For a small unit like a drill: Is it better to charge after any use or wait until the lithium battery is exhausted ?
After a materials study of lithium ion batteries it was discovered that maximum lifespan was obtained by not discharging beyond 82% recharging up to 12% of maximum power capacity. Exceeding these levels proved that deposition of lithium metal crystals became detrimental to conductance capacity over time and even encouraged leaching of mineral from the solution without electronic current.
After 300 cycles of 90%> discharge to 100% charge the lifespans were critically affected but controlled charge inside 18% of max and min capacity extended life by as much as 72%.
From this they concluded that full discharge and complete recharge was detrimental to life expectancy of lithium ion batteries in regular use.
Research white papers from Samsung research I've read agreed this research too. So to answer your question, don't let potential energy drop below 12% and, try not to maintain charge above 82% capacity.
It depends on the battery. You can discharge some batteries until 0-10 % and battery life won't be reduced. Examples: NCA (Nickel-cobalt-aluminum) and LTA (Lithium titanate oxide) lithium-ion batteries. The final state of charge (SOC) is 0-10 % and the depth of discharge (DOD) is 100-90 %.
There are other batteries in which is better to charge them after any use because their life gets shortened when the DOD it too high.
Check this papers: El-Sayed recommends a 20 % DOD:
El-Sayed, M. A. H. “Lithium-ion energy storage battery in PV-smart building application”, Renewable Energy and Power Quality Journal, no. 19, April 2019.
Abdon, A., Zhang, X., Parra, D., Patel, M. K., Bauer, C., Worlitschek, J. “Techno-economic and environmental assessment of stationary electricity for different time scales”, ELSEVIER Energy, vol. 139, pp. 1173-1187, November 2017.