Hi Pedro, that voltage is way too low for any lithium battery in good nick and with more that 20% capacity.
Can I make a suggestion.
Try running your system on one battery at a time.
The simplest way to do this is to remove the negative ( - ) lead from one battery and run everything off the other battery for a day or two.
Then take the negative ( - ) lead off that battery and reconnect the negative ( - ) lead back to the other battery and again, run your system of that battery and see what voltages you get.
I have been running a combination of a 100Ah lithium battery continually connected to a FAULTY cranking battery for well over two years and the lithium battery ( actually both batteries ) has never had a voltage reading below 13.15v.
NOTE only last week I replaced the cranking battery with a new one and have removed the lithium battery for testing.
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