HP97 battery pack rebuild

[Jlouis]

I couldn't find any information about this.

I suppose four AA does not fit, so, how can I rebuild one? Or it's better buy one on TAS?

Thanks for any enlightenment on this subject.

Cheers

JL

Edit I just found Katie's article about it, but I'm not sure If I can find sub C batteries in my country. Any Idea?

[d b]

Google up "tabbed sub c battery" and look for who is there or will ship to you.
I don't know which supplier is best. My favorite is now gone.

[Jlouis]

Thanks Den, I found some on eBay and a nice article there too.

One question: Is it ok to buy NiMH instead of NiCAD?

TIA

[d b]

(02-10-2016 07:41 PM)Jlouis Wrote:  Is it ok to buy NiMH instead of NiCAD?

In this case they are probably interchangeable.
In general i think they want a slower, longer charge. Katie has a good equation to figure what that is.

[rprosperi]

(02-10-2016 09:26 PM)Den Belillo (Martinez Ca.) Wrote:  

(02-10-2016 07:41 PM)Jlouis Wrote:  Is it ok to buy NiMH instead of NiCAD?

In this case they are probably interchangeable.
In general i think they want a slower, longer charge. Katie has a good equation to figure what that is.

I agree with Den, it will work, but with these relatively large NiMH cells, it would probably take 3-4 DAYS to fully charge (assuming using the normal HP-97 charger.

[TASP]

It's slow (yeah, 3-4 days) to charge my NiMH N cells in the svelte official HP N cell charger.

But it does work. I'm only using one HP41 at a time these days (usually flagship, but the full XM one gets called upon too. I'm monkeying with the card readers quite a bit and I'm quite happy with how it's all working.

I have several sets of N cells, and do check the occasional 41 that turns up mysteriously in the mailbox . . .

[Hans Brueggemann]

(02-11-2016 12:12 AM)rprosperi Wrote:  I agree with Den, it will work, but with these relatively large NiMH cells, it would probably take 3-4 DAYS to fully charge (assuming using the normal HP-97 charger.

The DC charge current into the battery pack is approx. Ich = 170 mA (measured on my HP-97) when using the standard HP-97 charger. for minimum overcharge of the battery pack with a given amp-hours of a single cell, you calculate the charging time as

Tch = 1.1 * Qcell / Ich (assuming a fully discharged pack!)

example: for the battery pack containing cells with 2400 mAh each, your time to fully charge the pack is
Tch = 1.1 * 2400 mAh / 170mA = 14.12 hours
(using HMS on your trusty HP-41C gives 14 hours, 7 minutes, 4 seconds)

[Dieter]

(02-11-2016 11:02 AM)Hans Brueggemann Wrote:  The DC charge current into the battery pack is approx. Ich = 170 mA (measured on my HP-97) when using the standard HP-97 charger. for minimum overcharge of the battery pack with a given amp-hours of a single cell, you calculate the charging time as

Tch = 1.1 * Qcell / Ich (assuming a fully discharged pack!)

This is not a minimum overcharge. Charging is a process with an efficiency more or less significantly below 100%. Typical values for average charge rates are 70...90%. That's why for a slow 0,1 C charge (in your example it's even less than that: 170/2400 = 0,07 C) a factor like 1,4 is common. This would yield 1,4 x 2400 / 170 = 20 hours here.

(02-11-2016 11:02 AM)Hans Brueggemann Wrote:  Tch = 1.1 * 2400 mAh / 170mA = 14.12 hours

Hmmm... my HP41 returns 15,53 hours for this. Or 15:31:46. ;-)

All this of course is only valid for a completely empty battery pack. Which cannot be checked exactly anyway. But the relative charging current is so low (0,07 C) that the batteries should not be harmed even if a half-empty pack gets the full charge.

Finally let's not forget: you may charge the NiMHs for the same time as you did with the old NiCds – and you'll get the same capacity. Longer charging times (up to 20 hours) will yield more capacity than the old cells, and this exactly is the advantage here.

Dieter

[Hans Brueggemann]

(02-11-2016 11:23 AM)Dieter Wrote:  

(02-11-2016 11:02 AM)Hans Brueggemann Wrote:  Tch = 1.1 * 2400 mAh / 170mA = 14.12 hours

Hmmm... my HP41 returns 15,53 hours for this. Or 15:31:46. ;-)

ha! you are correct one of my HP-41's ROMs must be bad!

(02-11-2016 11:23 AM)Dieter Wrote:  Finally let's not forget: you may charge the NiMHs for the same time as you did with the old NiCds – and you'll get the same capacity.

i concur

(02-11-2016 11:23 AM)Dieter Wrote:  Longer charging times (up to 20 hours) will yield more capacity than the old cells, and this exactly is the advantage here.

i agree with you that longer charging times will get you more mileage. however, i managed to kill several of those battery packs over time by being "too generous" on charging times. so, i always keep charging below 120% of rated amp-hours.

[Dieter]

(02-11-2016 11:57 AM)Hans Brueggemann Wrote:  i agree with you that longer charging times will get you more mileage. however, i managed to kill several of those battery packs over time by being "too generous" on charging times. so, i always keep charging below 120% of rated amp-hours.

Actually 1,2 is the "official" factor for NiCd and NiMH for average charging rates (let's say 0,1...0,5 C). Since new high-quality batteries usually have somewhat more true capacity than nominally rated (+5...10% is quite common) and low charging rates around 0,1 C are quite relaxed, the manufacturers often state an 1,4x factor which leads to the ubiquitous "14 hours at 0,1 C" recommendation.

Example: Although rated at 2450 mAh ("minimum capacity") Panasonic's HHR260SCP has an average capacity of 2600 mAh and the recommended standard charge is 260 mA (=0,1 C) for 16 (!) hours. This would translate to 24 h in the HP97.

It's good to hear that you measured the 97's actual charging current. For a 2400 mAh battery, these 170 mA are merely 0,07 C. If you take a look at the manufacturers' data sheets you'll often find the statement that permanent trickle charge can be applied with 0,03...0,05 C without harming the battery. Some battery types (especially those designed for high loads and fast charge) even have no problems with a permanent 0,1 C charge.

So I think in this case we can see things quite relaxed here. For hard facts simply consult the manufacturer's data sheet of the battery type actually used.

Dieter

[rprosperi]

So, my totally unsubstantiated, off the top of the head without data to support it reply has been shown to be exaggerated. Thank you Hans and Dieter for your thorough explanations.

I guess my basic point was that yes, NiMH cells, which are generally known to be of higher capacity and often self-discharge better, will work fine, but the charge time is often much longer (due simply to charger being designed for older, lower capacity cells), which may be undesirable, depending on how the anticipated use/charge cycles will be. As such, I should have just said it that way.

I am frankly surprised it is only in the range you calculate; subjectively it has seemed longer than that, but this is on other devices. Maybe someone that does this (rebuild a Topcat battery pack with NiMH cells) could test it and share the results here.

[Jlouis]

(02-11-2016 03:08 PM)rprosperi Wrote:  So, my totally unsubstantiated, off the top of the head without data to support it reply has been shown to be exaggerated. Thank you Hans and Dieter for your thorough explanations.

I guess my basic point was that yes, NiMH cells, which are generally known to be of higher capacity and often self-discharge better, will work fine, but the charge time is often much longer (due simply to charger being designed for older, lower capacity cells), which may be undesirable, depending on how the anticipated use/charge cycles will be. As such, I should have just said it that way.

I am frankly surprised it is only in the range you calculate; subjectively it has seemed longer than that, but this is on other devices. Maybe someone that does this (rebuild a Topcat battery pack with NiMH cells) could test it and share the results here.

Thanks Bob, I understood your explanation. Hans and Dieter technical explanation was perfect, too.
Thanks to Den, too.

I'm expecting this Topcat soon, and I know the printer and card reader does not work with a battery pack exhausted. Let's see if I get lucky and get a full operational unit, after rebuilding the pack. Otherwise, some repairs will be needed.

Cheers

JL

[Dieter]

(02-11-2016 03:08 PM)rprosperi Wrote:  I guess my basic point was that yes, NiMH cells, which are generally known to be of higher capacity and often self-discharge better, will work fine, but the charge time is often much longer (due simply to charger being designed for older, lower capacity cells), which may be undesirable, depending on how the anticipated use/charge cycles will be. As such, I should have just said it that way.

Charging times are roughly proportional to the charged capacity. What was the value for the original NiCds in the late Seventies? 1000 mAh? 1200? A current NiMH-cell offers twice or three times as much. If you want to take advantage of this, the charging time simply is 2x or 3x the original time. Of course can also charge NiMHs the way you did in the Seventies – and you'll get the same operating time as back then. So there is no specific disadvantage of NiMHs. With NiMHs you can only win in that they allow much longer calculator operation – after a proportionally longer charge.

(02-11-2016 03:08 PM)rprosperi Wrote:  I am frankly surprised it is only in the range you calculate; subjectively it has seemed longer than that, but this is on other devices.

Well, twice or three times the capacity simply means a doubled or tripled charging time. It's really that simple. ;-)

Dieter

[rprosperi]

(02-11-2016 11:15 PM)Dieter Wrote:  Well, twice or three times the capacity simply means a doubled or tripled charging time. It's really that simple. ;-)

Yes, I get that larger cells take longer, it just feels like it takes even longer than the supposedly linear behavior suggests. It seemed like it took 12+ hours to charge an original Topcat pack from dead, so 2-3 times that clearly is 3-4 days, right?

If only I had a calculator near me for moments like this...

[Jlouis]

I just bought from TAS 4 2800mAh sub C batteries, with tabs.

But I found in my country a seller advertising a (allegedely) 6000mAh NiMh sub c!

As the TAS ones should get here in 1 or 2 months, I'm gonna try the NiMh. I want to check it out.

It is really fun, isn't it?

Cheers,

JL

[d b]

Jlouis;
It's a type of fun that our wives and sweethearts swear there should be a 12 step program for.

[Dave Frederickson]

A suggestion for charging those high-capacity battery pack rebuilds - modify a Reserve Power Pack to accept a smart-charger similar to this one which can charge packs at 900 or 1800 mA.

[Jlouis]

Just a follow up of this thread:

I received the HP97, with the case (mint) and the HP original AC charger (bad condition, already repaired, but working, 13v AC without connect and 8v AC after conected to the calculator).

The calculator is extremely dirty, I will clean it later, as the will to turn the beauty on is huge.

It's Made In Brazil, year 1978.

It did not work with the ac charge pluged in (exausted battery pack on). It was erratic, sometimes on, most of the time off. As my sub C batteries have not arrived yet, I managed to plug four rechargeables AA bateries using an adapter and alligator connectors...and... it worked!

Batteries are total of 5.13v, so I think this is a good voltage.

Printer is dead so the card reader. Funny thing is when I turn it on, there is a buzz noise, like a little dc motor running, I suspect it is the card reader motor. I don't think it is normal, I'm gonna check the service manual, but if some of you have a clue of what is going on, I will apreciate.

Keys are working, but sometimes they double or triple the number keyed! But if my keystroke is fast, it is normal.

I tried to remove the case, but the screws were stuck. I sprayed just a little of lub. I'll try to remove it later. I think this calculator has never been opened before.

I'm amazed of the crisp, bright and BIG display!!! What a pleasure!!!

Well, I supose that I will be busy for the next weeks!!

Cheers

JL

P.S.: Sorry for my bad english

[Dieter]

(02-16-2016 04:11 PM)Jlouis Wrote:  It did not work with the ac charge pluged in (exausted battery pack on).

This might not have been a good idea.
Quote from the museum's battery section here and here:

"Warning! Most models can be damaged if run from the charger without good batteries installed. Only the manuals for the HP 35 and 45 models say that it's OK to run without batteries, but many collectors feel that even those should have batteries installed for maximum protection and long life."

You wouldn't be the first one to fry his calculator this way.

(02-16-2016 04:11 PM)Jlouis Wrote:  It was erratic, sometimes on, most of the time off.

Uh–oh...

Dieter

[Jlouis]

(02-16-2016 07:28 PM)Dieter Wrote:  

(02-16-2016 04:11 PM)Jlouis Wrote:  It did not work with the ac charge pluged in (exausted battery pack on).

This might not have been a good idea.
Quote from the museum's battery section here and here:

"Warning! Most models can be damaged if run from the charger without good batteries installed. Only the manuals for the HP 35 and 45 models say that it's OK to run without batteries, but many collectors feel that even those should have batteries installed for maximum protection and long life."

You wouldn't be the first one to fry his calculator this way.

(02-16-2016 04:11 PM)Jlouis Wrote:  It was erratic, sometimes on, most of the time off.

Uh–oh...

Dieter

Dieter, it was like that before I pluged in the batteries. It is working fine now, running with four AA rechargeable.

This warning does not apply to the 97..

I read many posts in the old forum, and many says although working erratically, power on a 97 with no or with exhausted batteries does not damage the calculator (this is also stated in the 97 service manual). There's a circuit that, although simple, protects the calculator.

The calculator now is functioning like a 67 with no card reader.

I'm planning to repair the printer and the card reader, but that's another chapter.

Thanks anyway.

Cheers

JL

Edited for better acuracy of information