It's not per battery, its holding capacity is that.

Doesn’t coffee turn bitter after being in a thermos all day?

No, it stays reasonably OK. A keep-warm plate in a coffee maker is definitely worse.

Still, it's best to let the coffe get cold immediately if you don't drink it right away. Keep it in fridge, and when you want hot coffee, heat it up with microwave. If you just want drinkable coffee, this is the method of least effort. People don't do this because for many of us, the act of making fresh coffee is a ritual, part of the enjoyment; it's about self-perception, and perception by others (keeping up with the Joneses; microwave-heated coffee? What a loser!)

Cold brew at Trader Joe’s makes it even easier.

Cold brew stays good much much longer (e.g. about one week in the fridge) and IMO is even better tasting.

[0] https://www.simplyrecipes.com/recipes/how_to_make_cold_brew_...

I find that coffee in a thermos can stay good for almost a whole work day. Coffee on the machine in some sort of “keep warm” mode goes rancid within 30 mins or so. I have a suspicion it’s some reaction with the oxygen in the air...

Tea in a thermos unfortunately goes bad pretty much instantly - something about being sealed in there in its own steam I think. With milk in it it goes even worse.

If that's true why don't they just carry a little bag of caffeine pills, rather than lugging this machine around?

Because coffee has other chemicals as well - to help with digestion for example and also high just from the pills is too rough. Furthermore you can't eat the pill for 10 minutes, so coffee is a good excuse for having a break.

Because if you're popping pills you've got a problem.

Could be worse - there are people who crush the pills and snort them...