I'm using a power boost to convert a 18650 battery to 5V:

enter image description here

How to detect low battery (i.e. battery voltage under a certain threshold), and send this information to a microcontroller?

(I'm currently using a RPi, which has no ADC, but the question may be valid for other computers/microcontrollers).

Note: this question is interesting but the main answers use an ADC. I'm more looking for a simple electronic solution.

  • 2
    \$\begingroup\$ If you are looking for a solution without ADC, I recommend you to mark it in the title. ("Low battery detection without ADC") \$\endgroup\$ – Bence Kaulics Jun 10 '15 at 7:41
  • \$\begingroup\$ If Vbat is 3.7V and Vo is 5V, when will Vbat ever be greater than or equal to Vo ? \$\endgroup\$ – EM Fields Jun 10 '15 at 7:50
  • \$\begingroup\$ @EMFields I modified my question to make it clearer, I removed the V0 part. \$\endgroup\$ – Basj Jun 10 '15 at 8:05
  • \$\begingroup\$ @BattleHamster I was against ADC because it looked complicated to me, but if there is an ADC 1/ cheap (< 5€) 2/ non-SMD 3/ small-sized < 3cm 4/ easy to use with a Pi, that could do this, I'm still interested :) Do you think this exists? \$\endgroup\$ – Basj Jun 10 '15 at 8:09
  • \$\begingroup\$ Unfortunately, I have never used external ADCs so I cannot recommend you a suitable one. But here is an article. \$\endgroup\$ – Bence Kaulics Jun 10 '15 at 8:52

What you are looking for is a voltage supervisor. (Note: I'm not recommending you buy them from that source, just providing an example of what you need to look for.)

The ones under that link are three pin devices that connect to your battery and ground, have an output that goes low when the battery voltage drops below a preset level. There are various models that have different cutoff voltages.

Since you mention price, the linked units are around $0.29 each if you buy 10 of them, which fits well with your price target.

Since you want non-SMD, I will mention that they are available in SMD and TO-92 (non-SMD) housings.

There also adjustable ones from various manufacturers.

There are also voltage detections circuits that you can build from discrete parts (example,) but making them work reliably might involve more complications than you want to deal with.

  • \$\begingroup\$ So basically you would avoid the solution given in your last link (using simple parts: resistors, transistors, ...)? Something else: where to find STM1061 non-SMD parts? I search by my usual dealer's website and didn't find any. \$\endgroup\$ – Basj Jun 10 '15 at 8:43
  • \$\begingroup\$ The STM1061 parts are SOT23 (SMD.) Further down the page are some TC54VC parts that are TO92. I'm not saying that those are the exact parts you need, but that they are the typo of thing you need. You'll have to find a source and select the correct part yourself. \$\endgroup\$ – JRE Jun 10 '15 at 8:48
  • \$\begingroup\$ Yes, I would avoid the solution using discrete parts. Getting it adjusted properly and making it work reliably would be messy - and it would be nearly as large as the size you gave as maximum. \$\endgroup\$ – JRE Jun 10 '15 at 8:50
  • \$\begingroup\$ Thanks! A small example of how to use it to trigger a "Low battery" to the Pi? There are just 3 pins, so I imagine: Pins #1 and #2 to the battey, pin #3 to the RaspberryPi GPIO? No other part needed? (resistor...) \$\endgroup\$ – Basj Jun 10 '15 at 8:53
  • \$\begingroup\$ Exactly right. One pin goes to the Battery + terminal, one to ground, and one to the GPIO pin on the Pi. There are data sheets for the parts that explain all of this. If there is a fixed supervisor that fits your needs (correct trigger level) then there are no other parts needed. The adjustable ones will need more parts, but are still fairly simple. \$\endgroup\$ – JRE Jun 10 '15 at 8:56

The simplest non-ADC solution is probably an analog comparator with a voltage reference, looking at your Vbatt; a Vref of about 3.0V (assuming lithium-ion, whose 'nearly flat' voltage is about 3.0V (consult datasheet, & dependent on load & temperature). That comparator can be powered from +5V from your boost-regulator, & will give you a digital signal to say flat/not-flat. That digital signal could be push-pull, or open-collector/open-drain, depending on details. Obviously you'll need to work out where to implement this circuitry - on your boost-regulator, or inbetween it & the battery. If you don't power this comparator from 5V, but instead directly from the battery itself, then you need to deal with the added complexity of maintaining a stable Vref with a falling Vbatt - doable, but somewhat more complicated for a novice. Basically we'd need a bit more info about your load & environmentals to get more specific.

  • \$\begingroup\$ Thanks! Load: approx 600mA. Would you have a small example circuit using an analog comparator for this purpose? A small picture would help to understand how to use it. \$\endgroup\$ – Basj Jun 10 '15 at 8:07

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