Battery, button or flat top
LiIon batteries exist in many sizes, but even within the same size, there are differences. Here I am looking at the plus pole on the battery, it can either be raised (button top) or not (flat top). This is a small difference, but sometimes it is the crucially factor between working or not working equipment. All pictures show 18650 batteries and equipment, but the flat/button top factor does also exist in other sizes.
The first two pictures show some classic button top batteries, the plus pole is raised above the rest of the battery.
These two batteries has much more flat plus pole, but only one is a true flat top, the other is a button top with a large diameter button.
A ruler on top of the batteries, on the first one the plus pole makes solid contact with the ruler, i.e. the plus pole is raised above the plastic. On the second battery the ruler makes contact with plastic first, i.e. the top is flat.
Here is the battery connection in 3 flashlight heads. The first one has a raised top, the second one has a spring, both will work with both button top and flat top batteries. But the last one has only a flat circuit board and will not work with a flat top battery.
Some flashlights has a ridge or some bumps around the plus pole as a mechanical polarity protection. This may restrict the size of the battery plus pole. I.e. only batteries with a small plus pole may be able to work in this kind of light.
When connecting batteries in series, i.e. equipment that needs two or more batteries, flat top batteries can also be a problem. The first picture shows a button top that connects with another battery. On the next picture is a typical battery bottom, it is flat and has some plastic around the edge, i.e. the plus pole of the connection battery must be raised more than the plastic thickness to make connection. The last picture shows the minus pole on AW's flat top battery, there are some bumps, making it possible to connect to a flat plus pole, i.e. these batteries can be used in series.
In my battery reviews I will use SB for the small diameter button top and LB for the large diameter button top. The flat top will be called flat.