New, modern and expensive cordless drills might also come equipped with Lithium-ion batteries. These batteries are different from the aforementioned Nickel based batteries in every possible way. The entire chemistry is different and so is the behaviour. Li-ion Panasonic cordless drill batteries have no memory effect and hate deep discharges.
The best way to maintain your Li-ion batteries is to:
Charge it frequently without letting it drop below 20-30% charge.
Operate it between 30%-90% charge without either charging or discharging it fully. This will increase the charge cycles of the battery many times.
Operate and store it in a cool environment as much as possible. Performance and capacity decreases in high temperatures.
Store the Hitachi power tool battery in a partially charged state of 40-50%.
Recondition it by using the drill normally till power drops and give it a full charge.
Li-ion batteries are smart batteries with a smart circuit built into them. Their chargers also usually know how to charge them properly.
Its not that important to charge the power tool battery for AEG till only 80-90%. You can charge it fully before use. It is more important to not discharge it completely. There are many ways of reconditioning a dead Li-ion battery. Here are some helpful resources.