A good quality li-ion like an 18650, 14500, 18350, or 21700, kept at around 3.6v will hold that charge for a long time. It is not remedied to leave a nicad, alkaline or nimh in a light (or anuthong) with potential parasitic draw because it can cause the batteries to leak. A nimh like an Eneloop or something similar thats low self discharge will also hold a charge for a while. Some flashlights have built in batteries, but enthusiast grade ones usually don’t.