I made a programmable Arduino that measures an input voltage. The Arduino will close or open an inverter using a relay to prevent the set of batteries from dropping below 50% to keep the batteries healthy. I currently have a set of 8 6V batteries configured to supply a total of 12V to one inverter. I used a set of 10k ohm and 100k ohm to safely measure the 12 volt input from the batteries. In the current configuration the total Amperage provided by the set of batteries is 400 amps. Is it safe to measure the voltage from a 12V/400A input or do I need some kind of protection?
It is perfectly safe.
The Arduino will see the voltage reduced by the divider you have. It will see whatever current it needs to see in order to sense the voltage - which is a tiny amount.
Remember: Current is drawn on demand by the load. It is never pushed by a supply.