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I am using an ATtiny85 to sleep for a long period of time, collect some data, send it over radio, and go back to sleep, after disabling everything possible. The ATtiny is currently powered by a coin cell. I can get quite some time off that coin cell, but it's simply not enough, and the coin cell cannot be changed once the board is deployed. Peak current draw is around 10-15 mA.

My aim here is to find an alternative power source for my board. Some good news is that the boards are never more than one or two feet from a receiver, so rf-like power transmission may be possible. Some other options may be motion or heat (it's a wearable device). The amount of power could be pretty small, as I only need it at peak periods, so I could just charge up a supercapacitor for a while and drain it when power is needed after the Attiny wakes up for less than a second.

The ideal situation would be to have a means of power transmission or generation that provides enough current to continuously power the microcontroller without the need for sleep, but I recognize that this is probably not feasible. However, due to the limited range it might be. Another concern is radiation to the wearer.

If anyone has any experience in this kind of power transmission or generation or just any ideas I would love to hear it. Thanks!

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    I can get quite some time off that coin cell, but it's simply not enough - please specify numbers. You get x days off the coin cell, but you want y days? What are x and y? I have a post about a solar powered Arduino which may be relevant to you. I can elaborate in an answer (rather than just giving a link) if you think it would be useful.
    – Nick Gammon ♦
    Commented Jan 12, 2016 at 5:02
  • I can get a few weeks depending upon usage, and I need a few months. Unfortunately, solar is out of the question for this project
    – Alex Wulff
    Commented Jan 12, 2016 at 12:24
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    a few weeks on a single coin cell is quite short. I think you should be able to get a lot more time out of it in software. e.g. disabling all unused modules; combining data, and sending it all at once; not sending data if data hasn't changed; etc. But without any specifics it's hard to say. Have you measured the standby current? How often do you collect/send data per hour? What radio are you using? Do you put the radio to sleep? What code are you using?
    – Gerben
    Commented Jan 12, 2016 at 15:51
  • As soon as you use a radio, you are looking at wasting power in RF circuits. Most power is wasted in PA in RF circuits. A Li cell like CR2032 can't survive in RF transmission tanks. Some power would always be reflected (and wasted) due to impedance mismatch irrespective of your perfect RF circuit.
    – Chetan Bhargava
    Commented Feb 13, 2016 at 10:07

3 Answers 3

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10mA is quite a lot for a coin cell. And of course the duration is really low.

As for wireless power transmission, I highly discourage you. Since you are using coin cell batteries I think that size is a must for you, and for wireless power you will need a really big antenna. If it is a wearable I think that the receiver is on the body too, and so much power is not a good thing.

In my opinion and according to my experience you have two possibilities:

  1. use alkaline batteries (e.g. AAA). Use a step up driver to get 3.3V from 1.5V (since the current you need is low you can use simple charge pump regulators, so with a 10pin TSSOP integrate and three small capacitors you are done) and you will get a really long lasting circuit. Drawback: AAA batteries are a bit large, so probably you'll have to make a bigger receiver

  2. use lithium batteries (usually I use LiPo). You will then be able to recharge the battery with any power supply (I usually use an IC which can charge LiPos with a 5V source - USB - and a couple of other components (two capacitors and a LED with its resistor).

Personally I struggled with coin cells for a long time before giving up and sticking with lipos forever.

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  • Unfortunately I can’t charge or access the devices once deployed. I was thinking of using a cr123a as a last resort, because it’s small but still delivers 3v. I might be inclined to agree with you about coin cells - they’re more trouble than they’re worth.
    – Alex Wulff
    Commented Jan 12, 2016 at 12:34
  • Instead of that I'd still prefer a lipo one.. In the end they have similar capacities and sizes, but once a CR123A expires you have to change it, while when the LiPo does you simply recharge it.. (note: I'm not a LiPo batteries seller XD)
    – frarugi87
    Commented Jan 12, 2016 at 13:29
  • @frarugi87 Do you have any charge pump regulators you recommend (i.e. recommended part nr)?
    – Gerben
    Commented Jan 12, 2016 at 15:55
  • Well, for my projects I used the MAX1759, which is an adjustable regulated charge pump, and the MAX1595, with fixed voltage (either 3.3V or 5V). The second one is smaller, delivers more current and requires less components, so if you want either 3.3V or 5V the second is better. I usually use these because they are regulated (and very stable), use small components (just three ceramic capacitors the 1595) and, most important, they deliver a reasonable amount of current (100mA the adjustable version, 125mA the fixed voltage one). And.. no, I'm not from Maxim ;)
    – frarugi87
    Commented Jan 13, 2016 at 9:27
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This question seems to be aligned to your other question about nrf24l01+ transceiver. The charge pump is a good idea, but if this is still for the nrf24l01+ project i would not recommend it because it would cause ripples in the supplied power and RF noise which is really not something you want together with the nrf24l01+.

if the unit is to be strapped on the wrist or ankles a concept like the one in a shake/faraday flashligt might be implemented, but that would be a diy custom job. and probably require size, even though they can fit a similiar contraption into a wristwatch.

But anyway I'm not sure if you are aware, but the nrf24l01+ has a function called enhanced shockburst which is designed for very low current consumption according to the datasheet it only requires 1% or 0.12ma at normal transmitting mode! this comes at the cost of transmitting power ofcourse but your receiver is only a few feet away so that is most likely not an issue.

https://www.sparkfun.com/datasheets/Components/SMD/nRF24L01Pluss_Preliminary_Product_Specification_v1_0.pdf page 14

I'm pretty sure you already have optimised your current consumption, but one setting you might have missed is the atTiny clockspeed which can be lowered to 1mHz instead of 8mHz this will lower the current to run the processor to 20 of the normal.

if these are not implemented already, then they might give you the needed battery life.

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  • Thanks for the info on shockburst, I’ll look into that. The Attiny is already running at 1MHz. I didn’t know that it could transmit with such low power...
    – Alex Wulff
    Commented Jan 12, 2016 at 15:35
  • I have not had an opportunity it test it, so I don't if they are overselling it so to speak - but I'm waiting for my nrf24l01+ and will most definitely test it when I get my hands on them
    – Mikael
    Commented Jan 12, 2016 at 17:42
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I just helped a friend install a Fitbit "Zip" model. In the process of doing that I noticed it is powered by a CR2025 (3V lithium button) battery.

Their ads claim it runs "up to 6 months" on this battery. It communicates wirelessly (ie. Bluetooth) as well as having an LCD screen. It also takes measurements.

Thus, what you are trying to achieve sounds technically possible, as someone has done it.

I can get a few weeks depending upon usage, and I need a few months.

So to solve your problem it would help to know what your existing current consumption is.

I have a project about an ATtiny85 torch locator. That does not do radio transmission but does flash an LED. I calculate the battery should last 3 years, and at this stage (almost a year after I made it) it is still working on the same battery.

You may get some ideas from that link.

I calculated (and confirmed by measurement) that in idle mode it uses 4.4 µA. You should be getting a similar figure during your sleep periods.

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  • I gotta say, that's pretty amazing that it can run for so long on such a small battery with all those radios and displays working. I've got the power up and power down much more efficient - now the peak current draw is less than 5mA or so (my DMM's fuse is blown so I can't read it accurately). The main purpose of this question was to see if there was a better option than battery power, but the general consensus seems to be that most things will require quite a bit of experimentation and to work. Thanks for your help.
    – Alex Wulff
    Commented Jan 12, 2016 at 21:51

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