WP-34S Battery Questions



#11

Hello,

I have a few questions on the batteries for the WP-34S.

1. At what BATT value (X.FCN->BATT) should the batteries be replaced at? I seem to be getting random resets of the calculator when BATT < 2.7.

2. Can the LIR2032 rechargaeble coin cells be used in place of the CR2032? The voltage per cell is 3.6V versus the 3V of the CR2032. Is there some limiter within the hardware which prevents damage from occuring with higher than spec voltage?

As an aside, I used the Serial I/O for the first time today, and was impressed by its functionality!

Edited: 28 Sept 2011, 6:39 a.m. after one or more responses were posted


#12

The 3.6V might be too much as the power goes directly to the processor.

The crashes worry me. Try the SLOW command which reduces the speed. I might need to adopt a different approach at the low battery detection thresholds.


#13

Yes, the SLOW command seems to prevent the resets from occurring. There does appear to be something going on at the lower battery levels. Let me know if I can assist in any way.


#14

My guess would be that we've not yet done enough calibration of the clock rates to the lower battery voltages. At some point we switch the 34S to slow mode. This point would seem to be too aggressive still.

The power management is Marcus's baby so I'm loathe to change much here.


- Pauli

#15

Hello again,

May I ask which activity caused the reset? (running a program, saving to flash, etc.)

And also:
- what is the brand of your batteries?

- have you experienced the problem with other batteries?

Because the HP platform current draw at full speed is so much out of the specified region of these batteries, I suspect that there might be significant variations between brands.

Regards,

Etienne


#16

I was executing math operations with the top row.
The batteries are Sony's, but I have seen it with Duracell as well at similar voltage levels. New batteries report BATT=3.0 or so, and no issues.

#17

AT91SAM7L128 supply voltages spec:

Normal operating range: 1.8V to 3.6V

Maximum operating: 4.0V

The ususal note from the data sheet:

Stresses beyond those listed under “Absolute Maximum

Ratings” may cause permanent damage to the device.

This is a stress rating only and functional operation of the

device at these or other conditions beyond those indicated

in the operational sections of this specification is

not implied. Exposure to absolute maximum rating conditions

for extended periods may affect device reliability.



I do not know what other components there are and their operating limits (LCD voltage should be regulated by the AT91SAM7L128).


-Bart

#18

Hello,

In response to your battery question:

Looking for a datasheet, I found a source Here .

The only conclusion is that I would strongly advise aginst it!

These are standard Lithium-Ion chemistry, which means that they are charged at 4.2V. So a fresh battery will expose your calculator to a little less than 4.2V, and will only decrease to a safer level of around 3.6V after having used 80% of its capacity (see figs. 7.1 & 7.2). I believe there is a strong risk of damaging your calculator, even after only a single try.

Moreover, to protect the LIR batteries against overdischarge, you would have to monitor "manually" their voltage quite regularly and take them out of the calc as soon as they reach 3V. Note that this is nearly the fresh voltage of a CR2032, so the software will not warn you at that point. Overdischarging these cells will ruin them very fast, and might even be a real hazard.

You can also see that their capacity is quite modest (approx. 1/6 of the CR2032).

OTOH, they woud not suffer from the 20mA load from the calculator.

I would like to see what would be achievable with an LiFePO technology. That should be voltage-compatible, but might offer an even slightly lower capacity.

Regards,
Etienne
[edit] clarifications


Edited: 28 Sept 2011, 7:13 a.m. after one or more responses were posted


#19

Thank you for the response. I was curious whether there were rechargeable alternatives to CR2032 cells, and stumbled upon these. I'll stay away from them !


#20

You're welcome.

You have been misleaded by the claim "rechargeable alternative to CRnnnn".

While looking for the technical data, I've encountered a lot of sites claiming the same, in several languages.

To me, that's almost a lie, as many devices could be damaged.

Happy calculating with your WP34S!


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