How to efficiently use 2 AA batteries with a 3V Arduino without losing capacity?

I would like to use 2 alkaline AA batteries for an Arduino-based wireless doorbell transmitter.
The Arduino I'm using is a modified 8Mhz 3v Pro Mini clone. The modification is that I removed the voltage regulator and power LED from the PCB to reduce the power consumption to ~200 nanowatts. (This is the power usage in sleep mode of course. But it will be in sleep mode 99.99% of the time and only wake up for a couple of milliseconds when an interrupt button is pressed.)

According to my calculations 2 AA batteries should have lasted me for years, but unfortunately it turns out that my Arduino requires at least 2.75V to start and at least 2.74V to not turn off. When I hit 2.75V the batteries still have almost 90% of the capacity left. That's very wasteful in my opinion and of course shortens the time between battery swaps massively.

Do you have any suggestions on how I could get more out of these batteries?

The first thing that would come to mind is of course a step up converter, which would allow me to completely drain the battery and supply the Arduino with steady 3V all the way through, but all the step down converters that I have are insanely inefficient to the point that it I would be better of not using one at all.

Using a lithium battery is also not an option because I'm too scared of fires because of the high energy density. It's a doorbell transmitter after all, so it will hang on the outside wall of a house and in the summer it might get extremely hot in the sun.

Use Lithium Energizer AA batteries, they will be at more than 1.5v for 95% of their life and 1.4V for around 99% of their life @ 3200mAhr.

The real limit in such low power setups is the self dischage of the batteries which may limit actual life to 3-5 years with Alkalines.

The Lithium Energizer AA batteries have an extremly low self discharge rate, they could last you 20 years or more.

There is no fire risk with these batteries that I am aware off.

However most batteries of this type (Alkaline and Lithium) are likley limited to circa 60c, which is easy to exceed for something that is outdoors in the sun …

my Arduino requires at least 2.75V to start

The brownout detection level is set too high. You can reduce it or turn it off completely.

See this excellent tutorial for how to properly set up a low power battery operated Arduino.

Also, double check the actual current consumption with a multimeter. Sometimes stuff can draws more current than it should be (wrong sleep mode, contamination or moisture on the board, whatever transmitter you have consuming current, ect).

As mentioned, the brown out detection (BOD) is most likely set incorrectly. Once that is squared away, you must measure the actual current consumed in sleep mode.

It has been recently documented that there are counterfeit atMega328’s being used in pro mini’s (and likely other Arduino variants using the ‘328) that draw 150ua in sleep mode. There is a sketch that can be loaded into the board that will give a reasonable yes/no probability that the device is genuine or counterfeit.

The details were documented on EEVBlog and YouTube. The origins:

The evidence:[

The Energizer primary lithiums would be a good choice. They are more expensive than alkalines, but work very well in the cold (alkalines - not so much), and maintain high voltage until almost discharged. I don't think there's any fire risk with lithium primaries.

Or, you could just use three alkaline AAs in series instead of two.

How is your transmitter going to react to being run on low voltage? Have you tested that?

And did you read this thread?

Thanks for your quick responses.

@ShermanP and @srnet What makes you think there is no fire risk with "Energizer primary lithiums"? I would say the higher the energy density, the higher the risk of fire when you boil or puncture or short it. That has been my observation so far, especially when it comes to lithium batteries. If there was a magical way to make high energy density lithium batteries that aren't dangerous, I'm sure they would be more common.

3 AA cells in series would mean almost 5V at full charge. I'm not sure if my 3V Arduino would like that.

@jremington I'll give that brownout thing a try.

@John_S Well, where do you think I got my numbers from? From a multimeter of course. :wink: So it's fine.

@WattsThat and @Paul__B I've seen those videos. ShermanP told me about it a quite while ago, but luckily I'm not affected because I have really old ones.

3 AA cells in series would mean almost 5V at full charge. I'm not sure if my 3V Arduino would like that.

A Pro Mini is specified to operate at 5 V (or in fact, up to 5.5 V). Nothing problematic. :roll_eyes:

The only concern may be the 3 V regulator being subjected to 5 V at its output but whether or not that might be a concern (mostly in regard to drawing current unnecessarily,) you have removed it.

Interesting. That's good to know, thank you.

So I just tried using the avrdude command that is suggested in the tutorial linked by jremington, but I just can't get it to work. Any ideas what I'm doing wrong?

As you can see I even tried adding the serial port using the -P flag, but it still wouldn't do it.

It's my understanding that it's the lithium rechargeables' chemistry that's dangerous, and that lithium primaries are about as safe as alkalines. But I see your point that if anything did happen to the battery, the higher energy density of a lithium might cause a bigger bang. Anyway, the Energizers are rated to 60°C. Maybe a little shading and ventilation would keep the temperature well below that level.

Your Pro Mini was "3.3V" only because the regulator was 3.3V. The processor in a 3.3V Pro Mini is exactly the same as the one in a 5V Pro Mini. Either will operate up to 5.5V. But you also have an 8MHz resonator instead of 16MHz. It will also work fine at 5V, but you need to check the datasheet to see what minimum Vcc is needed for 8MHz - you'll need to know that if you decide to change the brownout level.

I think three alkaline AAs may be the best choice for you. They make 3-cell holders. And you may find that you don't need to change the brownout level. Do you know if your radio transmitter will work below 2.75V?

Do you know if your radio transmitter will work below 2.75V?

Or at 4.5 V if it is specified at 3.3 (applies only to 2.4 GHz "smart" radios)?

I have a sensor with two old alkaline AA cells - just ATMega, nRF24L01+ and some other electronics for the measuring. Average current drain < 10uA, it has a 1000uF electrolytic cap to cover the increased current drain when transmitting (I don’t know if it is really needed). It works well even during winter (-10°C or so). What I have read cold only greatly increases internal resistance of the batteries (as well as the capacitor). With my low currents it is not significant.

If you put the pushbutton between battery and a converter, standby current is zero. Efficiency will not be a problem, considering low drain and self discharge.

You could also have seasonal batteries. You could use alkalines in the summer, then switch to lithiums in the winter.