Battery power Xenon

Hardware
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I’m building a sensor node around a Xenon that will have to operate around -20 degrees C. I don’t want to bother with a rechargeable battery, because LiPo doesn’t do well at those temperatures and I expect a battery life of at least a year (I plan to choose the sensing frequency based on that requirement).

I believe my best option is to power it from the Energizer Lithium AA batteries because of the excellent low temperature performance, but I can’t find any information about powering 3rd gen boards from AA batteries. 2 batteries in series connected to the 3.3V pin won’t work since the 3V minimum is too high as the batteries get used up.

If I put 3 in series on the +5V input will that work? I’m not sure the voltage will be high enough for the regulator. And if it will, how does the regulator work, is the power from the excess voltage wasted as heat like a linear regulator or will the input current be lower than the output current to efficiently use the power?

Thanks.

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The datasheet blockdiagram shows the regulator used on the Xenon.
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The collected datasheets for the Xenon can be found here

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Use something like this with a 4 x AA pack (or even 2 x AA pack).

Buck Boost convertor

Minimal energy loss for a simple solution.

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@shanevanj Thank you for the response. I thought about going that route, but I was hoping to find out if it was actually necessary with the particle hardware.

In the datasheet, Particle keeps using different terms which makes it very hard to read, but it appears that the "LiPo Battery voltage" should nominally be 3.3 to 4.4V, and the "Battery Input Voltage" should have an absolute maximum of 6.5V. Assuming (isn't it great to have to assume when reading technical datasheets) that these two terms both refer to the "Li+" pin on the Xenon, then I should be able to connect 3 AA batteries in series to Li+. New batteries should be 4.8-4.9V, which will pass the 4.4V nominal range, but still be well within the 6.5V absolute maximum. By the time the batteries drop to 3.3V (1.1V per cell), they can be considered depleted, so the low end is fine.

But I'm not comfortable with these assumptions, I still don't know if Li+ will accept 3.3 to 6.5V without frying the device, and I don't know where this "supply input voltage" with an absolute maximum of 6.2V fits in. I also don't know if the extra power from the higher voltage will be efficiently converted on the Xenon (as it will with the part you linked) or dissipated as heat in the regulator, so it would be nice if someone more familiar with the boards could help. Particle could take lessons in documentation writing from Pololu.

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@ScruffR Before posting here, I read the relevant Xenon documentation, and the only useful thing I could find was the part number of the regulator used. Then I read the datasheet for the regulator and wasn't able to find any answers to my questions. For example, with the regulator set for 3.3V, what is the minimum input voltage? It mentions problems if the input voltage isn't enough above the output, but doesn't say what enough is. I gather it depends on the current.

It also describes the theory of operation at a level that assumes a great deal of knowledge and I wasn't able to tell from that anything at all about efficiency. I mean buried within the 3 pages on theory of operation it does mention

The DC/DC converter characteristics depend greatly on the externally connected components as well as on the characteristics of this IC, so refer to the specifications and standard circuit examples of each component when carefully considering which components to select

which means Particle should know the characteristics in this particular application better than the part manufacturer. And since there's nothing about regulator efficiency in the particle docs, asking the question here seemed appropriate.

Between that and googling directly for an answer to my question, I spent a good hour trying to answer the question on my own before posting here. But thanks for those links.....if the answer is actually in there could you please be a bit more specific/noob friendly? I did spent a lot of time with those materials and could not find it.

Googling for "power particle xenon from AA batteries" yields no results (along with similar seaching for particle iot products. So it really doesn't seem like such a simple thing.

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I have built quite a few prototypes using older Arduino 3v3 and 5v dev boards and the on board regulators are just “adequate” - pretty soon I went to the Polou solution for stability and heat management for input power as it was just one less thing to worry about.

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I see now you have done some research before posting, but this sentence in your original post ...

... suggested that you weren't aware of the used regulator as it is a switching regulator which don't dissipate the excess voltage as heat in any way like a linear regulator. Sure some energy will be transformed to heat, but not as the "main" means of regulating the voltage.

The fact that the efficiency can't be given as a single figure in any of the datasheets (Particle's or Torex's) is rooted in the same problem - neither company knows what the enduser will demand from the device. While Particle could give a figure for the Xenon for a default state (e.g. "all external GPIOs in Hi-Z state, mesh radio on, everything else off, no LiPo connected") there would be little use in that figure in any other case - especially when connecting external components like sensors, switches, LEDs, ...
The efficiency also changes with the input voltage, hence the used battery chemistry characteristics impacts the figure but is unknown to Particle or Torex.

However, the Torex datasheet does provide some charts where the end-user can get the points that are most likely of interest for each of the intended/likely use-case states

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Pages 17/28 and following of the Torex datasheet are where you want to look at mainly

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2.8V minimum was mentioned in another Thread, here. But as you said, the 3.6V max (Open Circuit Voltage) may be too high to direct drive the Xenon with (2) Energizer L91's, IDK. The Voltage will be closer to 3.3V when loaded, but I've never tried this with a Xenon.

3xAA L91's (5.4 OCV) would work when feeding Vusb, and might be fine for Li+ Pin, maybe someone can confirm.

Considering you plan to operate at -20 degrees C, I'd vote to go w/ the higher starting Voltage (3xAA 5.4 OCV) and use the most efficient Path available, or an external Buck/Boost converter as previously mentioned. Either way, you'll want to test the actual Sleep & Operating Currents @ -20 degrees C to calculate your Power Budget.

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@shanevanj that is a good point to consider. But the official 5V Arduino Uno uses a NCP1117ST50T3G voltage regulator which is a linear regulator meaning it will burn off excess voltage as heat. Afaik, there are no official 3.3V Arduino boards so it’s hard to know what you were using. The first board I looked at was the Sparkfun Arduino mini 3.3V board. It uses an MIC5205 which is also linear. The Particle boards may effectively have the Pololu type of regulator on board which is what I’m hoping to learn.

@ScruffR Thanks for your more detailed answer. I did read that the XC9258 is a switching regulator, but I didn’t know what that meant, nor did I understand that that was the important part to have googled for more information. Of course i’ve heard of switching power supplies as all the modern adapters we use, but that knowledge didn’t help. I’m looking at the graph you linked, and I should be above 85% when the device is fully on which will be fine. I guess in standby mode when the power is hopefully well below 1mA, I don’t have to worry too much if efficiency is below 50%?

It doesn’t help though with knowing if it’s okay to power the device with 3AA batteries that will likely give me around 5V when new and minimally loaded. I really don’t want to destroy the Xenon. I guess it’s too much to hope for official docs on powering the devices besides “use USB or a LiPo” :slight_smile: Is there really nobody interested in AAA to D batteries, Alkaline or NiMH?

@Rftop I could be wrong about this too, but I believe powering via VUSB will consume enough power in standby mode to make standby mode moot and long term battery power not viable. I haven’t yet dealt with low temperature or the need for long runtime, but usually I just connect 4 1.2V NiMH cells to USB power. USB spec is 4.75-5.25V so that hasn’t been a problem.

I don’t like the idea of direct powering the 3.3V line off batteries. Years ago, I destroyed a handful of PIC MCUs by not paying attention to no load voltage even though voltage under load was well within spec. The nRF52840 is far more delicate and less forgiving than the PICs and the Xenons are more expensive to replace too. I definitely have to go through a regulator, I just hope the onboard one is suitable.

If I do need the external buck/boost route, I will probably go with the Adafruit breakout of the LM3671 buck converter and 3 of the energizer lithium cells. Does that sound like a reasonable choice? These options seem pretty expensive, but a lot better than dead Xenon.

You’re right about the sleep and operating currents too. At this point, I’m still at the level of back of napkin calculations, but I’m hoping for under 50mA operating, under 0.1mA sleeping. And under 10 seconds to wake up, connect to mesh, take and transmit the sample, and go back to sleep. I don’t know yet how realistic that is.

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The Torex datasheet does specify the max operating voltag with 5.5V (p. 6/27)

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That should be your red line. The absolute max. rating of +6.2V is nothing you should consider getting near.

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I have a low temperature sensor application where I plan to use 2xAA L91 batteries and a Pololu buck boost converter to supply 3.3 vdc to the 3V3 pin on the Xenon with the X9258 regulator disabled. I initially considered using 3xAA L91’s(5.4 OCV) connected to the Li+ pin. While the 5.4 OCV is less than the maximum input voltage for the X9258 regulator, this same voltage is also applied to the R15/R16 voltage divider circuit that is directly connected to the AIN3 (BAT_DET) input on the nRF52840. The resulting ~3.8vdc applied to the analog input could be a problem.

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@tsand That’s an excellent point I never would have thought of. It looks like R15 and R16 are 806k and 2.1M, so AIN3 will have 72% of of Li+ on it. And From page 612 of the Nordic datasheet, the absolute maximum for all IO is Vin+0.3V or 3.6V. That would limit Li+ to an absolute maximum of 4.9V without damaging the SoC, and a recommended maximum of 4.5V.

The Xenon datasheet says the maximum for “Battery input voltage – V(LiPo)” is 6.5 Volts. Is this an error on the datasheet?

@ScruffR I see you’re right about 5.5V being the red line for the regulator, but for the reasons above, would the red line for the Xenon actually be 4.5V, despite the datasheet saying 6.5?

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True, but IIRC that only applies when feeding via the Li+ pin.
VUSB is not the same rail as Vbat - I don’t think I was talking about the Li+ pin but meant to refer to VUSB (sorry if that wasn’t clear :blush:)

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@ScruffR Okay, I understand what you’re saying now. The Xenon datasheet says

The pin is internally connected to the VBUS of the USB port. The typical output should be around 4.5 to 5 VDC when the device is plugged into the USB port and 0 when not connected to a USB source. You can use this pin to power peripherals that operate at such voltages. Do not exceed the current rating of the USB port, which is nominally rated to 500mA. This pin is also protected with an internal fuse rated at 1000mA.

There is nothing in the datasheet to lead me to believe I can power the Xenon via that pin. I wonder if that’s where it means by “Supply Input Voltage 6.2V” though. I saw that spec but didn’t know where it expected me to connect a supply voltage. And I didn’t think that mattered because of the V(LiPo) 6.5V spec.

Anyway, at this point, it seems that my best bet is to use an external regulator as others have suggested. The Pololu part that was discussed looks good, but the best way I can find to get my hands on one in Canada is $7 plus $11 shipping.

I’m looking at This regulator on AliExpress. Will it be suitable for the Xenon with 3 AA cells? I’m kind of concerned about the reliability of parts from AliExpress, and it may be draining the batteries while I have the Xenon in sleep mode.

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i’m curious if the multiple references to using a single cell battery in the datasheet for the xenon can be safely ignored? thanks.

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There is. You may just not have understood that this sentence: "The pin is internally connected to the VBUS of the USB port.", implies exactly that.
I also linked to the datasheet and schemaitc in the second post of this thread where you can also see what that sentence means.

What do you mean by that?

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the xenon datasheet states "If you want to make your projects truly wireless, you can power the device with a single cell LiPo/Lithium Ion (3.7V). " which i take to mean that one should use a lipo with 1 cell vs. a lipo with multiple cells or multiple lipo batteries. am i misunderstanding this?

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A single LiPo cell has a nominal voltage of 3.7V.
Multi-cells usually have the cells connected in series which would give you cell-count times 3.7V.
You could of course have mulit-cells in parallel to double current but for that you’d need cell balancing charging circuitry.

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is it ok to use different chemistry, like Ni-MH? I have some 1.2V AA when charged are ~1.3V.

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You should be able to use NiMH to power the device, but I’m to sure the charging chip would work with that chemistry (haven’t checked tho’).

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@ScruffR

There is. You may just not have understood that this sentence: “The pin is internally connected to the VBUS of the USB port.” , implies exactly that.

Implies means you have to guess, which is extremely dangerous with a datasheet. The datasheet also explicitly says you can connect up to 6.5V on V(LiPo), which thanks to the schematic, we know is a "release the magic smoke" class of mistake. I don't think guessing what the datasheet is implying is a good idea with this document.

The schematic is helpful, but it's not the most easy to read, it would would if the pins were more clearly marked for example. To say "it's in the schematic so you should know" is not fair to the average user, especially when it contradicts the text of the datasheet.

@dkryder this ties in closely with the subject of this thread. The absolute maximum for the battery pin is 4.9V and the recommended maximum is 4.5V. 4 NiMH cells (nominally 1.2V each) will take you above that 4.9 and likely damage the Xenon. So you have to use 3 NiMH cells. The minimum voltage is 3.3V and 3.4-3.5 is better for stability. NiMH cells do an excellent job holding their terminal voltage as they drain, but that's still probably asking too much of them.

You may want to consider 4 cells connected to the USB power input, but you'll have to find out of that has any implications on power drain. Also don't charge the NiMH using the onboard LiPo charging circuit.

I think it's pretty bad that Particle isn't willing to say anything or release any documentation on powering their devices with standard batteries. This would be a much more common usage case for most people than LiPos. Considering how good, cheap,and convenient NiMH's are, I can't imagine ever using LiPos, which cost more than the current price of a Xenon around here anyway.