Author Topic: Powering from 1 or 2 AAs  (Read 1003 times)

LK

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Powering from 1 or 2 AAs
« on: December 17, 2019, 12:26:47 PM »
Hi all (I hope this is in a suitable section),

I've had a good read around the forum for this, but not yet come up with an answer. I was looking to power nodes from 1 or 2 alkaline AA cells. I have already used nodes powered by 3 or 4 cells, feeding that into the MCP1703 regulator to get a well-regulated 3.3 V, and generally reliable results. The end result, though, is either a bit awkward (AAs are generally sold in 2s or 4s, not in 3s) or a bit bulky (4 AAs take up a fair amount of space).

I'd really like to run nodes off 2 AAs, as that seems like a good capacity / size compromise, but would also like to ensure a well-regulated and low-noise 3.3 V for the processor, radio and any attached sensors. I read that using an LDO regulator is the best way to get that (and also a very low quiescent current), and that a boost circuit is not a good way, as the output would be noisy, which may affect the radio parts. But it seems that boosting the 2 x AA voltage to 5 V (or thereabouts), just to regulate it down to a smooth 3.3 V again would waste about 40% (or more) of the power in the cells.

So I was wondering if anyone had run Moteinos (or similar hardware) off a boost circuit, providing it directly with 3.3 V (not regulating the voltage back down again through an LDO)? Some of the boost ICs seem to have quite low quiescent currents, and some appear to claim low ripple/noise, though "low" is a relative term, and I don't know how low that needs to be to not interfere with the radio parts. Does anyone out there have any experience of trying this?

I'd rather stick with inexpensive, long-lasting alkaline AAs than use AAAs (seem to be a worse size-to-capacity ratio), "exotic" AAs (expensive, less available) or LiPos and all of the extra (charging/safety/transport) issues those bring.

TomWS

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Re: Powering from 1 or 2 AAs
« Reply #1 on: December 17, 2019, 07:00:55 PM »
Your best bet is to use the 8MHz Moteino so that you can run reliably to and below 3.0V.  This will allow you to use 2 AAA cells to power you for a year or two (depending on your other tasks/hardware).  The 8MHz Moteino bypasses the LDO and powers the processor directly from the batteries.  Personally, I use Eveready UltraLithium which give me a wider power margin and much longer life, but Alkalines are commonplace and will work in this application.

Also, a booster will be much less energy efficient than a switching buck regulator and there is no real need to run at 3.3V (if you run the processor at 8MHz).  A booster is useful when you have an alternate power source, like Solar, that needs to be boosted to the processor's operating voltage.
« Last Edit: December 17, 2019, 07:05:55 PM by TomWS »

LK

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Re: Powering from 1 or 2 AAs
« Reply #2 on: December 19, 2019, 06:56:08 AM »
Thank you for the advice, Tom. I thought you might suggest that, based on some of the existing posts I found! :)

The idea of running directly from cells, with no wastage in a regulator is appealing, but I suppose it depends on the sensors being used and what their tolerance is to low voltage running. I note that elsewhere (e.g. https://lowpowerlab.com/forum/jobs-freelancing/review-of-a-current-pcb-designimprovements/) others have said "at 8mhz you can run down to 2V without any issues", so I assume that means that the radio can run down to that level (despite the RFM69 datasheet's spec. of 2.4 to 3.6 V working range) and the processor can run down to 2 V, as long as the BOD is set to a lower level than "normal" (e.g. 1.8 V)? I note that the SPI flash memory used on Moteinos (W25X40CLSNIG) states a voltage range of 2.3 to 2.7 V, or 2.7 to 3.6 V if using higher frequencies of "fast read" instructions. Not sure if that's the case on Moteinos or not? But even the lower range only goes down to 2.3 V.

Looking around the datasheets of a few sensors I've used, most are OK at down to about 2.1 V, some lower. But then some others not so much. For example DS18B20s (very handy for lots of temperature measurements with only one digital pin, OK with long cables, etc.) state that they won't run below 3.0 V. So running at low voltages would also mean finding suitable replacement sensors, as boosting voltages just for one or a few sensors seems overkill.

I was mainly wanting to run at 3.3 V, as it's a well-established voltage that should work for the processor, and a good variety of sensors, with stable results, plus it means that analogue measurements (of remaining battery voltage or of sensors that give an analogue voltage) is easier as the voltage input to the system is stable. Things like capacitive sensing, for instance, might be upset if the input voltage to those sensing element does not remain constant?

TomWS

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Re: Powering from 1 or 2 AAs
« Reply #3 on: December 19, 2019, 06:30:04 PM »
If you're running on Alkalines, you're going to fall off a cliff when they get below about 2.2V for two cells so operating below 2.3 is risky business anyway.

The radio will operate down to 1.8V, I know, I and others on the forum have done that.  Interestingly, the TX power output is fairly stable from 3.3V down to low 2.x volts so that's good to go.

Most of today's sensors will operate into  the 2's and you can get footprint compatible flash memory that will operate below 2v if you're willing to deal with the minor code changes to the flash code.  The commonly used SPIFlash library is not very (as in not at all) adaptable to sensing and adjusting command based on Flash ID.

Having said that, I am a steadfast fan of Ultralithiums. I have run hundreds of motes and haven't had a battery issue (other than battery sockets rusting) since I've switched.  No VR needed and plenty of reserve power even near the end of life.

Felix

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Re: Powering from 1 or 2 AAs
« Reply #4 on: December 20, 2019, 10:48:08 AM »
Tom, I think you mean Eveready Ultimate not Ultra (Lithiums).

Hey how about AA batteries with an integrated USB port for recharging!

LK

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Re: Powering from 1 or 2 AAs
« Reply #5 on: December 20, 2019, 12:42:04 PM »
Those rechargeable ones look fancy! Bit pricey maybe.

So Energizer Ultimate Lithium AAs? Though it seems like Energizer Holdings, Inc. makes batteries under Energizer and Eveready brand names...

TomWS

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Re: Powering from 1 or 2 AAs
« Reply #6 on: December 20, 2019, 04:58:58 PM »
Tom, I think you mean Eveready Ultimate not Ultra (Lithiums).
Whoops, my bad!  Yes, you are partly right.  I should have said Energizer Ultimate lithium.  Man, it's tough getting old!
Quote
Hey how about AA batteries with an integrated USB port for recharging!
IMO, the only place to use rechargeable batteries is for power backup on a mains (or other, eg solar) powered mote.  (Or a mote that is temporarily in flight...)