pfr I Cant say for sure that it's possible to disable AC power while plugged in but If TLP can stop the AC from charging beyond a certain point then it must have the capability to control when AC power kicks in again?
Depends on what you mean by "control" here. Lemme explain:
Almost all removable batteries available nowadays are what're called Smart Batteries. When you plug these batteries in, the microcontroller/firmware built-into them determines the SoC (state of charge) of each of the cells of the battery pack, then applies a varying charge current to charge them: a large currents if the cells are drained and smaller amounts (to each cell individially) as the cells approach full capacity (100%)--which itself diminishes over the life-time of the battery. At 100% nominal SoC, the battery controller signals the laptop power supply to "shut-off" the current. It then monitors the individual cells and supplies a "topping-up" charge to keep the cells at 100%.
When off AC power, the battery controller monitors the SoC and sends a warning signal to the OS (at a set percentage--16% on mine), and also a critical signal at a very low SoC (1.6% on mine) so that the OS can do a clean emergency shutdown. (The microcontroller may also signal the OS to control the battery LED: colour, blinking, etc.)
TLP, on Thinkpad laptops, you might be able to set different thresholds instead of 100%-16%-1%. In your use-case scenario, you might set a full-charge threshold of 80% and a critical threshold of 30%, (giving you 50% of actual run-time off-line--3 hrs, instead of 6hrs, let's say) to prolong battery-life, and the controller will do the rest. Note that only the thresholds have been changed, not the operating mode of the controller. The laptop will still have to be unplugged and used, otherwise it will keep topping-up at 80% and never discharge down to 30%.
What, however, you want to do is to keep the Thinkpad on wall-power always and have the controller drain the battery while it is on AC power and while respecting your custom thresholds. This is operating the controller in an unusual mode of operation, and is what I wanted to know if is possible on modern Smart Batteries. It may be possible; it may not be (which is more likely).
TLP documents are not much help, and I don't have a Thinkpad to experiment on, which is why I asked for clarification.