The ST16 batteries are Lithium-Ion, the same as used in power tools.
They should be topped up after each use, to be ready for the next time.
Besides, they take a long time to recharge if run low.
If you run it below 20%, you're getting close to an unexpected low volt cut-off.
For clarification the transmitter uses lithium-ion cells, the H+ itself uses lithium polymer. But the advice is sound, never go below 20% left. I tend to be a bit more conservative and use 35% as my time to land number.