues...), but many of the vintage calculators that I buy come with batteries inserted which are still good for many hours of operation.



And here, most important of everything! , mark the polarity of the batteries in your holder. The calculator is not protected against overvoltage (therefore don't use the charger) and it is not protected against reverse polarity either.



Enjoy your little calculator - these are among my all-time favorites!

Max

[rprosperi]

Nice post Max!

These tips cannot be repeated too often, and the photos illustrate the points clearly and will be helpful to folks hesitant to modify the battery case.

One other option is HP sold a charging cradle for the Woodstock battery packs, model HP82028A, seen here. They are convenient and totally safe, but also quite rare, it took me years to find one.

[Ignatz]

Thanks, Max, Bob... that charger is the bee's knees... Max... I can’t see any pictures, just placeholders... is there a forum setting that I need to toggle?

[Maximilian Hohmann]

Hello Tim,

(11-20-2019 04:37 PM)Ignatz Wrote:  I can’t see any pictures, just placeholders... is there a forum setting that I need to toggle?

I don't know, but here are direct links to the pictures which should work in any case:
http://www.bombie.de/tmp/IMG13540_1024px.jpg
http://www.bombie.de/tmp/IMG13541.jpg
http://www.bombie.de/tmp/IMG13542.jpg
http://www.bombie.de/tmp/IMG13543.jpg

[Ignatz]

Max... is there any way you can PM me the photos? And the battery voltage needs to be 1.2 - 1.5 V, right?

[Maximilian Hohmann]

Hello Tim,

(11-20-2019 05:21 PM)Ignatz Wrote:  Max... is there any way you can PM me the photos? And the battery voltage needs to be 1.2 - 1.5 V, right?

I sent you a PM and yes, anything from 1.2 ... 1.5V should work.

Max

[Ignatz]

Thanks, Max... that’s weird... they’re there now....

[Martin Hepperle]

(11-20-2019 02:21 PM)Maximilian Hohmann Wrote:  [...]

[...]
Max

Hi,

I have a related question: is there a source for a replacement for the conical double spring as shown on the left? I have a battery box with a broken one (due to corrosion) and would like to replace it with something with the same looks.
All the modern battery boxes I have found so far only have single springs and a hollow rivet to the next cell in their plastic case.
Or do I have to wind my own from spring steel wire?

Martin

[Ignatz]

HP25 Batteries by Tim Danaher, on Flickr

Already supplied / butchered :0)

[Maximilian Hohmann]

Hello Martin,

(11-21-2019 08:45 AM)Martin Hepperle Wrote:  is there a source for a replacement for the conical double spring as shown on the left? I have a battery box with a broken one (due to corrosion) and would like to replace it with something with the same looks.
All the modern battery boxes I have found so far only have single springs and a hollow rivet to the next cell in their plastic case.
Or do I have to wind my own from spring steel wire?

I have not been able to find those double springs either. On eBay they sell single battery springs (e.g. article number 274094650108 on eBay Germany or search for "Batteriefeder AA" or "battery spring AA") which can easily be soldered together. The result does not look as nice as the original but it works well.

Somewhere (we had a an overnight guest recently and I had to clean up my mess in the guest room in a hurry by throwing everything into the closest boxes) I have a bag of these springs. As soon as I find them I can send you some it you want to try it out.

Regards
May

[Massimo Gnerucci]

(11-21-2019 08:45 AM)Martin Hepperle Wrote:  

(11-20-2019 02:21 PM)Maximilian Hohmann Wrote:  [...]

[...]
Max

Hi,

I have a related question: is there a source for a replacement for the conical double spring as shown on the left? I have a battery box with a broken one (due to corrosion) and would like to replace it with something with the same looks.
All the modern battery boxes I have found so far only have single springs and a hollow rivet to the next cell in their plastic case.
Or do I have to wind my own from spring steel wire?

Martin

Like these, do you mean?

More choices.

[Maximilian Hohmann]

Hello!

(11-21-2019 01:12 PM)Massimo Gnerucci Wrote:  More choices.

Beware that most of these (or all? I didn't check all listings) are for "18650" Lithium batteries which are 1 1/2 times as thick as AA cells and therefore will not fit the battery holder. The first link should be fine, but the cost+shipping is a bit on the high side...

Regards
Max

NB: The measurements of the original spring are: Total width: 28mm, coil diameter (= height): 15mm, distance of the centres: 15mm, uncompressed spring length: 10mm

[Dave Britten]

I'm using a pair of these in my 25, and they work great in the 3D-printed battery holder:

https://www.batteriesplus.com/productdet...a=2100nmft

Batteries Plus part number AA-2100NMFT.

The pack is still reading 2.519 volts after about a week of use on the off-the-shelf charge.

[Martin Hepperle]

Thank you for your replies concering the contact springs - I think I will modify (solder together) some of the single springs which I already have.
But I will keep an eye on eBay etc. using the examples you have shown with some new search terms.

Greetings,
Martin