According to [0] the limit in the US is 100Wh, so you could fit a battery that is three times bigger (assuming 3.3V). I don't know what the relevant numbers are elsewhere.
[0] https://www.tsa.gov/travel/security-screening/whatcanibring/...
[0] https://www.tsa.gov/travel/security-screening/whatcanibring/...
For context 100Wh is only about 2x most normal laptops. A few systems with secondary batteries (Thinkpads for example) come within 10-15Wh of the limit when configured into their max battery configurations (although in my case that is two batteries).
I'm betting that is something you could actually walk through security with. I can't imagine the guys running the scanner are making estimates of battery capacity. For sure I've carried more than 200Wh worth of batteries before, considering for a while I was regularly traveling with two fairly high capacity laptops, a tablet, charging battery and cellphone. I'm also not the only odd ball with more than one laptop. Now that i've slimmed it back down to one, I regularly notice other people dragging out more than one.