HP 41CL - Power Consumption

[Michael Fehlhammer]

Dear HP 41CL users,
I've got current supply problems with several of my CL calculators.
I have equipped them with lady cell rechargeable batteries which should have a significant higher capacity than HP's original battery pack, but a lower one than NIMH batteries, of course.
In several but not all of my CLs a calculator with fully charged batteries doesn't survive more than two weeks on the shelf, without being used. Then not even the battery indicator goes on, the calculator simply is dead. Investigation of the individual cells shows that always just one (the leftmost) is totally empty, whereas the other cells are ok. Of course we experimented with different cells.
Something must severely go wrong here. I would expect a CL with fully charged lady cells to keep its memory (without being run!) for at least two months instead of two weeks with rechargeable batteries (and several months instead of actually only 8 weeks max. with new non-rechargeable NiMH batteries). And I don't understand the onesided depletion of the set of battery cells.
Maybe electronics experts can give a hint what could be the reason of the current drain, and how to fix.
Also I'd be interested in CL owners' experience regarding battery durability (in "OFF" state, with rechargeable or with NiMH batteries).

[AndiGer]

Michael,
You should clarify ... NiMH ARE rechargeables ... Do you mean alkalines?

Andi

[Michael Fehlhammer]

Ah, yes I mean Alkaline Batteries.

My technican has taken a photo of this strange behavior. I this case, the 3rd NI-MH has a negative voltage and the other ones are ok.

[mfleming]

Rechargeable batteries (particularly NiMh) tend to have a very steep drop off in voltage at the end of their discharge curve. A 41CL will often behave erratically at the end rather than show the low battery indicator. I've tried several brands of rechargable N cells and have yet to find anything reliable. I've also seen the one cell dead and unrecoverable problem, which is why I gave up on rechargable cells.

HTH,
~Mark

[rprosperi]

I've had basically the same experience that Mark reports. I've noticed that when the overall set "dies" that typically, one of the batteries is usually more "dead" than the others, though honestly never checked them in-place to note if that one is always in the same battery holder position. I also don't recall a negative voltage battery. I guess that's past dead??

For my CLs, I use Alkaline cells and check them about monthly, and indeed a set will always last many months, though I admit I rarely use these for long run-time calculations, so perhaps a different usage scenario.

I can also report that I've used similar-looking mint-green NiNH cells and have found their quality to be uneven, though none of them have ever leaked. So far.

[Hans Brueggemann]

in a chain of cells, the weakest one (i.e., the one that drops to zero volts first) experiences reverse charging from the remaining cells, hence the reversed polarity on the dead cell.
when chosing cells, specifically when it comes to NiMH, the "mission profile" of the calculator becomes a factor of cell "mileage", too: when used on a regular basis, cell self-discharge is less important than total available charge (mAh), whereas in the case of sporadic usage, a cell with minimum self-discharge is preferable over those with max available charge (completely disregarding use of the cardreader, of course). so, you may actually get better "mileage between charges" by using cells that have less than the maximum available charge for a particular brand.

edit:
my HP-41CL (first gen., cardreader attached) draws approx.
115 µA when OFF, and
3.5 mA when IDLE.

[Monte Dalrymple]

(08-23-2021 03:46 PM)Hans Brueggemann Wrote:  my HP-41CL (first gen., cardreader attached) draws approx.
115 µA when OFF, and
3.5 mA when IDLE.

These numbers are consistent with those shown on page 6 of the 41CL Calculator Manual.

[Michael Fehlhammer]

Thank you guys for your answers.

Now we measured power consupmtion in "off" state of
- a version 4 CL with time module: 135 µA
without time module: 124 µA.
- a version 5 CL with time module clone: 390 µA. (!!!)
These numbers are not far away from those listed in the CL user manual.

If I had been aware of the drastic increase in power consumption with the time module clone, I wouldn't have ordered it and would use a classical time module instead.

But I still don't understand what is going on:
The NiMH lady cells are specified to provide 500 mAh.
Divided by 135 µA (or 390 µA, respectively), I would assume a set to last 154 days or five months (53 days or almost two months for V.5/ time module clone).
The actual life cylce is around two weeks only. That is too short by a factor of 4 to 10!

Even if there is some strange behaviour at the end of the discharging cycle which reduces the ideal life span, I can't believe that such effects (alone) are responsible for the observed ridiculous durability.

[mfleming]

Many of the no-name batteries don't actually meet their advertised ratings. Some of the better chargers can actually measure the rating through a discharge profile. Test, or have someone test, a few of your cells.

[Sylvain Cote]

(08-25-2021 05:53 PM)Michael Fehlhammer Wrote:  If I had been aware of the drastic increase in power consumption with the time module clone, I wouldn't have ordered it and would use a classical time module instead.

I hear you Michael.
In Monte defense, the Time Module Clone (TMC) consumption is clearly indicated at page 6 of the 41CL user manual. (as Monte said previously)
Also, consumption values for his Flexible Hardware Module (FHM), on which the TMC is based on, was published in his HHC 2018 presentation.
Sylvain
PS: I do not know which TMC revision you have, but some had issues, look on the status page and on Time Clone Errata.

[Hans Brueggemann]

(08-25-2021 05:53 PM)Michael Fehlhammer Wrote:  The NiMH lady cells are specified to provide 500 mAh.
Divided by 135 µA (or 390 µA, respectively), I would assume a set to last 154 days or five months (53 days or almost two months for V.5/ time module clone).
The actual life cylce is around two weeks only. That is too short by a factor of 4 to 10!
Even if there is some strange behaviour at the end of the discharging cycle which reduces the ideal life span, I can't believe that such effects (alone) are responsible for the observed ridiculous durability.

as has been pointed out before, in a failing battery pack there's always that one cell that capsizes first, while the others are still delivering. you may be able to identify that cell and replace it with a known good one to find out whether that brings your entire battery pack up to the numbers that you calculated.
one more point to note here: the mainboard's power supply (PS) is a regulated switch-mode converter, which will pull proportionally more current with decreasing battery voltage (which is to be expected: the load of the converter is constant, hence the input power has to be maintained constant despite decreasing input voltage). that said, once a cell goes down in a battery pack, the PS will pull more current in order to stay in regulation, thus bringing down the remaining cells even faster.
for example, a HP-41CX w/ CLONIX attached and RUNning a program consumes about 14 mA when the battery pack is fully charged at 6 VDC, but 25 mA when the battery pack is almost flat (i.e., 3.7 V, just before the LOW BAT warning kicks in).
so, your math is correct, but the actual value of the measured current you are using for runtime calculations depends heavily on the battery voltage.

[Michael Fehlhammer]

Thank you for your hints!

We will measure the capacities of our batteries and try to form matched groups of similar ones which discharge uniformly, in order to prolong battery life as long as possible.
And I will inform you about our proceedings in a couple of weeks.

[Geoff Quickfall]

Well, thought it was mine only. But alkaline N cells last about a month in ‘off’ and then die.

Version 2 board.

Any ideas? Could it be the standby power cap used for retaining the memory during a battery change?

Geoff

[Hans Brueggemann]

well, not an idea in terms of what a probable fault could be. but here you may find an idea about how to sense the supply current of your calculator:
Current sensing in battery compartments
it is just a stripe of Kapton foil "sandwiched" between two self-adhesive copper strips, so both strips are fully isolated from each other. the copper strips each have a super-flex wire soldered to them and are then "short-circuited" via a DMM set to mA mode.
(running out of data space here in the forum, and don't want to delete anything. hence the link to my post on eevblog.)

[Neve]

I have been using these exact same NiMh rechargeable batteries for years now, and they seem to hold up at least 1.5 to 2 months when not in use. I usually recharge them every month, which is the rate at updates are usually released. But when I forget, the next month they’re still good, albeit they fail in the middle of the update cycle.
One thing I did notice though, I tend to put them in my charger in the same order I take them out of the calculator, and the second one from the left, is always the one with the lowest reading and, consequently, the longest to complete charging.

PS: I recently updated a V5 CL board with a Time Module Clone from Monte. I haven’t noticed any significant changes so far. But again, it hasn’t been long enough for me to really notice anything.