I'm travelling to Germany from Bangladesh (transit at Istanbul) on a Turkish Airlines flight. I wonder if I can carry a powerbank of 10000mAH in cabin luggage.
Assuming a typical 3.7V Lithium-ion battery voltage, 10Ah is 37Wh (watts are volts times amperes), which is well under the 100Wh limit. Even if you assume it's 10,000mAh at the 5V output (and for not-so-subtle marketing reasons the makers don't tend to do that- the voltage should be indicated on the label) it's still 50Wh, which is half the limit. But it won't be, it will be no more than 37 and if it's an off-brand possibly substantially less- but the label is what matters here, not reality.
Huawei and other quality brand-name makers offer 20Ah power banks which can be brought aboard (74Wh).
You specifically say cabin luggage, which is fine, but for anyone else reading this question, a reminder never to put lithium batteries or devices containing lithium batteries into checked bags, and that counts double for dodgy power banks from unknown makers.