AVR Freaks

When I google "constant 3.3v from lithium cell" the top hit is:

http://electronics.stackexchange...

Seems to have lots of useful answers.

While I'm just a bit mangler I thought it was a buck/boost regulator you would need as you actually face two issues: it's true that at the start of the discharge the lithium will start at 4.2V but it's going to end at 2.7V (as long as you are careful) so you face both a period where the supply is above the 3.3V you require and then a period when it is below the 3.3V. Hence buck/boost.

I don't think a Li-Ion battery will last a year. They usually have rather high self-discharge. Also, this type of battery doesn't like to rest at high or low charges. So I'd recommend to look out for other options.

Buck-Boost regulators have usually a high power consumption. So your battery will be empty within days. So I'd go for a linear regulator with low quiescent consumption and low dropout voltage.

I currently power my remote sensors (xmegaE5,  si7020 and nrf24l01+) from 3 AAA batteries and use MPC1703 as voltage regulators. This setup is running since the end of November and if the voltage keeps dropping at the current pace I won't have to replace the batteries for the next few months and I could still decrease the average current consumption.

Edit: Typo and bracket removed

http://ww1.microchip.com/downloads/en/DeviceDoc/mic5205.pdf

(page 3)

(idles at about 80microA typical)

The MIC5205 has a PNP pass transistor and therefore will have some ground current.

There are a plethora of LDO voltage regulators with PMOS pass transistors that reduce the ground current by an order of magnitude or two.

The ground current is the regulators own current consumption which usually varies with output current. So the total consumption is this current plus whatever your circuit draws. That means you want it to be as low as possible.

The regulator on the Arduino is not ideal but not that bad either. The MCP1703 is far better in terms of power consumption though. The problem is that it doesn't fit on the Arduino, so you will have make a second board for it. Or you design everything to run on a wide enough supply voltage and omit the regulator entirely.

-Patrick

There are hardly any worse choices than a 9V battery. Their capacity is very low for their price and the increased voltage doesn't make it last longer. On the contrary even. Also, you waste nearly two thirds of their energy.

You could consider three AAAA(four As, no typo here) batteries. They were very rare, but since many computer manufacturers including Microsoft use them in their touchscreen pens, they became easier to find.

Things change however if you decide to use a switching regulator since they don't waste the excess voltage. Then a 9V block seems reasonable.

-Patrick

This only works as long as you don't use a voltage regulator between your batteries and the atmega. The voltage divider is absolutely necessary if you want to measure voltages higher than Vcc, which is the case here. And yes, the capacitor goes between the ADC pin and ground. Sorry for not making that clearer the first time.

What do you mean by design? If you're going to 3D-print something, you could incorporate the battery holder into the design. The contacts could be salvaged from some cheap battery holders.

I suggest to read the Wikipedia article. There are limitations to this battery technology, but I think it's reasonable if done correctly. For example, you should use brand new cells as used cell have higher self discharge rates.

-Patrick

Why would you need boost if your battery is empty @3V7?

Lots of Buck converters have too high quitescent current for this. Some Lineair regulators have very low quitescent currents.

But preferaby, make you circuit so it can be powerd from you battery without a regulator (Maybe add a good old 1N4007 to drop the voltage a bit?)

Also research "jeelabs" "jeenode". He has blogged extensively about battery powered sensor gadgets.