Fogstar Energy 4000 21700 Battery

Fogstar FS4000 21700

The Fogstar Energy 4000 is a 21700 lithium-ion cell built for regulated vape mods that take external batteries. It runs a 4000mAh capacity with a 40A maximum discharge rate, making it a solid choice for sub ohm and DTL devices that pull harder than a standard 18650 can comfortably handle. The 21700 format is larger than an 18650 - check your mod's battery bay before ordering.

This is an unprotected flat-top cell with no internal circuit board, so it is intended for use in regulated mods with built-in protection, not mechanical mods unless you fully understand battery safety and Ohm's law. Each battery comes in a protective carry case.

Key Features

• 4000mAh capacity
• 40A maximum continuous discharge
• 3.6V nominal voltage, 4.2V maximum charge voltage
• Flat-top design
• Unprotected cell
• 21700 format - 21.2mm diameter, 70.1mm height
• 66g weight
• Sold with a protective carry case

Capacity

At 4000mAh, the FS4000 gives you noticeably more runtime than the typical 3000mAh 18650 battery, and holds its own against most competing 21700 cells. For sub ohm vaping at 50-80W, that usually means longer sessions between charges. The 21700 format was specifically developed to offer more capacity in a cell that still fits dual or single battery mods designed around it.

Discharge Rate

The 40A maximum discharge rate gives this cell considerable headroom for high-wattage regulated use. Most vapers running a regulated dual-battery mod at 80-100W will draw nowhere near that ceiling, which means the FS4000 operates comfortably within spec and generates less heat than a cell being pushed to its limit. If you are running a single-battery mod at high wattage, check the amp draw against your coil resistance using Ohm's law before use.

Voltage

The FS4000 operates at a nominal 3.6V and charges to a maximum of 4.2V. Your regulated mod handles output voltage conversion, so the raw cell voltage itself is less relevant to your day-to-day vaping than the capacity and discharge rate. What matters is that you use a dedicated lithium-ion charger that stops at 4.2V - overcharging a lithium-ion cell degrades it faster and is a safety risk.

Flat-Top Design

The flat-top configuration is what the vast majority of regulated vape mods require. Button-top cells are typically only needed for specific torches and some older devices. If you are unsure which format your mod takes, check the battery contacts - flat-top cells have a flush positive terminal, while button-top cells have a small raised nub. Most modern dual and single 21700 mods take flat-top.

Unprotected Cell

The FS4000 is an unprotected battery, meaning it has no internal protection circuit. This is standard and correct for use in regulated vape mods, which have their own built-in protections against over-discharge, short circuits, and overheating. Do not use unprotected cells in devices with no electronic protection. Inspect the battery wrap regularly and replace any cell with a damaged or torn wrap immediately - bare lithium-ion cells can short against metal contacts.

Safe Storage and Handling

Each FS4000 comes in a hard plastic carry case for safe storage and transport. Never carry loose 21700 cells in your pocket, bag, or anywhere they can contact metal objects like keys or coins. Charge on a dedicated lithium-ion charger rated for 21700 cells and do not leave batteries unattended while charging. Fogstar recommend recycling cells after 200-500 charge cycles, when you notice noticeable capacity drop-off.

What's in the Box

• 1 x Fogstar Energy 4000 21700 Battery
• 1 x Protective carry case