Battery power with mixed voltages - How do you decide your approach to power?

[Aston01]

I have a project I would like to do that will require 3.3v & 12v to be supplied with battery power and I am having a hard time getting my head wrapped around the mixed voltages on a single battery aspect.

Using this project as a reference, the power allocation would be the following:

• ESP32 (3.3v) 160~260mA w/ WIFI
• GPS RTK (3.3v) 68mA - 130mA - future
• Soil Sensor (12v) 1 mA Idle, 25 mA for 2s Active

• In a scenario like this how do you generally decide on the battery voltage to use?
• When converting to different voltages what aspects drive your decision-making on when you start opting for different or more $$$ methods of conversion?

FWIW - I am aware this is a bit of a general question, but despite having done a fair amount of looking into this area, if someone put me on the spot and asked me why you'd opt for one method or another I still couldn't give an informed answer.

[Phil1977]

Can you somehow specify how large the battery should be?

If the battery is more on the small side I´d take a medium voltage between 6 and 9V. So the conversion ratio for the DC/DCs is not too high which is good for the efficiency.

If the battery should be larger I´d go for something more usual like a 12V-LiFePo - then you could power the soil sensor directly from V_bat.

Deciding about the type of converter is an endless topic. Just take something that´s good enough, because there´s always something better   

[Aston01]

Can you somehow specify how large the battery should be?

Back-of-the-envelope math with GPS etc ...a weeks worth of usage would probably be ~2700mAh @3.7v

If the battery is more on the small side I´d take a medium voltage between 6 and 9V. So the conversion ratio for the DC/DCs is not too high which is good for the efficiency.

That is an interesting point I hadn't thought of. Would you go as far as to prioritize the conversion efficiency based on the relative load at each voltage?

[BeBuLamar]

Can you somehow specify how large the battery should be?

Back-of-the-envelope math with GPS etc ...a weeks worth of usage would probably be ~2700mAh @3.7v

If the battery is more on the small side I´d take a medium voltage between 6 and 9V. So the conversion ratio for the DC/DCs is not too high which is good for the efficiency.

So you're already decided. Use a 3.7V battery with both up and down conversion.

That is an interesting point I hadn't thought of. Would you go as far as to prioritize the conversion efficiency based on the relative load at each voltage?

[Phil1977]

Yes of course, conversion efficiency from 3.7V to 3.3V can even be higher if you use a high-quality synchronous converter that can work with 0.2V of dropout.

Dropout voltage is the buzzword here. A single Li-Ion can go down to 3.4V, then you always have some cable and plug resistance. That means, if your GPS and ESP work as well with 2.8V as with 3.3V then it may be a good idea, but if you want a lot of margin for the converter and a stable 3.3V - then I´d go for a higher voltage battery.

Look into the conversion efficiency curves of a common step-down IC. Efficiency vs. input voltage is usually quite the same up to a ratio of 1:2.5 (by my gut feeling, please confirm with the part of your choice).