Button top or flat kind of depends on the mod, though most will take either. You should have a multimeter, or an Ω meter that can also check voltage so you can check your
batteries. You should be checking them when they come off the charger, making sure they're around 4.2v, and then checking them when you take them out of your mod to make sure you haven't drained them too low, most regulated mods cut off at 3.2v I think. If you notice a battery isn't holding a charge as well, it's time to replace it.
Edit: your regulated mod can read your battery voltage for you, forgot what you had.
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