How it's work?

Have relay and use it battery disconnect load(deep discharge protecti) if voltage < 10.5V + have level indicator with 3 LEDs:

PB0 1 LED 100% Charge level - >12.5 V;

PB1 2 LED 65% Charge level - >11.7 V;

PB2 3 LED 35% Charge level - >11.1 V;

PB3 voltmeter, 50 kOhm and 10 kOhm + 1 kOhm variable resistor;

PB4 Relay.

Source code(for Arduino IDE - "ATtiny13BatteryProtect/ATtiny13BatteryProtect.ino") here:

https://github.com/techn0man1ac/ATtiny13BatteryProtect/tree/main/Code

It's project full open source(PCB and code).