I want to check how full my batteries are, is there a small piece of asm code (like the EF0850 for turning the Ti off) to do this?
The only thing you can actually check is if the batteries are "low". This is what gets displayed when you turn the calculator on.
This will return 1 in Ans if the batteries are good, and 0 if they are low. I'm not entirely sure what this might be useful for, but it's the best you can do.
Edit: the actual assembly, for the curious, basically puts bit 0 of port 2 into Ans:
in a, (2) and 1 bcall _SetXXOP1 bcall _StoAns ret
what exactly is low, completely empty or they're almost completely empty? How many hours can you max go on with them without having your ti fall out?
And is there no way to tell that they're 50%, 75%, 25%, …?
There is no way to check the actual battery life left. Returning a 0 if batteries are low is the equivalent of displaying the "Your batteries are low. Recommend change of betteries." message, which means that there are up to one or two weeks of functioning left, depending on usage.
Can I check Bateries with the languages Axe, Mirage OS, Ion or Assembler?
MirageOS and ION aren't actually languages, but yes, you can use all of those to check the battery level. To do it in assembly, you can use the hex code on the hexcodes page.
47%? Take a look and try to imagine how cool 100% will be. This has won zContest 2011 and made news on TICalc. This compromise between Assembly and BASIC parses like BASIC and is fast like assembly. Grammer 2
And yes, it is possible on the 84's (at least) to check the battery life at increments of 25%.