Battery Protection 3S 11.1v Li-ion Raspberry PI project

Hi, I’m Mark. I’m working on a Raspberry Pi car.

I have three 18650 li-ion batteries (3.7v each) in series to for an 11.1V battery. They are not protected. This is OK to an extent because I plan on charging them outside of the project with a standalone charger. What I want to prevent is over discharge.

I need a low voltage soft power down at somewhere between 9 and 9.6v. I don’t want cutout as it might corrupt my SD card.

I can’t quite see the point of an UPS (Uninteruptible Power Supply) as it’s overkill for what I need it for and introduces more weight/bulk/wires/fiddle.

I don’t want to mess about with a 3S BMS (battery management system) and soldering to the 18650 (which is risky with my equipment/skills), plus no-one sells them apart from ebay/amazon so I don’t trust them to look after the components I have, or the battery.

The other problem is the voltage input. 3S is rare, but I need the voltage to run the motors with my motor controller. I also use the battery power to step down for powering the PI and a servo hat. These all have kick back protection so I’m only worried about monitoring the voltage and shutting down at a certain level.

Are there any other possibilities? One of my preferred outcomes is for a solution that applies across multiple projects and is cheap. I’ve found a cheap voltage cutout from RC hobby websites, all the way to the PICO controller or PIJuice which I’m not sure would do what I’m looking for.

I’ve been searching for ages but I don’t know what exactly to search for. What I’ve put here is what I’ve learnt so far!

Thanks in advance