Would it be possible to safely power the bbc micro:bit using coin cell batteries? Out of the box it requires x2 AAA batteries, although an earlier prototype did use cell batteries.

I was wondering if connecting x4 3V CR2032 batteries in parallel would work? I believe this would increase the typical capacity to approx 900 mAh and maintain a 3V output. Would this be safe if one of the cell batteries was faulty and discharged? Would using rechargeable cell batteries in parallel reduce any safety concerns?

This is for a home hobby project.

Many thanks

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    \$\begingroup\$ Find a datasheet for a CR2032 and look at the equivalent internal series resistance. Then figure out what the internal voltage drop will be for the current you require. You may be surprised at how the voltage collapses. \$\endgroup\$ – Transistor Apr 12 '16 at 21:49
  • \$\begingroup\$ @transistor: Thanks for your comment. I will endeavour to find out more about the micro:bit's typical current draw and then follow your advice. \$\endgroup\$ – Simon Cragg Apr 13 '16 at 8:53

From a voltage standpoint, yes it could be done.

However, voltage is not everything. You need to look at the datasheet for the coin cells to see if they can supply enough current. In almost all cases (apart from some Li-Ion rechargeable ones), the rated continuous current sourcing capability is only ~2mA at most - typically only 1mA. Though it should be possible to run at a higher current at the expense of capacity - but probably not more than 10mA or so at a guess, depending on the battery.

With four in parallel, you can probably only supply 8mA to the board without significant voltage drop. The downside of running them in parallel is that if the batteries have slightly different terminal voltages, the lower voltage ones start acting as a load rather than a source).

By comparison, an AAA battery can typically source as much as 1A continuously, and also have much higher capacity.

Coin cells are designed for battery backup, or ultra-low power applications. They are not designed as a replacement for AAA batteries.

Having said all that, it is hard to tell what the requirements of the micro:bit are - in a quick look around I can't seem to find any information on power supply requirements/current draw. However looking at the processor on it, that requires at most around 11mA which may be within the capabilities of the coin cells. I'm not sure what else is on the board, however if you start trying to drive the LEDs, that will increase the current consumption far beyond the limits of the coin cells.

As a final note, you are getting your units mixed up - current and capacity are different things:

  • Current is a measure of the flow of electrons, with units of Amps [A] (mA = milliamp = 0.001 Amp).

  • Capacity is a measure of energy stored in the battery, and is measured in Joules, although frequently quoted in Amp-Hours [Ah] (mAh = milliamp-hours).

  • \$\begingroup\$ I disagree with your max current numbers. That's only for the stated capacity at the target voltages. You can draw significantly more than 2mA continuously without harming the battery, as long as you understand it will result in a lower mAh life. \$\endgroup\$ – Passerby Apr 13 '16 at 3:40
  • \$\begingroup\$ @Tom: Thanks for your response! Indeed, I was meaning 900 mAh as the measure of capacity. I will try and amend the question. As you've probably guessed, I'm not from an electrical engineering background :) I will endeavour to find out more about the current draw from the micro:bit and go from there. \$\endgroup\$ – Simon Cragg Apr 13 '16 at 8:48

Just the core CPU - yes, it'll easily run from a single coin cell.
BLE bluetooth radio - yes. Although you'd want to aim for a fairly low level of activity on the radio link or you ruin the battery life.
LEDs - not well. You need to be careful, pwm the brightness and only light one at a time. If you try turning them all at once it's not going to work.

You will also want to make full use of things like sleep modes to minimise power consumption. That probably means skipping the graphical programming interfaces and using the mbed c++ libraries so that you can do a lot more power management.


Found this link that might be a solution to this: http://www.electronicsweekly.com/blogs/gadget-master/boards/kitronik-powers-bbc-microbit-with-coin-cell-battery-2016-05/

  • \$\begingroup\$ please do not post bare links. If the link breaks, your answer will be rendered worthless. \$\endgroup\$ – Ariser Feb 14 '17 at 9:33

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