Protected vs Unprotected Battery Guide: Sofirn, Wurkkos, ZebraLight & More

There is a particular kind of frustration that only happens when you have just unboxed a new flashlight, peeled the plastic off the lens, unscrewed the tailcap, and inserted a freshly charged battery — only to find that the tailcap won't screw down all the way. The O-ring is visible. Half a thread is still exposed. The light works, but it's not weather-sealed anymore, and you know you've just discovered that your battery is too long. I have been in this exact situation with three different lights over the years: a Sofirn SP31 V2, a ZebraLight SC64w HI, and a Wurkkos TS10. Each time, the culprit was the same — a protected button-top cell in a light designed for unprotected flat-tops.

This guide takes the general principles of protected versus unprotected batteries and applies them to the specific flashlight models that dominate enthusiast forums, EDC pockets, and emergency kits. By the end, you'll know exactly which battery fits your Sofirn SP35, whether your Wurkkos FC11 needs a button top, and why putting a protected cell in a ZebraLight SC64 is an exercise in futility.

The Measuring Tape Comes First

Before we talk about specific brands, let's establish the two numbers you absolutely need to know before buying any battery for any flashlight.

Compartment depth: With the tailcap fully tightened (or as close to fully tightened as you can get without a battery inside), measure the distance from the positive contact pad at the head of the light to the negative contact surface at the tailcap. A digital caliper is ideal, but even a ruler with millimeter markings will get you close enough. Write this number down.

Spring travel: If the positive contact at the head is a spring, use a non-conductive probe — a wooden chopstick, a plastic pen cap — and gently compress the spring until it's fully collapsed. Measure how far it moved. This is your compression reserve. A deep-compression spring might let you run a cell that is 2–3 mm longer than the compartment's static depth; a shallow or stiff spring will not.

The standard lengths you will encounter:

• Unprotected flat-top 18650: ~65.0 mm

• Unprotected button-top 18650: ~66.5–67.0 mm

• Protected button-top 18650: ~69.0–71.0 mm

• Unprotected flat-top 21700: ~70.0 mm

• Unprotected button-top 21700: ~71.5–72.0 mm

• Protected button-top 21700: ~74.0–76.0 mm

If your compartment depth is less than 67 mm, you are in flat-top-only territory. Between 67–70 mm, unprotected button-tops will fit. Above 72 mm for 21700 or 70 mm for 18650, you can start considering protected cells — but always check the specific cell's listed length.

Brand by Brand: What Works in What

Sofirn

Sofirn has built a reputation for budget-friendly lights with surprisingly modern features, and most of their current lineup includes built-in low-voltage protection (LVP) in the driver firmware. This means unprotected cells are electrically safe in nearly every Sofirn light made in the last four years.

Sofirn SP35 (21700): This is one of Sofirn's most popular EDC lights. The SP35 uses a dual-spring design — springs at both the head and tailcap — which makes it unusually flexible on battery length. It can accept unprotected flat-tops (70 mm), unprotected button-tops (~72 mm), and even some shorter protected 21700s up to about 73 mm. However, the SP35 has LVP built into its Buck driver, so there is no electrical reason to use a protected cell. My recommendation: Samsung 50S flat-top unprotected. It delivers 5,000 mAh and 25A continuous, more than the SP35's driver will ever demand, and it fits with room to spare.

Sofirn SP31 V2 (18650): This is one of Sofirn's few lights that ships with a physical reverse-polarity protection ring at the head — a plastic washer with a raised lip that only allows a button-top to make contact. The light has LVP, so protection is electrically unnecessary, but a button top is mechanically required. My recommendation: unprotected button-top Samsung 35E or 30Q, depending on whether you prioritize capacity or peak output. Do not use a flat-top unless you remove the polarity ring (which some users do, but it voids water resistance if not done carefully).

Sofirn IF25A (21700): This is a multi-emitter quad light that pulls substantial current on turbo — roughly 8–10A. It has LVP and springs at both ends. Fits any 21700 up to about 72 mm. Recommendation: Samsung 50S unprotected flat-top. The 25A rating gives it comfortable headroom above the driver's turbo draw.

Sofirn SC31 Pro (18650): One of the most popular entry-level Anduril lights. It has a spring at the head and tail, full LVP via Anduril, and can accept flat-tops without issue. Recommendation: Samsung 35E unprotected flat-top for maximum runtime, or Samsung 30Q unprotected flat-top if you spend a lot of time on turbo.

Wurkkos

Wurkkos shares a lot of DNA with Sofirn and similarly equips most of its lights with LVP drivers.

Wurkkos FC11 (18650): The FC11 is probably the single most recommended budget EDC light on Reddit. It ships with a Wurkkos-branded 18650 that is actually a button-top unprotected cell. The FC11 has a spring at the head and a spring at the tail. It works with flat-tops and button-tops, unprotected or protected — but any protected 18650 over 68 mm will be a tight fit. My recommendation: if you're buying a spare, Samsung 35E button-top unprotected is the closest match to the stock cell's fit and performance. If you want a protected cell, measure your specific FC11's tube depth first; there is some batch-to-batch variation in tail spring length.

Wurkkos TS22 (21700): A high-output single-emitter 21700 light with a boost driver and LVP. The TS22 uses a single spring at the tail and a flat contact pad at the head. Button tops work, but flat-tops are a more secure fit. Recommendation: Samsung 50E or 50S flat-top unprotected. The boost driver means current draw is modest, so the 50E's 5,000 mAh is fully accessible.

Wurkkos TS10 (14500): A tiny but hot-rodded triple-emitter light that takes a single 14500 cell. The TS10 has very tight internal tolerances and explicitly requires an unprotected flat-top 14500 — a protected 14500 will simply not fit. Wurkkos ships it with a branded unprotected 14500 for good reason.

ZebraLight

ZebraLight is famous — notorious, really — for its extremely tight battery compartment tolerances. ZL machines its tubes to fit unprotected flat-top cells only, and the margin for error is measured in fractions of a millimeter.

ZebraLight SC64 series (18650): The SC64c LE, SC64w HI, and their descendants have one of the tightest 18650 compartments in the industry. The stated maximum cell length is 65.2 mm. That is it. No button top will fit. No protected cell will fit. Even some thick-rewrapped flat-top cells that measure 65.5 mm will be difficult to insert and extract. ZebraLight builds its own LVP into the driver, and the light uses a pogo-pin positive contact that demands a flat-top. Recommendation: Samsung 35E or 30Q unprotected flat-top, in the thinnest factory wrap available. The Panasonic NCR18650B also fits well due to its minimal wrapper thickness. If you want to use protected cells, look at ZebraLight's SC600 series instead, which has a slightly longer tube.

ZebraLight SC700 series (21700): The SC700d and SC700Fd use the same tight-tolerance design philosophy in the 21700 format. The stated maximum cell length is 70.5 mm. Unprotected flat-tops only. Recommendation: Samsung 50E or 50S unprotected flat-top. The Vapcell F60 at 70.2 mm also fits but is effectively at the absolute limit; any wrapper damage during insertion could make it too tight.

Emisar / Noctigon (Hanklights)

Emisar and Noctigon lights, affectionately called "Hanklights" after their creator Hank Wang, are enthusiast hot-rods that run Anduril 2 firmware with full LVP. They use FET+1 or boost drivers, depending on the model, and current draw can be extreme.

Emisar D4V2 (18650): This is the quintessential hot-rod quad. On a fresh cell with a FET driver, it can pull 15–20A on turbo. The tube accepts flat-tops and button-tops, but the turbo current draw means you need a high-drain unprotected cell — a protected cell's PCM will trip on turbo. Recommendation: Samsung 30Q flat-top for balanced capacity and current, or Molicel P28A flat-top for maximum output. Do not use protected cells.

Emisar D4K (21700): The 21700 version of the D4V2. Same driver options, similar current demands. Fits flat-tops and button-tops up to about 71 mm. Recommendation: Samsung 50S flat-top for the best balance of 5,000 mAh capacity and 25A continuous discharge. Molicel P42A or P45B for maximum turbo output at the cost of some runtime.

Emisar DW4 (18650): The right-angle headlamp version of the D4V2. Same tube, same driver options, same battery requirements. Unprotected flat-top high-drain cells only.

Convoy

Convoy lights, designed by Simon Mao, span a wide range of configurations and are popular among DIY enthusiasts. The battery requirements vary significantly by model.

Convoy S2+ (18650): The classic budget 18650 host. The S2+ uses a mechanical tail switch with no LVP on most configurations (unless you install a driver with LVP). It accepts flat-tops and button-tops. Because most S2+ builds lack LVP, a protected cell is actually a reasonable choice here — especially if you're using the light intermittently and might forget to check the voltage. Recommendation: Protected button-top Panasonic NCR18650B for maximum runtime with safety, or unprotected Samsung 30Q if you've installed an LVP-equipped driver and want higher output.

Convoy M21 series (21700): These larger Convoy lights use a mechanical tail switch without LVP on most models. Some drivers support LVP, but many do not. The tube accepts flat-tops and button-tops. Recommendation: Protected button-top Samsung 50E if your M21 lacks LVP; unprotected Samsung 50S if you've confirmed your driver has LVP.

Acebeam, Fenix, Nitecore, and Olight

These premium brands often take a different approach: they ship their own branded batteries, and many of their lights require button-top or proprietary cells.

Acebeam: Many Acebeam lights use a dual-spring design and accept both flat-tops and button-tops, but some high-output models require button-tops for reliable contact. Check the manual — Acebeam is good about specifying. Most current Acebeam models have LVP.

Fenix: Fenix traditionally requires button-top cells, often protected, for their mechanical-switch lights. Their newer models with digital regulation and LVP are more flexible, but the conservative approach is to use a protected button-top 18650 or 21700 unless the manual explicitly says flat-tops work.

Nitecore: Nitecore sells its own NL-series protected button-top cells and designs most of its lights around them. Many Nitecore lights will not work with flat-tops at all. If you own a Nitecore light, buy Nitecore's own cells or a third-party protected button-top with matching specs.

Olight: Olight uses proprietary battery packs in most of its modern lights. You cannot use standard 18650 or 21700 cells. Use Olight's own batteries.

A Product That Covers the Spectrum: Samsung 50S 2-Pack

The Samsung 50S 21700 is the most versatile high-performance 21700 cell on the market, which is why we offer it in a 2-pack with multiple configuration options to match exactly the light you are powering:

• Flat-top unprotected: Fits Sofirn SP35, Wurkkos TS22, Emisar D4K, ZebraLight SC700, and any other 21700 light with LVP and a spring contact.

• Protected button-top: Fits Convoy M21 series, Fenix lights, Nitecore lights, and any light without LVP that requires a button top. Features a built-in PCM that protects against overcharge, over-discharge, and short circuits.

• Spot-welded with pre-wired leads: For DIY projects, custom battery packs, and repairs. Saves you the cost of a spot welder.

All three versions use the same Samsung 50S cell underneath — 5,000 mAh, 25A continuous (45A temperature-limited), and low internal resistance for minimal voltage sag under load. You can find all the options here: Samsung 50S 21700 2-Pack – Multiple Options.

For the 18650 users, our Protected 18650 batteries category lists cells from Keeppower, Nitecore, and other brands with precise measured lengths, making it easy to match a protected cell to a Convoy S2+ or Fenix light with a known compartment depth.

Quick Reference Table: What Battery for What Light

*Button top required unless polarity ring is removed.
**Protected cells may exceed length limits on some batches.

When Protected Cells Are Non-Negotiable

There are a few scenarios where I will always recommend a protected cell, regardless of the flashlight:

• Loaner lights: Any flashlight you lend to non-enthusiast friends or family should run a protected cell. They will not check voltage, they will not notice the light dimming, and they will leave the light on until the cell is dead. A protected cell saves them from destroying your battery and potentially creating a safety hazard.

• Emergency / glovebox lights: A flashlight that lives in a car or a kitchen drawer for emergencies may sit for months between uses and may be activated accidentally. A protected cell prevents deep discharge from killing the battery during storage, and protects against a short if the switch gets pressed against something in a glovebox.

• Mechanical switch lights without LVP: Any light with a clicky tail switch and no voltage-based step-down or cutoff needs a protected cell, full stop. Without LVP, the light will happily drain the cell to zero volts, at which point the cell is permanently damaged and unsafe to recharge.

When Unprotected Cells Are Required

• FET-driven hot-rods: Emisar D4V2, D4K, DT8, and similar lights with FET or FET+1 drivers will pull 15–25A on turbo with a high-drain cell. A protected cell's PCM will trip at 8–12A, cutting power entirely. You must use unprotected high-drain cells.

• Tight-tolerance lights: ZebraLight SC64 and SC700 series, as discussed. Protected cells literally will not fit.

• Multi-cell packs with a BMS: If you are building a 2S, 3S, or 4S battery pack with an external BMS, every cell in the pack should be unprotected. The BMS handles all protection functions for the pack as a whole.

The Bottom Line

The protected vs unprotected decision for flashlights comes down to three questions, asked in order: Does my light have LVP? What is my light's maximum current draw? What is the exact internal depth of my battery tube?

If the answer to LVP is yes, current draw is under 8A, and your tube depth is generous, you have maximum flexibility — choose unprotected for simplicity or protected for an extra safety layer. If LVP is no, use a protected cell period. If current draw is high, use unprotected regardless. If the tube is tight, use unprotected flat-top or risk damaging your light.

When you are ready to buy, Browse All Flashlight Batteries in our store, filtered by format, terminal type, and protection status. The right cell for your Sofirn, Wurkkos, or ZebraLight is a few clicks away — and a lot closer than the frustration of a tailcap that won't close.