Battery Charging Currents of Assorted HP-Calcs & Peripherals

[Hans Brueggemann]

as discussed in this thread, exchanging cells in battery packs with newer ones with higher capacities (Amp-Hour ratings), be it NiCad, NiMH, low-leakage NimH (for example "Eneloop") or Cold Fusion, always leads to the question as to how long you should charge your new battery pack in order get the longest run time of your HP-item without sacrificing lifetime of the batteries.
in order to calculate the required charging time (as discussed here), you need to know the charging current that your battery pack is subjected to. however, it is not sufficient to know what type of charger you are using, as the charging current may vary considerably depending on the HP-item where your battery pack lives in.
to facilitate calculation of charging times, find below a list of HP-items and their associated charging currents. all currents were measured with an HP-3468B _directly_ at the battery packs. be prepared for surprises:

HP-34C: 120 mA
HP-97: 170 mA
HP-65: 55 mA
HP-67: 55 mA
HP-75C: 100 mA
HP-82120A: 14 mA
HP-82161A: 350 mA
HP-82162A: 250 mA

[Dieter]

(02-11-2016 06:41 PM)Hans Brueggemann Wrote:  to facilitate calculation of charging times, find below a list of HP-items and their associated charging currents.

Ah, thank you very much for this useful list.

The only result I could possibly contribute is the charging current of the 82180A battery pack for the 41-series. Here I got 15...16 mA while your result is 14 mA. So rounding the listed value to 15 mA might be a good idea.

Dieter

[rprosperi]

(02-11-2016 06:41 PM)Hans Brueggemann Wrote:  be prepared for surprises:

HP-34C: 120 mA
HP-97: 170 mA
HP-65: 55 mA
HP-67: 55 mA
HP-75C: 100 mA
HP-82120A: 14 mA
HP-82161A: 350 mA
HP-82162A: 250 mA

Thanks Hans for testing and publishing these stats. Very interesting and some surprises indeed!

I would expect different, higher current for the later Topcat units, but I would never guess more than 2:1 difference.

Do you have a Topcat Reserve Power Pack (82037A)? I'd be curious what the current is from there?

At last, a good use for my 82161A ! (though I must admit I was very happy it worked when I found my set of tapes from the 80's a few years ago).

[Sadsilence]

(02-11-2016 10:24 PM)Dieter Wrote:  The only result I could possibly contribute is the charging current of the 82180A battery pack for the 41-series. Here I got 15...16 mA while your result is 14 mA. So rounding the listed value to 15 mA might be a good idea.

By system (when battery voltage goes high during charge time) all charging currents in HP calculators and peripherals decreases to the end of a charging cycle. So 16mA at start is right and 14 mA at end is right, too ;-).

HP67 normally starts with > 60 mA, and my HP82162 starts with around 300 mA. Both values decreases to TO's values.

[Hans Brueggemann]

(02-11-2016 11:18 PM)rprosperi Wrote:  Do you have a Topcat Reserve Power Pack (82037A)? I'd be curious what the current is from there?

i don't have the HP82037A, but from the HP-97 service manual, the HP82037A has a charge-current limiting resistor of 12 Ohms, whereas the HP-97 internally uses two resistors in series that add up to 12.9 Ohms while the HP-97 is switched off(!)
on top of that, the HP82037A's LED indicator adds about 18 mA to the charge current, hence i'd guesstimate the charge current of the HP82037A to be 200 mA.

[Dieter]

(02-12-2016 08:52 AM)Sadsilence Wrote:  By system (when battery voltage goes high during charge time) all charging currents in HP calculators and peripherals decreases to the end of a charging cycle.

In general I agree – especially if the charging circuit essentially consists of a simple resistor. ;-) But are you sure this also applies to the 82120A battery pack? It contains a PCB that is described here: There is a 78L12 voltage regulator and a resistor which should provide a (nearly) constant charging current. But I'm not at all an expert in electronics, so you or someone else may be able to say more about how constant the resulting charging current will be with this design. Please note the linked PDF at the bottom of the mentioned URL, here the last page says 16 mA.

On the other hand discussing about 1 mA more or less does not make much sense assuming components with ±5% or even ±10% tolerance... #-)

(02-12-2016 08:52 AM)Sadsilence Wrote:  So 16mA at start is right and 14 mA at end is right, too ;-).

Fine. So let's make it 15 mA on average. ;-)

Dieter

[Dave Frederickson]

(02-11-2016 11:18 PM)rprosperi Wrote:  Do you have a Topcat Reserve Power Pack (82037A)? I'd be curious what the current is from there?

I measured mine at 450 mA.

[rprosperi]

(02-13-2016 12:28 AM)Dave Frederickson Wrote:  

(02-11-2016 11:18 PM)rprosperi Wrote:  Do you have a Topcat Reserve Power Pack (82037A)? I'd be curious what the current is from there?

I measured mine at 450 mA.

Wow! I found mine today and will try it tomorrow. Not sure I have a dead Topcat battery pack, but I suppose the current shouldn't change that much until the end of the charge.

Not being a EE type guy, I have a dumb question. I'm sure the current is only valid if I test with the battery actually connected, so how do you physically measure it? Run some lead wires from the cradle terminals outside the case, then drop the battery onto the terminals/leads, and use a DMM to test the ends of the leads?

[Dave Frederickson]

(02-13-2016 01:10 AM)rprosperi Wrote:  I'm sure the current is only valid if I test with the battery actually connected, so how do you physically measure it?

I just connected a DMM directly to the Power Pack terminals. When I checked my Classic Power Packs this way I measured the expected 55 mA. With a battery connected to the Power Pack, using clip leads, I measured 220 mA earlier today and now after the battery has charged some it's down to about 100 mA.

I plan on modifying my Power Packs to accept a smart charger to faster charge the packs I've rebuilt using high-capacity cells. Optimum charging is done between 0.5 and 1 C, so even my 1000 mAH classic packs can be charged at 900 mA.

Dave

[rprosperi]

(02-13-2016 03:05 AM)Dave Frederickson Wrote:  With a battery connected to the Power Pack, using clip leads, I measured 220 mA earlier today and now after the battery has charged some it's down to about 100 mA.

Thanks Dave - can you post a pic showing the kinds of leads best for doing this?

(02-13-2016 03:05 AM)Dave Frederickson Wrote:  Optimum charging is done between 0.5 and 1 C, so...

C ?

[Dave Frederickson]

(02-13-2016 03:38 AM)rprosperi Wrote:  

(02-13-2016 03:05 AM)Dave Frederickson Wrote:  With a battery connected to the Power Pack, using clip leads, I measured 220 mA earlier today and now after the battery has charged some it's down to about 100 mA.

Thanks Dave - can you post a pic showing the kinds of leads best for doing this?

(02-13-2016 03:05 AM)Dave Frederickson Wrote:  Optimum charging is done between 0.5 and 1 C, so...

C ?

Any type of test lead with some sort of grabber on the ends would work. Pomona makes a wide selection, but the alligator leads I used I got from Radio Shack when I was in high school.

C: Charge Rate. Section 1 in this Wikipedia entry on battery chargers.

Dave

[rprosperi]

(02-13-2016 06:35 AM)Dave Frederickson Wrote:  Any type of test lead with some sort of grabber on the ends would work. Pomona makes a wide selection, but the alligator leads I used I got from Radio Shack when I was in high school.

Thanks. I have some of these, likely nearly as old; just wondering how you connect to both the tabs in the cradle and the recessed battery terminals at the same time. Initially I was thinking of how to do it with battery actually in the cradle, on top of the clips/tabs, but it's likely easier with the battery outside the cradle. I need to fiddle a bit with different leads.

[Sadsilence]

(02-12-2016 07:46 PM)Dieter Wrote:  In general I agree – especially if the charging circuit essentially consists of a simple resistor. ;-) But are you sure this also applies to the 82120A battery pack? It contains a PCB that is described here: There is a 78L12 voltage regulator and a resistor which should provide a (nearly) constant charging current. But I'm not at all an expert in electronics, so you or someone else may be able to say more about how constant the resulting charging current will be with this design. Please note the linked PDF at the bottom of the mentioned URL, here the last page says 16 mA.

On the other hand discussing about 1 mA more or less does not make much sense assuming components with ±5% or even ±10% tolerance... #-)

Fine. So let's make it 15 mA on average. ;-)

Dieter

Of course you are right. Around 15 mA is a perfect compromise ;-).

Just for info: Curcuit plan from that fabulous site tells the story. Current is limited by R1 resistor, which is given with 365 Ohm. So theoratical current depends on Battery voltage which increases during load time. Let assume a totally empty pack with 4V remains voltage. Icstart = (Usupply - Udiode - Ubatstart)/ R1 = (12V - 0.7V - 4V) / 365 Ohm = 20 mA. When completely loaded Ubat has around 1.5V/cell for NiCD and 1.6V/cell for NiMH, in total 6V or 6.4V. But battery voltage is limited by a 6.2V Zener diode. So worst case Icend = (Usupply - Udiode - Ubatend)/R1 = (12V-0.7V-6.2V)/365 Ohm = 14 mA. But as you mentioned. There are a lot of components with tolerances...

Best regards,

Werner

[Sadsilence]

(02-11-2016 11:18 PM)rprosperi Wrote:  Do you have a Topcat Reserve Power Pack (82037A)? I'd be curious what the current is from there?

My results from weekend

HP 82037A Reserve Pack: Ic(dc) = 215 mA ... 200 mA
HP 82028A Reserve Pack (Woodstock): Ic(dc) = 45 mA ... 40 mA

But I feel at least for HP 82037A that is only half of the story as my 2850 mAh rebuilt Topcat pack (discharged in a racing car charger completely before) was fully loaded within 12 h (I checked cell voltages at the same time). That is impossible with NiMH technology.

Useing a truerms DMM (Agilent U1252B) reveals that there is a great AC ripple of 170 mA(!) on charging current, which leads to an effective load current of around 270mA.
Sorry no oscilloscop here. Maybe charging curcuit capacitors within 82037A do not work any more or only partially due to age. Have to check.

[rprosperi]

After digging out my Topcat charger, and then some remedial DMM lessons from Dave, I can also confirm 200mA. I didn't really doubt it, just wanted to use the opportunity to learn a little. And I did.