HP-16C current consumption

[Sadsilence]

Last days I played around with my HP-16C after batteries had been empty. Voyager series is known for it's extremely low current consumption at time of invention. But how low is extremely low?

Instead of a new set of nonrechargeable batteries I obtained 3x Varta V40H NIMH rechargeable batteries of matching size. Really nice cells which even had a capacity of 43 mAh being above data sheet.

With a usable capacity of about 40mAh (my HP-16C starts flashing battery indicator at 3,533V) HP-16C runs following minute counter program for about 68hours:

Code:


001 - 43,22,A    LBL A
002 - 42,45,0    FLOAT 0
003 -   43 35    CLX
004 - 43,22,0    LBL 0
005 -    45 2    RCL 2
006 -   44 32    STO I
007 -      33    Rv
008 - 43,22,1    LBL 1
009 -   43 23    DSZ
010 -    22 1    GTO 1
011 -       1    1
012 -      40    +
013 -   43 34    PSE
014 -    22 0    GTO 0
015 -   43 21    RTN

266 STO 2
GSB A

Results:
Battery capacity [mAh]       40
Start time                   10.07.2017 11:33:00
Stop time                    13.07.2017 07:57:20
Run time                     68:24:20
HP-16C minute counter        4106 = 68:36:00
Precision counter [%]        0,284
Batterie indicator limit [V] 3,533
Av. current consumption [uA] 585

About 600 uA is a really impressive value for a 35 years old calculator. Other asthonishing result is, that HP 16C is only able to execute 266 DSZ loop counts per minute. 266 is calibration constant that worked best for my calculator to reach one minute cycle.

[Dave Frederickson]

How are you measuring the current? I understand that DMM's have a burden resistor that affects low current measurements. Consequently, there are devices that overcome that limitation. For example:
https://www.eevblog.com/projects/ucurrent/

[Ardvan]

Don't forget about the self discharge of these rechargeable batteries.

The specification sheet says: Charge Retention at 20C: 90%
So your rechargeable batterie looses 10% of its energy per month in addition to the current your calculator will need.

If you use the calculator regularly then this will work. If you leave the calculator off for many months this will not work, because then the calculator will probably be discharged when you need it.

[Sadsilence]

(07-13-2017 05:01 PM)Dave Frederickson Wrote:  How are you measuring the current? I understand that DMM's have a burden resistor that affects low current measurements. Consequently, there are devices that overcome that limitation. For example:
https://www.eevblog.com/projects/ucurrent/

Honestly I did not measure current at all. It is simply a matter of calculation. A battery with a capacity of 40 mAh (that is what I measured) runs out in 68h:20min:

40 mAh / 68,33 h = 0,585 mA = 585uA

Of course there may be an error of a few percent, because battery capacity is taken from previous runs. Nevertheless exact result is surely between 550uA and 600uA which is more than impressive.

Measuring current of a running processor with a DMM is difficult cause current consumption changes within ms or us (nowaday within ns) and may irritate DMM's low path filter.

[Sadsilence]

(07-14-2017 06:49 AM)Ardvan Wrote:  Don't forget about the self discharge of these rechargeable batteries.

The specification sheet says: Charge Retention at 20C: 90%
So your rechargeable batterie looses 10% of its energy per month in addition to the current your calculator will need.

If you use the calculator regularly then this will work. If you leave the calculator off for many months this will not work, because then the calculator will probably be discharged when you need it.

Of course you are right and beginning with Voyager series it does not make sense to use rechargeable batteries within pocket calculators. This time I accidentally had seen those V40H cells and grabbed change to measure current consumption in described way. Just curiosity. Honestly it does not make sense to own 20 or more pocket calculators, too ;-).

Normally I have 2 sets of rechargeable batteries for every device and change them every 3 months doing a discharge/charge cylce. So even non LSD NiMH batteries can be used for many years. For classics series up to 30 series I use LSD cells anyway. Hopefully there may be some 1/3 AAA cells with LSD feature for HP41 rechargeable battery pack.