Choosing either 1S4P or 2S2P configuration with a 21700 battery (3.7V) for 3.3V Supply

I'm trying to design a circuit that is powered by 21700's. I lack experience in dealing with batteries, so looking for advice.

The batteries are responsible for powering a device that is recording and storing video footage.

If all the peripherals on the device require at most 3.3V, that means I need to supply a minimum of 3.3V obviously. Now a 21700 fully charged is 4.2V, nominally 3.7V however is classified dead at roughly ~3V (which is below the supply requirement).

I am trying to decide what the best approach is. If I only do 1S4P that means the batteries can no longer supply 3.3V necessary when it starts to deplete down to <3.3V. I can implement a low dropout fixed voltage CMOS LDO (like TC1262) to ensure the voltage is always 3.3V or is it smarter to use a 2S2P configuration and result in a minimum supply of 6V's, and use a buck converter to regulate it down.

I am looking for the most efficient way, is stepping up the voltage smarter, or step down the voltage with a buck converter the best idea.

• What is the maximum operating current draw? Commented Nov 25, 2022 at 4:52
• @BruceAbbott just under 1A. When recording 4K and writing to a file its roughly 3.5W Commented Nov 30, 2022 at 5:31

If you are looking for a cheap solution, an LDO or linear regulator of sorts should do the trick. But then also consider how you will manage charging and ensuring the battery does not get too low.

I would suggest use a dedicated IC, such as TI's https://www.ti.com/lit/ds/symlink/bq24210.pdf for these purposes, and you can use your microcontroller to help monitor the battery. You can also use these to tell your MCU if the battery is charging, and possibly even at what current it is charging.

I wouldn't let your battery get below 3.3V, you should put your MCU to sleep and wait for the battery to charge up again. Even below 3.7V I would consider "low battery"

Using a voltage divider (say 2 x 100k resistors if your MCU runs on 3.3V) from the battery source into an analog input on the MCU you can help monitor your battery voltage.

Here is a block diagram of some ideas, some things you can do to help manage your power. You could also add a control pin from the MCU to the LDO or other mosfets/power controllers to essentially force-cutoff power to the circuit if the battery gets too low, so as not to damage it.

There's always more than one way to achieve similar results, it is up to your application, budget and design constraints on how best to achieve this.

Both solutions can be good, this is more a price (and availability) problem to find an efficient buck converter while using the 2S battery or a buck boost to use 1S configuration.

Another important question:
How do you plan to recharge the battery?
A 2S battery might need balancing the cells, whereas there is not issue with parallel batteries if they are always connected together.