[34S] battery warning indicator / question

[John Galt]

(04-01-2015 04:15 PM)Marcus von Cube Wrote:  Whenever a computation takes longer than a fraction of a second, the speed is increased (in two steps) up to the maximum speed of some 30 MHz.

Even in SLOW mode?

[Marcus von Cube]

(04-02-2015 07:57 PM)John Galt Wrote:  Even in SLOW mode?

SLOW mode limits the frequency to about half the maximum in all cases.

Quote:@Marcus: Do you know the voltage threshhold at which the calculator resets?

No, I don't. I assume that depends on the internal resistance of the batteries. If the resistance is high, high currents lead to a dramatic voltage drop. If the cells can no longer provide the current requested, the processor fails.

[John Galt]

(04-02-2015 07:19 PM)Bernd Grubert Wrote:  @Marcus: Do you know the voltage threshhold at which the calculator resets?

I performed some measurements that may interest you. The WP 34S will run reliably all the way down to 2.35 V.

Unfortunately that value is completely irrelevant and totally useless, for the very reasons Marcus explained. Depending on the particular battery's internal condition, when its no-load voltage becomes that low, a very small load can cause it to decrease precipitously. The calculator cannot tolerate the slightest power interruption, and will reset at about 2.33 V, even if it goes that low for an instant.

[Bernd Grubert]

(04-02-2015 09:39 PM)John Galt Wrote:  I performed some measurements that may interest you. The WP 34S will run reliably all the way down to 2.35 V.

Thanks for providing the voltage values. This means that a large capacitor might help. If e.g. the residual voltage of a weak battery is 2.8V a voltage drop of 0.45 is tolerable. If the capacitor has an internal resistance of 30 Ohm and 1.5F capacity, it can deliver 10mA for about 22s until the voltage gets below threshold.
I will try to get one and check whether it helps.
Regards
Bernd

[John Galt]

OK, but why do you need 22s? The capacitor required will be enormous.

I think you would be better off installing bigger batteries, but that's just my opinion. I don't intent to discourage experimentation, I enjoy doing that myself.

[Bernd Grubert]

(04-03-2015 09:59 PM)John Galt Wrote:  OK, but why do you need 22s? The capacitor required will be enormous.

No, it is not. It's dimension are 19mm dia. x 5mm height. A little bit more than the batteries that are use inside the HP 30B.

(04-03-2015 09:59 PM)John Galt Wrote:  I think you would be better off installing bigger batteries, but that's just my opinion. I don't intent to discourage experimentation, I enjoy doing that myself.

May be you're right, but I don't want to use AAAA-batteries. Taking apart 9V batteries and soldering them in anytime you have to replace the batteries seems uncomfortable to me.

Regards
Bernd

[matthiaspaul]

(03-31-2015 09:02 AM)Bernd Grubert Wrote:  What about adding a gold cap as a back-up power source for demanding calculation?

My experiences with super capacitors are limited to double-layer capacitors (EDLCs). In general, they are great to deliver the relatively high-energy peak currents observed f.e. in conjunction with GSM wireless modems, however, I have found them to be not without their own set of shortcomings as well.

For example, they are extremely sensitive to over-temperature and over-voltage (both no problems in a calculator, though). Compared to ceramic capacitors, they have higher leak currents reaching the low micro-amperes range (not a problem in many applications, but certainly an issue in a calculator without a hard power switch and powered by small capacity coin cells).

Further, for a supply voltage of ca. 3V two capacitor cells would have to be used in series (as the maximum allowed cell voltage is typically in the range of 0.9-2.8V only), which would require the cells to be balanced. Passive balancing would increase the "leak" current by a factor of 10 or more, but even with active balancing the leak current could not be reduced below about 2 micro-ampere.

Another potential issue is the fact, that the charge current must be current limited, as an uncharged capacitor represents almost a short circuit to the power supply - and depending on how much current the supply can deliver, it may take seconds to charge the capacitor. Unless power planes are decoupled from each other, this can easily cause brown outs / resets in other devices connected to the same supply. Parts or PCB traces could be permanently damaged due to over-current. Without some sort of current limiter, the batteries could overheat (and in the worst case explode) when inserting fresh batteries. Much less of a problem in a coin-cell powered calculator, as the batteries would most probably be empty before bursting, nevertheless, this is an issue that needs to be taken into account.

One possible solution might be to add some circuitry to disconnect the super capacitor when the calculator is powered off (to eliminate the leak current when it hurts) or the battery voltage exceed the allowed capacitor voltage (even as little as 0.1V too much will damage the capacitor mid-term), to slowly (in order to avoid resets) charge it once the calculator is on and running at low speed, and to directly connect it to the power plane once it is fully charged *and* the calculator wants to run at a higher speed. With the super capacitor under firmware control, unless the battery voltage is above a certain threshold value, the calculator would not attempt to run at high speed until the capacitor is fully charged. Depending on if the capacitor is connected or not, the firmware would use different thresholds to determine when it needs to switch back to slow speed or has to power down. An approach like this might be suitable to get the most out of those little coin cells, but it's a bit more than simply dropping a capacitor into the existing design.

However, I think, the first and primary goal should be to find out, if the WP 34S suffers from similar high current spikes beyond the normal peak current (which we must assume as unavoidable under normal operation) as observed in the HP 15c LE, and if so, to eliminate them.
Even very short spikes would cause the calculator to reset, so if they'd exist and we could avoid them, we would already gain alot of headroom until the voltage under normal peak load drops low enough to reset the calculator. Such spikes may be too short to even measure them (with the calculator's hardware, that is), so it would be next to impossible to take them into account in any possibly more sophisticated power management schemes, except for by adding some empirically derived and undesirably large safety factor. As demonstrated above, a suitable ceramic capacitor may be enough to accomplish this goal already.
Most important is low leak current (since the capacitor remains connected at all times, and even a low leak current would eventually exhaust the batteries), followed by low ESR (so it can stabilize the power plane enough), followed by capacity (larger is better, but to eliminate the current spikes, something between 10-100uF is already enough, and typically, the other parameters weaken with increasing capacity).
Perhaps this would already stabilize the power plane long enough for the voltage detection code to get more "deterministic" readouts and thereby make the low battery indicator more reliably.

Greetings,

Matthias

[matthiaspaul]

(04-04-2015 07:40 AM)Bernd Grubert Wrote:  

(04-03-2015 09:59 PM)John Galt Wrote:  I think you would be better off installing bigger batteries

May be you're right, but I don't want to use AAAA-batteries. Taking apart 9V batteries and soldering them in anytime you have to replace the batteries seems uncomfortable to me.

This is exactly my thinking as well.

In general, I much prefer a calculator to be powered by AA- or AAA-sized batteries (even if it needs to be a few millimeters thicker), simply because they are readily available and this would allow me to use low self-discharge rechargeable NiMH batteries (Eneloop etc.).
I would also accept the much rarer AAAA batteries, if I could thereby avoid using coin cells, however, I would have no fun extracting them from 9V blocks and soldering them in every couple of months - even in DIY projects I prefer "clean" and professional-level solutions. This, unfortunately, rules out this otherwise nice idea for me, at least in the WP 34S.

Now, if a AAAA battery holder could be fitted... But that's just wishful thinking...

Greetings,

Matthias

[MarkHaysHarris777]

(04-05-2015 05:06 PM)matthiaspaul Wrote:  

(04-04-2015 07:40 AM)Bernd Grubert Wrote:  May be you're right, but I don't want to use AAAA-batteries. Taking apart 9V batteries and soldering them in anytime you have to replace the batteries seems uncomfortable to me.

This is exactly my thinking as well.

So, we've established that its generally a bad idea to power a calculator with coin cells. I'm planning to power my generic with lithium ion batts; have a charge cradle (you need to lay it down sometime when you're not using it) which doubles as a docking station for instrumentation bus and pen plotter (still working on making the pen plotter wifi...

Cheers,
marcus

[d b]

" which doubles as a docking station for...", and answers that nagging question, "where did i sit my calculator?" Now, if I could just train myself to put my keys next to the cradle, I'd be in business.

[Bernd Grubert]

@Matthias:
Thanks for your detailed answer. The capacitor I chose could be used with voltages up to 5.5V. Therefore there is no necessity for putting two in series. On the other hand the leakage current reduces the life time of the batteries, which is truly a disadvantage. The high current when charging the first may cause a problem, too. May be small capacitor for smoothing the spikes is a better solution. I will try this.

Regards
Bernd