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Scraping together some parts, I managed to build a 22AH 12V battery pack, but I don't want to damage the batteries.

I am using 4x 12V 5.5AH UPS batteries wired in parallel.

I have an Arduino Uno, and don't want to spend money on additional equipment. Is there a way that I can use the Arduino to detect low voltage and kill the power?

I've done very little light current electronics, but have a basic understanding. First time working with Arduinos. I bought the Arduino for a different project, but if I can use it for this it would be great.

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The Atmega used on the Arduino has an ADC on board. Use it to measure the battery voltage and when the voltage falls below a threshold use a digital output to open a relay to cut off the load from the battery.

Now you can start designing your circuit. If you have more detailed questions during that process feel free to come back here.

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The Arduino Uno has ADC inputs however, you cannot plug 12V straight in, you'll have to divide the monitored voltage to the acceptable ADC range then scale back in your code to retrieve the actual voltage. The ADC range is 0-5V so, after fully charging your batteries, measure the UPS voltage and size the divider resistors values so the voltage you are measuring comes right under 5V past the divider. With this method, you won't have a great resolution on your battery voltage because you won't be using the full ADC range but I believe it should be usable for your project. Else you'll need to use a dedicated voltage monitoring IC.

To kill the power you may want to look into relays? There are tons of them out there to interface with Arduinos: https://www.google.com/search?q=arduino+relay+board&oq=arduino+relay+board

Make sure the relay can handle your full current load and voltage.

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