Every battery listing on our site says either "flat top" or "button top," and it's one of the most common questions we get before someone buys a cell. The two look nearly identical, and nobody explains which one your flashlight actually wants. Here's the whole thing in one place.
What the difference actually is
An 18650 (or 21700, 14500, whatever) is a metal can. The body is the negative terminal, and the positive terminal sits at the top end, separated by a thin insulating ring. On a flat top, that positive terminal is flush with the shoulder of the cell - a flat disc, nothing sticking up. On a button top, the factory has welded a small raised nub onto that disc, usually a millimetre or two tall.
Same chemistry, same capacity, same discharge rating. A Molicel P28A with a button on it is still a P28A. The button is a mechanical detail, not a performance one - but it's a mechanical detail that decides whether the light turns on.
Why your flashlight cares
It comes down to how the positive contact inside the light's head is built.
A spring on the driver. The cell's positive end presses into a coil spring, which compresses to take up whatever length you feed it. Flat top, button top, protected, unprotected - the spring doesn't care. The single-cell Convoys we carry work this way: the S2+, the S6 and the C8+ all have a spring at both ends of the cell, which is why we pair them with flat-top high-drain cells.
A flat contact pad or a raised ring. Plenty of lights use a solid brass pad on the driver instead of a spring, and many surround the pad with a raised plastic or metal ring as mechanical reverse-polarity protection, so a backwards cell can't reach the contact. The catch: a flat-top cell inserted the right way can't reach it either. A button top pokes through the ring and makes contact. A flat top sits on the ring and does nothing. The light is dead, the cell is fine, and you'll spend twenty minutes convinced the charger lied to you.
Multi-cell lights are the third case, and the strictest. In a light like the Convoy 4X18A or 3X21A, the cells sit against a common contact plate, and each one needs its positive terminal to physically reach it. Flat tops leave a gap and the circuit never closes. Those lights want button tops, full stop - or the fix in the next section.
The rule of thumb
- Spring in the head (look down the tube with the cell out): either style works. Buy the cell with the best specs and don't overthink the terminal.
- Flat pad or raised ring in the head: button top. No exceptions.
- Two or more cells in one light: button tops, and a matched set - same brand, same age, charged together and kept together.
- Light shipped with a battery: replace like for like. The Wuben lights that take a swappable 18650 ship with Wuben's own button-top cell, and our Wuben 2600mAh/10A button top is the drop-in.
- Genuinely unsure, single-cell light: button top is the more universal answer. It works in springs and pads alike; the only risk is a very tight host where the extra millimetre keeps the tailcap from seating.
Bought flat tops for a button-top light? Don't return them
A flat top becomes a button top with a battery spacer - a small PCB disc with a conductive pad that sits on the positive terminal and adds the missing height. They're about a dollar each, sized for 14500, 18650, 21700 and 26650, and they're reusable. Set it on the positive end of the cell before you close the light, and the "dead" light turns on.
Even in a spring-loaded light, a short flat-top cell can rattle in the tube, and a rattle is a flicker waiting to happen under a drop or a hard tap. A spacer takes up the slack. If your light blinks when you smack it against your palm, that's the fix, not a new driver.
For the big Convoy multi-cell lights there's a dedicated part: the flat-top-to-button-top converter plate, one version for the 4X18A's four 18650s and one for the 3X21A's three 21700s. It's a single plate with a contact for every cell position, purpose-built for those two lights - and only those two. Single-cell light? You want the spacers, not the plate. We see that mix-up often enough to say it plainly.
"Button top" is not the same as "protected"
These get conflated constantly. A protected cell has a small circuit board wired under the wrap that cuts the cell off on over-discharge, overcharge or short. It needs a strip running the length of the cell, so protected cells are always longer - and they almost always come as button tops, because the manufacturer is adding length anyway. But the button is the terminal shape; the protection is a circuit. You can have either without the other.
The high-drain flat tops we stock - the Molicel P28A, the Samsung 40T and 58E, the Vapcell K25 and K55 - are all unprotected. That's normal for performance cells, and it's fine in a light whose driver has its own low-voltage cutoff - Convoy's do, and so do most current lights from the brands we carry. The protection lives in the light, not the cell. The Wuben button top above is a protected cell, which makes it the safer pick if you're not going to babysit charge levels.
The length trap
Terminal style is one variable, and length is the other. A bare flat-top 18650 runs about 65mm. A button top adds a millimetre or two. A protected button top adds a few more. A cell with a USB-C port built into it is the longest of the lot - the Wuben USB-C 18650 is 70mm, and the Nitecore NL1836R is longer than standard as well. Built-in-charging cells are excellent in the lights they're designed for and physically will not fit some others - the Nitecore cells don't work in any Convoy we carry. Our battery pages flag terminal style and length where it matters.
Chargers don't care (mostly)
Any slot charger with a sliding contact - the LiitoKala Lii-S2, the Vapcell Q2S-C, Q4 and Q8 - takes flat and button tops interchangeably; the slider moves to meet the cell. Just make sure your longest protected or USB-C cell actually seats in the bay.
Bottom line
Look in the head of the light. Spring: buy whatever cell has the best specs. Pad or ring: button top. Multiple cells: matched button tops, or flat tops plus the right converter. And if you've already got a drawer of flat tops and a light that ignores them, a dollar's worth of spacers puts them back to work. Everything here lives in the 18650 batteries collection and the accessories beside it. Still not sure which terminal your light wants? Email us the model before you order - we'd rather answer the question than process the return.