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April 20, 2012 AT 8:57 am

ASK AN EDUCATOR! “Can I use a 3V lithium battery in a 3.3V circuit?”

Kenton asks:

Can I use a 3V lithium battery in a 3.3V circuit? I know it depends on the circuit and its dropout voltage, but as a general practice is it acceptable? The other option is to use a significantly larger 3.7V rechargable battery.

It all depends…

For those of you who don’t know, ICs are designed to work within a relative wide range of voltages. If we look at Arduino’s ATMEGA328P, the data sheet states that supply voltage can range from 1.8 – 5.5V. While this is true, care must be taken, especially if you are doing data transfer, that the clocks are calibrated to the voltage. Then if we look at a device like the XBee 2.5, it is capable of operating from 2.1 – 3.6V. So hypothetically, you could connect your AVR and an XBee in a simple non-regulated circuit with just your 3V battery……although you might get mixed results. Circuits like to have stable voltages, so regulation is often recommended.

An alternative approach would be to use a device known Boost Converter, which is a type of switching regulator whose output voltage is actually greater then the input voltage. I recently used Linear Technologies LTC3525 Step Up DC/DC converter in one of my designs with great results. The device runs on 2AAAs which total 3V. The converter boosts the voltage to a happy 3.3V @ 400mA with very little noise and drop off.

With regards to using a 3.7V Li-ion, you run into the same problem. If your circuit is designed to run at 3.3V, you will need to buck, or drop the voltage down. The most common way to do this is with a LDO or low drop off regulator, like the Micrel MIC5205. The disadvantage to is that you only have ~0.4V of use out of the Li-ion battery. Li-ion batts are designed with a working voltage of ~3.0V to 4.2V, so you are loosing a bit of capacity.

I hope this helps steer you in the right direction and good luck with your project!

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.


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2 Comments

  1. Don’t forget that with 3.7V Li-Ion batteries you are gonna get the most mileage by using a buck-boost regulator. MCP1252 (150 mA max) and LTC3112(>1A) come to mind.

  2. I recommend MCP1640, which I’ve used in a project called AASaver. It’s really convenient and much cheaper than the other boost converters mentioned above.

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